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Advantages and drawbacks of boron-fueled propulsion
Acta Astronautica Vol. 29, No. 3, pp. 181-187, 1993
0094-5765/93 $6.00+ 0.00 Copyright © 1993 Pergamon Press Ltd
Printed in Great Britain. All rights reserved
ADVANTAGES A N D DRAWBACKS OF BORON-FUELED PROPULSION ALON GANY and YAAKOVM. TIMNAT Faculty of Aerospace Engineering and Space Research Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel
(Received 17 September 1991; revised 26 February 1992; receivedfor publication 16 November 1992) Abstract--The theoretical energetic advantages and the practical problems and characteristics associated with three modes of ramjet-type propulsion systems are discussed. It is shown that the use of boron may result in a remarkable theoretical energetic gain compared with hydrocarbon fuels, especially in volume limited systems. However, the poor combustion characteristics of boron and the high ignition temperature required (1900 K), impose special constraints on the combustor system in order to obtain high efficiencies. Solid fuels and propellants containing a high percentage of boron tend to exhibit irregular combustion, which may cause thrust modulation, while agglomeration of boron particles may result in incomplete combustion due to the insufficient residence time in the combustor.
1. INTRODUCTION The use of boron in air-breathing propulsion devices such as various types of ramjet and ramrocket engines, including the possible use in the first stage of space launchers, exhibits considerable theoretical energetic advantage over conventional hydrocarbon (HC) fuels[l]. Energetic performance of a propulsion system is commonly expressed by its (fuel) specific impulse Isw For similar flight conditions and thrust level, the specific impulse of a ramjet-type device is, in turn, approximately directly proportional to the heat of combustion per unit mass of the fuel, AH~[1,2]. Frequently, volume rather than mass may be the system's main constraint. For volume limited systems the volumetric specific impulse, plsp, and correspondingly the heat of combustion per unit volume of the fuel, pAH~ (rather than AH~ alone), are the key factors. The specific thrust of a given system (i.e. the thrust per unit mass flow rate of air), which is an important design feature, can be characterized by the air specific impulse, Isp,a= F/(rh~g), where F is the thrust and rha is the air mass flow rate. The air specific impulse is related to the fuel specific impulse through Isp.a = Isp 'f, w h e r e f i s the fuel/air ratio. The maximum value of Isp.a is usually attained for the stoichiometric fuel/air ratio, f~t. Correspondingly, the maximum relative specific thrust may be represented by AHR f~t, the maximum heat release per unit mass of air. Table 1 summarizes the theoretical gravimetric and volumetric heats of combustion as well as the product AH~ 'f~t for selected elements. The values of a typical hydrocarbon fuel, "CH2", representing, e.g. kerosene or polymeric materials, which are the most commonly used liquid or solid fuels, are also shown for comparison. The table reveals that, como
.
pared to HC, boron combustion energy per unit mass is approx. 40% higher and per unit volume it is about three times higher (the highest volumetric energy of all elements). Boron also exhibits a high value of A H ~ f (about twice as high as that of HC), although the highest A H ~ f , corresponding to the highest specific thrust, is demonstrated by Mg. Note that beryllium, which also exhibits outstanding theoretical performance, is very toxic, and therefore is not considered here for practical use. Realization of the high combustion energy of boron and especially its remarkable volumetric performance, implies the use of elemental boron, usually in powder form, in order to make use of its relatively high density (2.35 g/cm 3 vs about 1 g/cm 3 of HC fuels). However, boron particles often exhibit poor combustion and ignition characteristics, which may lead to low combustion efficiencies. The inherent problem associated with boron particle combustion is the formation of a molten oxide layer on the surface, which serves as a barrier between the boron and the air and slows down the boron--oxygen reaction. Only at sufficiently high temperatures is the vaporization rate of this layer high enough to enable fast oxidation and sustained combustion. It has been found that in order to achieve sustained combustion of boron particles, the surrounding gas temperature should be of the order of 1900 K[3]. It should be noted, however, that achieving the highest possible combustion temperature is not necessarily a desired goal, because the formation of gaseous boron oxide (normal boiling temperature 2316 K) results in a substantial decrease in the heat of combustion (compared to the case of liquid boron oxide). Three modes of ramjet operation, using boron, are considered, and the practical problems and combustion characteristics associated with each of 181
182
ALt)N
GANYand YAAKOVM. TIMNAT
Table 1. Thermochemicaldata of combustionof selectedelementsin air Element
p (g/cm 3)
Oxide (selected state)
H (liquid) Li
0.071 0.534
H20 Lifo
(g) (s)
Be B
1.85 2.35
BeO B20 ~
AH~
2.25 1.74 2.70 2.33 2.16--2.3t 2 4.5 5.96 7.20 7.87 7. [ 33 6.49 12.02 16.6
CH 2
0.8-1.08
A,H R ,f
(kcal/cm 3)
/~t
kcabg)
CO,, MgO AI~O~ SiO`, P4Om SO,, TiO z V,O 5 CrzO ~ FeO ZnO ZrO2 PdO Ta20~
(s) (s) (I) (g) (s) (s) (1) (g) (g) (s) (s) (s) (s) (s) (s) (s) (s)
28.9 10.26 9.52 15.88 14.12 13.87 7.83 5.91 7.41 7.72 5.45 2.21 4.71 3.64 2.43 1.13 1.27 2.87 0.193 1.38
2.05 5.48 5.08 29.38 33.19 32.60 17.62 10.28 20.01 17.99 11.8 12.6 4.42 21.20 21.69 17.50 8.89 9.08 18.63 2.32 22.90
0.0292 0.2009 0.2009 0.1305 1).1043 0.1043 0.0869 0.3519 0.2604 0.2033 0.1793 0.2321 0.3467 0.2950 0.5018 0.8084 0.9462 0.6602 1.5401 1.0477
0.844 2.06i 1 ~)13 2072 1.47', 1.440.680 2.080 1.930 1.569 0.977 0.513 1.633 1074 1.219 0.913 1.202 1.895 0.297 1.446
COz,H20
(g)
10-10.5
8.~11.3
0.0677
0.68-O.71
(I)
C (graphite) Mg AI Si P S Ti V Cr Fe Zn Zr Pd Ta
pAH~
(kcal/g)
these modes are discussed. The modes are: liquid fuel ramjet (LFRJ), ducted-rocket (DR), and solid fuel ramjet (SFRJ). Liquid fuel ramjets can employ slurry fuels containing up to 80% boron in kerosene. In solid ductedrockets (ramrockets) boron powder (up to 60-70% mass) is introduced into a mixture of an oxidizer (e.g. ammonium perchlorate) and a polymeric binder to form a fuel rich propellant of self-deflagrating characteristics, whose initial combustion products can further burn with air. In the solid fuel ramjet boron particles (as high as 70% mass) are usually incorporated in a polymeric matrix. The fuel grain does not contain an oxidizer and can only burn when air flows through the combustion chamber. 2. THEORETICAL CALCULATIONSAND EXPERIMENTAL DATA
2.1. Liquid fuel ramjets The general geometry and the main stages of the flow and combustion processes in an LFRJ combustor employing slurry fuels are shown in Fig. 1. One can distinguish different stages. At station 1 fuel is injected into hot air, resulting from the inlet
aerodynamic compression. This mixture undergoes a sudden expansion, step 2. The sudden expansion causes the formation of a hot recirculation zone, in which ignition and partial evaporation of the liquid fuel take place. Between stations 2 and 3 the liquid fuel burns, while the solid particles are heated and eventually undergo combustion between stations 3 and 4. Finally, a conventional nozzle flow takes place. Theoretical performance of slurry fuels was presented by Pinns et al. [4] and by Peleg and Timnat [5]. Due to the complex dependence of the specific impulse on flight and ambient conditions, Peleg and Timnat defined sonic air specific impulse, Sa, and sonic fuel specific impulse, Sf, which are the values obtained in static test conditions with a sonic converging exit nozzle expelling the jet to zero ambient pressure. Calculations of S a and Sr were performed for boron slurries containing 50, 70 and 80% boron using Gordon and McBride's[6] computer code. The value of Sa increases with fuel-air ratio, reaching 160-170 s for stoichiometric mixtures. This value is similar to
So[s} 200
R ECIRCULATION ZONE
Q
®
FuEL AIR
®
®
:
---,.- t ~ - ~ : - --" ~o"-r~'r.
'
o,-
I
-.:-.,,.-:~'..:.:
I" 0 . . ' r : m . '..
LIQUID FUEL VAPORIZATION AND COMBUSTION
-
.
::,, , . ~
, ,':,1.... -:.-.,..'---J
PARTICLE / COMBUSTION | PARTICLE HEATING
,oo Y ~ - 8 o ~
Pe'O"MP°
I /
Fig. 1. Schematic of a slurry-fueled LFRJ combustor.
0
I
o
I
s
I
I
io
I
I
m
I
I
f [o/.] 20
Fig. 2. Calculation of Sa vs f for an LFRJ slurry combustor [5].
Boron-fueled propulsion 5000, PSF[
183
f =0.05
cm3 ]
AIR TEMP [ ° C ] ÷ I00 • I00 Z~ I 0 0 - 200 13 2 0 0 - 3 0 0
,,,~ O
o~-
4000 ! t f = 0.075
PRESSURE [ M P o ] 0,1 0,2-0.3 0.2-0.3 0 . 2 - 0.3
I00
4SO
3 OOO
~e
60
**+
A
oOe
Z~L~
/%
40 20
2000
0
= 0
olo2'
' ' 008 ' ' oi,o 'oi, 006
olo,'
f Fig. 4. Experimental results of the combustion efficiencyof boron slurries[10].
I000
O
120
I
I
I
14.0
160
180
So[,] Fig. 3. Calculation of pS r vs Sa for an LFRJ slurry combustor [5]. that obtained for liquid kerosene and for aluminum slurries (see Fig. 2). Nevertheless, compared to kerosene, sonic volumetric specific impulse, Sfp, can be doubled for boron slurries, reaching values of 28005000 s. g/cm 3 for practical fuel-air ratios (see Fig. 3). In practice the usefulness of a given fuel depends not only on its theoretical performance and burning efficiency, but also on handling characteristics, storage stability and injection properties. The main problems to be overcome for slurry fuels are high viscosity and poor storage stability. The latter can be improved by decreasing the particle size and adding appropriate stabilizing compounds. Thanks to the non-Newtonian character of slurries, it is also possible to overcome the viscosity limitations. Existing experimental evidence indicates that the particulate material within each slurry droplet tends to form an agglomerate during the evaporation process of the volatile ingredients[3,7]. Since the burning time of large agglomerates[8] may be longer than the typical residence time within the combustor, the finer the spray droplets the higher the achievable combustion efficiency. With respect to injection characteristics, when using an air augmented injection technique, it appears that the behavior of slurry fuels is similar to that of liquid fuels as regards the formation of droplets by the air spray and their injection into the combustion chamber. The particle size, which appears to have a significant effect on the performance, is determined by the ratio between the air flow used for spraying and fuel flow. Therefore, using this technique pressure drop across the injector has a smaller effect on the droplet size than for pressure injection. Other properties that influence the injection characteristics are
the viscosity and the surface tension. It is not easy to determine their values, because of experimental difficulties encountered in measurements performed in slurries, and since they depend on temperature and aging. The effect of viscosity is the stronger one, in particular when spraying air to fuel ratio is less than two. The influence of the surface tension is somewhat smaller and diminishes for spraying air to fuel ratios above two. Lipinski et al. [9] found that slurry droplet size generally decreases with increasing the fuel temperature. Experiments performed in a special facility built for studying slurry fuels (see details in [10]) demonstrated the influence of initial temperature and pressure on the combustion efficiency r/of boron slurries having fuel/air ratios between 0.03 and 0.12. The results, presented in Fig. 4, show that the efficiency is low for fuel-air ratios below 0.05. This is a consequence of the fact that in this range of fuel-air ratio the temperature of the combustion gases is too low to ignite the boron particles (below 1900 K), so that only the kerosene burns. For higher fuel-air ratios the efficiency goes up, reaching about 80% around stoichiometric conditions (fuel-air ratio of 0.091). Similar behavior was demonstrated for the different chamber pressures and initial air temperatures. 2.2. Solid ducted-rockets
The solid ducted-rocket (ramrocket) is an air augmented propulsion device consisting of two GAS GENERATOR
.___
,
"GAS
GENERATOR:;2Z22MBUSTO~R
Fig. 5. Illustration o f a ductcd rocket.
184
,'\LON GANYand YAAKOVM. TIMXA{
combustion chambers (see illustration in Fig. 5). The first one serves as a gas generator• It contains a selfdeflagrating fuel-rich solid propellant, whose partially burnt combustion products are injected into the second chamber (the ram-combustor), in which they can further burn with the incoming air. The general characteristics and an up-to-date overview on the status of boron-fueled DR development are given in[ll]. The solid ducted rocket is simpler than the liquid fuel ramjet, and this is one of its major advantages. However, due to the fact that the gas-generator solid grain contains some amount of oxidizer (although much less than a solid propellant rocket), its theoretical energetic performance is somewhat lower than that of other ramjet engines which employ pure fuel without oxidizer (i.e. the LFRJ and the SFRJ). The other advantages of the DR are its relatively high thrust at low speeds and the better ignition and flame-holding characteristics in the ram-combustor, due to the high temperature of the gas generator products injected into it from the first combustor. The overall combustion efficiency and engine performance depend on the combustion and mixing features in the second combustor. However, the combustion characteristics in the solid propellant gas generator set the initial conditions in the ramcombustor and have a significant effect on the overall motor behavior. The combustion characteristics of fuel-rich solid propellants were studied experimentally (see Laredo and Gany[12]), using a windowed strand burner and high speed photography. The investigation revealed that, when using propellants containing 40-50% boron, irregular combustion took place• The surface regression was non-uniform both temporally and spatially. The solid propellant combustion exhibited a sort of periodic behavior (Fig. 6) with a characteristic time scale of about 100-300 ms, during which large variations in the regression rate occurred. The period included three major stages: (a) slow, quiescent, nonluminous deflagration including penetration of glowing particles under the condensed surface: (b) ejection of a segment or a layer of condensed material to the gas phase: and (c) a high flux of burning and TUNNEL
l
~ ~
-
I minT JL
SLOW PROPAGATION AND QUIESCENT COMBUSTION. FORMATION OF "TUNNELS" INSIDE THE SPECIMEN
COMBSI~L~ON -
EJECTION OF A SEGMENT OF PROPELLANT
AGGLOMERATES
~
--
LUX AND
OF BURNING AGGLOMERATES PARTICLES
Fig. 6. Schematic diagram of the main stages in a typical combustion cycle with a high boron content, fuel-rich solid propellant (after [12]),
t.L
'° i .
.
.
.
.
.
.
.
.
.
~0.5
_J 0 W FJ
I~
o
Ioo
200
300
APPARENT AGGLOMERATE SIZE (/~m)
Fig. 7. Cumulative volume fraction of agglomerates ejected during the combustion of a fuel-rich propellant containing 40% boron vs apparent agglomerate size[12]. non-burning particles• The boron particles, originally of sub-micron size, formed agglomerates of typical dimensions between 60 and 150 #m. The ejected agglomerates size distribution was of the log-normal type (Fig. 7). It should be noted that the agglomeration phenomenon is well known from solid rocket metallized propelants. It has also been demonstrated that during the combustion of solid propellants with high aluminum content (e.g. 20%), irregular phenomena such as flaking of the burning surface take place, especially at low pressures• As the burning rate and combustion product characteristics in the gas generator affect the instantaneous fuel-air ratio, the particle dispersion process, and the necessary particle burning time in the second combustor, it is anticipated that the irregularities of propellant regression rate and the relatively large boron agglomerates, which are ejected from the condensed surface to the gas stream in the gas generator, may cause undesirable thrust modulations and poor overall combustion efficiencies, especially in small size motors.
-9.3. Solid fuel ramjets The solid fuel ramjet is the simplest air breathing propulsion device. It consists of a cylindrical combustion chamber in which a fuel grain, often with a cylindrical bore, is placed, where the incoming air flows through its port. A gas phase diffusion flame between the volatile fuel decomposition products and the air takes place within the boundary layer above the condensed surface. Boron or other metal particles, which are incorporated in the polymeric HC fuel matrix, leave the condensed surface after being exposed to the gas as a result of the fuel regression. Ignition and combustion of the particles take place during their motion in the gas stream• A schematic diagram of a boroncontaining SFRJ combustor is shown in Fig. 8 (see also [ 13,14]). The theoretical energetic performance of the SFRJ is superior to that of the solid DR; however,
Boron-fueled propulsion REATTACHMENT ZONE
EDGE OF DIFFUSION BOUNDARY LAYER FLAME
185 AFT MIXING CHAMBER
SOLID
PARTZCLES
Fig. 8. Schematic diagram of a boron-containing SFRJ combustor. it exhibits lower thrust at low flight speeds and may suffer from low combustion efficiencies and difficulties in the flame stabilization. The main flame-holding mechanism in the SFRJ combustor is associated with the sudden expansion geometry, which generates a hot fuel-rich flow recirculation zone immediately downstream of the inlet step. Comprehensive theoretical studies on boron partitle behavior in the flowfield of an SFRJ (see details in[13,14]) lead to the conclusion that the specific conditions within the SFRJ combustor are inherently unfavorable for the peculiar requirements for ignition and combustion of boron particles. Boron particles which emerge from the condensed surface have to pass in the vicinity of the hot gas phase reaction zone in order to heat-up and attain sufficiently high temperatures which allow ignition. The physical ignition zone in respect to the particle ejection point stretches between the surface and the combustor centerline, downstream of the ejection location[13] (Fig. 9). The larger the particles, the smaller the ignition zone. However, in order for the particles to burn extensively, they have to reach the high oxygen concentration zone, further from the surface along the combustor hi (J tt. (l¢
core. Satisfaction of both ignition and combustion requirements corresponds to a very limited particle ejection velocity range--a parameter which it is very hard to control directly. Hence, in a typical SFRJ combustor, although most boron particles are likely to attain ignition conditions, the overall degree of boron combustion is expected to be very poor, as indicated by Fig. 10 (see details in[13-16]). Furthermore, high speed photographs of the combustion processes of boron-containing fuels in two-dimensional SFRJ combustors [17,18] reveal that the condensed material ejected from the fuel surface is often neither in the form of individual boron aprticles, nor in the form of boron agglomerates. Typically, fragments and flakes of material leave the surface sporadically and may later disintegrate in the gas stream to form smaller pieces and agglomerates, which are much larger than the original boron particles. This fact worsens the situation, since the combustion time of large agglomerates may be much longer than the residence time in the combustion chamber.
INCREASING
LIJ Z I00 n..
PARTICLE SIZE
gO
z
0 l,l-
LI.
u Z I-, (/')
PAR EJECTION POINT
f - CONDENSEO FUEL SUR7:ACE
o
40
Z 0 P
20
30p.m
,
\
!
<~
,,
Q
DIAMETER 40/J.m ~
80
on..o,m 6 0
t/) IE
PARTICLE
I
0
5
I0
15
20
25
EJECTION V E L O C I T Y [ m / s I AXIAL
DISTANCE
Fig. 9. Particle ignition zone within the SFRJ flowfield with respect to the ejection point.
Fig. 10. Fraction of boron mass burned inside a 1-m-long combustor as a function of the particle ejection velocity [13-151.
186
ALON GANY and YAAKOV M. TIMNAT
A promising solution to overcome the poor combustion efficiency of boron in SFRJ combustors has resulted from a theoretical investigation by Natan and Gany [15,16]. The idea is to introduce an aft-mixing-chamber with bypass air. In this way the combustor is divided into two main parts, each of them fulfilling a different function: (a) the main combustor, where the solid fuel is placed, provides the fuel ingredients to the flow and is the major site of boron particle ignition. (b) The aft-mixing-chamber, whose main role is to enable efficient combustion of the already-ignited boron particles. Using such an arrangement, the theoretical predictions reveal good combustion efficiency and overall performance. Recent experimental data[19] support this approach. One may also consider the enhancement of ignitability and combustion properties of boron containing fuels by the inclusion of certain additives in the solid fuel formulation. Note, however, that in this way the theoretical energetic performance is always lowered. Hence, one should use only minimal amounts of additives, which ensure that the improvement in combustion efficiency is larger than the reduction in theoretical heat of combustion. Different ingredients yielding energy release at the fuel surface or energetic interactions with the boron can promote boron ignition because of preheating of the particles prior to their ejection into the gas stream. Certain polymeric materials and energetic binders can be useful[20,21]. Fluorocarbon polymers such as polytetrafluoroethylene (PTFE, Teflon) or a copolymer of vinylidene fluoride with perfluoropropylene (Viton A) act like oxidizers in their vigorous exothermic reaction with boron yielding gaseous BF 3. Such reactions can initiate boron particle heating while in the condensed phase, enhance material ejection from the surface, and increase the burning rate. The combination of boron and energetic binders, in particular copolymers of azidomethyl oxatane, e.g. 3,3 bis azidomethyl oxatane (BAMO, (CsH8N60),) and nitrate esters, e.g. 3 nitratomethyl 3 methyl oxatane (NMMO, (CsH9NO4),), was shown to produce energetic surface reactions, lower surface temperatures (of the order of 500 K vs 700 K of HTPB binders), higher regression rates, and better dispersion of the boron particles to the gas phase. Metal powders may be other boron combustion promoters[17,18]: magnesium which is easy to ignite improves the general ignition characteristics of the fuel and generates intense heating in the vicinity of the boron particles, thus supporting their ignition and combustion. Another metal, titanium, may be added because of its exothermic condensed phase reaction with boron to yield TiB2, which can cause preheating of the boron in its preignition stage. Experiments with fuels containing mixtures of B and Ti particles [18] reveal surface reactions appearing as a greenyellow glow beneath part of the large surface flakes. The promising potential of boron-titanium interactions for enhancing boron ignition has also led to
the idea of using titanium-coated boron particles [22]. Another interesting concept, which has recently been suggested by Gany[14], proposes the use of composite particles of conglomerates consisting of a mixture of smaller boron and titanium particles. The internal heat source resulting from the exothermic gasless reaction within the composite particle thus augments the external heating. Analysis accounting for borontitanium sintering processes predicts pronounced shortening of the overall ignition time. 3. CONCLUDING REMARKS
The use of boron in ramjet-type propulsion systems may offer remarkable energetic advantages, particularly for volume limited systems, along with high specific thrust levels. On the other hand, the achievable specific impulse (on fuel mass basis) of practical boron-containing compositions demonstrates a little gain over that of HC fuels, attributed in part to incomplete combustion. The most severe problem in using boron is the inherent difficulty concerning its ignition and sustained combustion resulting from the formation of an oxide layer around the particles. In addition, boron particle agglomeration may take place in all three operation modes considered, i.e. liquid fuel ramjet, solid ductedrocket and solid fuel ramjet, leading to long required burning times compared to the residence time within the combustor. LFRJ employing a boron slurry has very high theoretical energetic performance as well as convenient thrust control capacity. The solid DR, which exhibits somewhat lower energetic performance, may provide better ignition and flame-holding characteristics along with a higher thrust at low speeds and simpler operation. The SFRJ is the simplest engine demonstrating theoretical energetic performance similar to that of LFRJ. Nevertheless, its flowfield characteristics lead to inherent difficulties to obtain efficient combustion of boron. The investigation revealed that this problem can be overcome by using an aft-mixing chamber with bypas air or by incorporating certain additives such as energetic binders or metal powders in the fuel grain, which can cause enhanced heating of the boron by exothermic chemical interactions. REFERENCES
1. A. Gany and D. W. Netzer, Fuel performance evaluation for the solid fueled ramjet. Int. J. Turbo Jet Engines 2, 157-168 (1985). 2. A. Gany, Theoretical considerations of the specific impulse of ramjet engines. Proceedings of the 15th Congress of the International Council of Aeronautical Sciences,
London, England. Paper ICAS-86-3.9.5. (1986). 3. G. M. Faeth, Status of boron combustion research. AFOSR
Specialists Meeting on Boron Combustion
(1984). 4. M. L. Pinns, W. T. Olson and H. C. Barnet, NACA research on slurry fuels. NACA Report 1388 (1958).
Boron-fueled propulsion 5. I. Peleg and Y. M. Timnat, Investigation of slurry fuel performance for use in ramjet propulsors. Israel J. Technol. 20, 206-213 (1982). 6. S. Gordon and B. McBride, Computer program for calculation of complex chemical equilibrium compositions, rocket performance, incident and reflected shocks, and Chapman-Jouget detonations. NASA SP-273, CFC-71 (1971). 7. F. Takahashi, F. L. Dryer and F. A. Williams, Combustion behavior of free boron slurry droplets. 21st Symposium (International) on Combustion, pp. 19831991. The Combustion Institute (1986). 8. J. T. Holl, S. R. Turns, A. S. P. Solomon and G. M. Faeth, Ignition and combustion of boron slurry agglomerates. Combustion Sci. Technol. 45, 147-166 (1986). 9. J. J. Lipinski, E. G. Coleman and B. R. Heath, Boron slurry fuel atomization evaluation. AIAA Paper 85-1184 (1985). I0. I. Peleg and Y. M. Timnat, Combustion of aluminum and boron slurry fuels in a dump combustor. 19th Symposium (International) on Combustion, pp. 557-563. The Combustion Institute (1982). 11. R. Strecker and H. L. Besser, Overview of boron ducted rocket development during the last two decades. 2nd International Symposium on Special Topics in Chemical Propulsion: Combustion of Boron-Based Solid Propellants and Solid Fuels, Lampoldshausen, Germany (1991). 12. D. Laredo and A. Gany, Combustion phenomena of highly metallized solid propellants. Acta Astronautica 10, 437-441 (1983). 13. B. Natan and A. Gany, Ignition and combustion of boron particles in the flowfield of a solid fuel ramjet. J. Propulsion Power 7, 37-43 (1991). 14. A. Gany, Combustion of boron-containing fuels in solid fuel ramjets. 2nd International Symposium on Special Topics in Chemical Propulsion: Combustion o f Boron-
15. 16.
17. 18.
19.
20.
21.
22.
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Based Solid Propellants and Solid Fuels, Lampoldshausen, Germany (1991). B. Natan and A. Gany, Effect of bypass air on the combustion of boron particles in a solid fuel ramjet. AIAA Paper 89-2886 (1989). B. Natan and A. Gany, Combustion characteristics of a boron-fueled solid fuel ramjet with aft-burner. International Symposium on Air-Breathing Engines, Athens, Greece, pp. 140-148, ISABE Paper 89-7013, (1989). A. Gany and D. W. Netzer, Combustion studies of metallized fuels for solid fuel ramjets. J. Propulsion Power 2, 423-427 (1986). A. Karadimitris, C. K., Scott II, D. W. Netzer and A. Gany, Regression and combustion characteristics of boron-containing fuels for solid fuel ramjets. J. Propulsion Power 7, 341-347 (1991). Also 26th J A N N A F Combustion Meeting, Jet Propulsion Laboratory, Pasadena, Calif. CPIA Publication 529, Vol. 1, pp. 355-369 (1989). B. Natan and D. W. Netzer, Experimental investigation of the effect of bypass air on boron combustion in a solid fuel ramjet. 2nd International Symposium on Special Topics in Chemical Propulsion: Combustion of BoronBased Solid Propellants and Solid Fuels, Lampoldshausen, Germany (1991). D. M. Chen, W. H. Hsieh, T. S. Snyder, V. Yang, T. A. Litzinger and K. K. Kuo, Combustion behavior and thermophysical properties of metal-based solid fuels. J. Propulsion Power 7, 250-257 (1991). W. H. Hsieh, A. Peretz, I. T. Huang and K. K. Kuo, Combustion behavior of boron-based BAMO/NMMO fuel rich solid propellants. J. Propulsion Power 7, 497-504 (1991). H. F. Calcote, R. J. Gill, C. H. Berman and W. Felder, Production of pure titanium coated boron powders. 2nd International Symposium on Special Topics in Chemical Propulsion: Combustion of Boron-Based Solid Propellants and Solid Fuels, Lampoldshausen, Germany (1991).
0094-5765/93 $6.00+ 0.00 Copyright © 1993 Pergamon Press Ltd
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ADVANTAGES A N D DRAWBACKS OF BORON-FUELED PROPULSION ALON GANY and YAAKOVM. TIMNAT Faculty of Aerospace Engineering and Space Research Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel
(Received 17 September 1991; revised 26 February 1992; receivedfor publication 16 November 1992) Abstract--The theoretical energetic advantages and the practical problems and characteristics associated with three modes of ramjet-type propulsion systems are discussed. It is shown that the use of boron may result in a remarkable theoretical energetic gain compared with hydrocarbon fuels, especially in volume limited systems. However, the poor combustion characteristics of boron and the high ignition temperature required (1900 K), impose special constraints on the combustor system in order to obtain high efficiencies. Solid fuels and propellants containing a high percentage of boron tend to exhibit irregular combustion, which may cause thrust modulation, while agglomeration of boron particles may result in incomplete combustion due to the insufficient residence time in the combustor.
1. INTRODUCTION The use of boron in air-breathing propulsion devices such as various types of ramjet and ramrocket engines, including the possible use in the first stage of space launchers, exhibits considerable theoretical energetic advantage over conventional hydrocarbon (HC) fuels[l]. Energetic performance of a propulsion system is commonly expressed by its (fuel) specific impulse Isw For similar flight conditions and thrust level, the specific impulse of a ramjet-type device is, in turn, approximately directly proportional to the heat of combustion per unit mass of the fuel, AH~[1,2]. Frequently, volume rather than mass may be the system's main constraint. For volume limited systems the volumetric specific impulse, plsp, and correspondingly the heat of combustion per unit volume of the fuel, pAH~ (rather than AH~ alone), are the key factors. The specific thrust of a given system (i.e. the thrust per unit mass flow rate of air), which is an important design feature, can be characterized by the air specific impulse, Isp,a= F/(rh~g), where F is the thrust and rha is the air mass flow rate. The air specific impulse is related to the fuel specific impulse through Isp.a = Isp 'f, w h e r e f i s the fuel/air ratio. The maximum value of Isp.a is usually attained for the stoichiometric fuel/air ratio, f~t. Correspondingly, the maximum relative specific thrust may be represented by AHR f~t, the maximum heat release per unit mass of air. Table 1 summarizes the theoretical gravimetric and volumetric heats of combustion as well as the product AH~ 'f~t for selected elements. The values of a typical hydrocarbon fuel, "CH2", representing, e.g. kerosene or polymeric materials, which are the most commonly used liquid or solid fuels, are also shown for comparison. The table reveals that, como
.
pared to HC, boron combustion energy per unit mass is approx. 40% higher and per unit volume it is about three times higher (the highest volumetric energy of all elements). Boron also exhibits a high value of A H ~ f (about twice as high as that of HC), although the highest A H ~ f , corresponding to the highest specific thrust, is demonstrated by Mg. Note that beryllium, which also exhibits outstanding theoretical performance, is very toxic, and therefore is not considered here for practical use. Realization of the high combustion energy of boron and especially its remarkable volumetric performance, implies the use of elemental boron, usually in powder form, in order to make use of its relatively high density (2.35 g/cm 3 vs about 1 g/cm 3 of HC fuels). However, boron particles often exhibit poor combustion and ignition characteristics, which may lead to low combustion efficiencies. The inherent problem associated with boron particle combustion is the formation of a molten oxide layer on the surface, which serves as a barrier between the boron and the air and slows down the boron--oxygen reaction. Only at sufficiently high temperatures is the vaporization rate of this layer high enough to enable fast oxidation and sustained combustion. It has been found that in order to achieve sustained combustion of boron particles, the surrounding gas temperature should be of the order of 1900 K[3]. It should be noted, however, that achieving the highest possible combustion temperature is not necessarily a desired goal, because the formation of gaseous boron oxide (normal boiling temperature 2316 K) results in a substantial decrease in the heat of combustion (compared to the case of liquid boron oxide). Three modes of ramjet operation, using boron, are considered, and the practical problems and combustion characteristics associated with each of 181
182
ALt)N
GANYand YAAKOVM. TIMNAT
Table 1. Thermochemicaldata of combustionof selectedelementsin air Element
p (g/cm 3)
Oxide (selected state)
H (liquid) Li
0.071 0.534
H20 Lifo
(g) (s)
Be B
1.85 2.35
BeO B20 ~
AH~
2.25 1.74 2.70 2.33 2.16--2.3t 2 4.5 5.96 7.20 7.87 7. [ 33 6.49 12.02 16.6
CH 2
0.8-1.08
A,H R ,f
(kcal/cm 3)
/~t
kcabg)
CO,, MgO AI~O~ SiO`, P4Om SO,, TiO z V,O 5 CrzO ~ FeO ZnO ZrO2 PdO Ta20~
(s) (s) (I) (g) (s) (s) (1) (g) (g) (s) (s) (s) (s) (s) (s) (s) (s)
28.9 10.26 9.52 15.88 14.12 13.87 7.83 5.91 7.41 7.72 5.45 2.21 4.71 3.64 2.43 1.13 1.27 2.87 0.193 1.38
2.05 5.48 5.08 29.38 33.19 32.60 17.62 10.28 20.01 17.99 11.8 12.6 4.42 21.20 21.69 17.50 8.89 9.08 18.63 2.32 22.90
0.0292 0.2009 0.2009 0.1305 1).1043 0.1043 0.0869 0.3519 0.2604 0.2033 0.1793 0.2321 0.3467 0.2950 0.5018 0.8084 0.9462 0.6602 1.5401 1.0477
0.844 2.06i 1 ~)13 2072 1.47', 1.440.680 2.080 1.930 1.569 0.977 0.513 1.633 1074 1.219 0.913 1.202 1.895 0.297 1.446
COz,H20
(g)
10-10.5
8.~11.3
0.0677
0.68-O.71
(I)
C (graphite) Mg AI Si P S Ti V Cr Fe Zn Zr Pd Ta
pAH~
(kcal/g)
these modes are discussed. The modes are: liquid fuel ramjet (LFRJ), ducted-rocket (DR), and solid fuel ramjet (SFRJ). Liquid fuel ramjets can employ slurry fuels containing up to 80% boron in kerosene. In solid ductedrockets (ramrockets) boron powder (up to 60-70% mass) is introduced into a mixture of an oxidizer (e.g. ammonium perchlorate) and a polymeric binder to form a fuel rich propellant of self-deflagrating characteristics, whose initial combustion products can further burn with air. In the solid fuel ramjet boron particles (as high as 70% mass) are usually incorporated in a polymeric matrix. The fuel grain does not contain an oxidizer and can only burn when air flows through the combustion chamber. 2. THEORETICAL CALCULATIONSAND EXPERIMENTAL DATA
2.1. Liquid fuel ramjets The general geometry and the main stages of the flow and combustion processes in an LFRJ combustor employing slurry fuels are shown in Fig. 1. One can distinguish different stages. At station 1 fuel is injected into hot air, resulting from the inlet
aerodynamic compression. This mixture undergoes a sudden expansion, step 2. The sudden expansion causes the formation of a hot recirculation zone, in which ignition and partial evaporation of the liquid fuel take place. Between stations 2 and 3 the liquid fuel burns, while the solid particles are heated and eventually undergo combustion between stations 3 and 4. Finally, a conventional nozzle flow takes place. Theoretical performance of slurry fuels was presented by Pinns et al. [4] and by Peleg and Timnat [5]. Due to the complex dependence of the specific impulse on flight and ambient conditions, Peleg and Timnat defined sonic air specific impulse, Sa, and sonic fuel specific impulse, Sf, which are the values obtained in static test conditions with a sonic converging exit nozzle expelling the jet to zero ambient pressure. Calculations of S a and Sr were performed for boron slurries containing 50, 70 and 80% boron using Gordon and McBride's[6] computer code. The value of Sa increases with fuel-air ratio, reaching 160-170 s for stoichiometric mixtures. This value is similar to
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Fig. 2. Calculation of Sa vs f for an LFRJ slurry combustor [5].
Boron-fueled propulsion 5000, PSF[
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So[,] Fig. 3. Calculation of pS r vs Sa for an LFRJ slurry combustor [5]. that obtained for liquid kerosene and for aluminum slurries (see Fig. 2). Nevertheless, compared to kerosene, sonic volumetric specific impulse, Sfp, can be doubled for boron slurries, reaching values of 28005000 s. g/cm 3 for practical fuel-air ratios (see Fig. 3). In practice the usefulness of a given fuel depends not only on its theoretical performance and burning efficiency, but also on handling characteristics, storage stability and injection properties. The main problems to be overcome for slurry fuels are high viscosity and poor storage stability. The latter can be improved by decreasing the particle size and adding appropriate stabilizing compounds. Thanks to the non-Newtonian character of slurries, it is also possible to overcome the viscosity limitations. Existing experimental evidence indicates that the particulate material within each slurry droplet tends to form an agglomerate during the evaporation process of the volatile ingredients[3,7]. Since the burning time of large agglomerates[8] may be longer than the typical residence time within the combustor, the finer the spray droplets the higher the achievable combustion efficiency. With respect to injection characteristics, when using an air augmented injection technique, it appears that the behavior of slurry fuels is similar to that of liquid fuels as regards the formation of droplets by the air spray and their injection into the combustion chamber. The particle size, which appears to have a significant effect on the performance, is determined by the ratio between the air flow used for spraying and fuel flow. Therefore, using this technique pressure drop across the injector has a smaller effect on the droplet size than for pressure injection. Other properties that influence the injection characteristics are
the viscosity and the surface tension. It is not easy to determine their values, because of experimental difficulties encountered in measurements performed in slurries, and since they depend on temperature and aging. The effect of viscosity is the stronger one, in particular when spraying air to fuel ratio is less than two. The influence of the surface tension is somewhat smaller and diminishes for spraying air to fuel ratios above two. Lipinski et al. [9] found that slurry droplet size generally decreases with increasing the fuel temperature. Experiments performed in a special facility built for studying slurry fuels (see details in [10]) demonstrated the influence of initial temperature and pressure on the combustion efficiency r/of boron slurries having fuel/air ratios between 0.03 and 0.12. The results, presented in Fig. 4, show that the efficiency is low for fuel-air ratios below 0.05. This is a consequence of the fact that in this range of fuel-air ratio the temperature of the combustion gases is too low to ignite the boron particles (below 1900 K), so that only the kerosene burns. For higher fuel-air ratios the efficiency goes up, reaching about 80% around stoichiometric conditions (fuel-air ratio of 0.091). Similar behavior was demonstrated for the different chamber pressures and initial air temperatures. 2.2. Solid ducted-rockets
The solid ducted-rocket (ramrocket) is an air augmented propulsion device consisting of two GAS GENERATOR
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Fig. 5. Illustration o f a ductcd rocket.
184
,'\LON GANYand YAAKOVM. TIMXA{
combustion chambers (see illustration in Fig. 5). The first one serves as a gas generator• It contains a selfdeflagrating fuel-rich solid propellant, whose partially burnt combustion products are injected into the second chamber (the ram-combustor), in which they can further burn with the incoming air. The general characteristics and an up-to-date overview on the status of boron-fueled DR development are given in[ll]. The solid ducted rocket is simpler than the liquid fuel ramjet, and this is one of its major advantages. However, due to the fact that the gas-generator solid grain contains some amount of oxidizer (although much less than a solid propellant rocket), its theoretical energetic performance is somewhat lower than that of other ramjet engines which employ pure fuel without oxidizer (i.e. the LFRJ and the SFRJ). The other advantages of the DR are its relatively high thrust at low speeds and the better ignition and flame-holding characteristics in the ram-combustor, due to the high temperature of the gas generator products injected into it from the first combustor. The overall combustion efficiency and engine performance depend on the combustion and mixing features in the second combustor. However, the combustion characteristics in the solid propellant gas generator set the initial conditions in the ramcombustor and have a significant effect on the overall motor behavior. The combustion characteristics of fuel-rich solid propellants were studied experimentally (see Laredo and Gany[12]), using a windowed strand burner and high speed photography. The investigation revealed that, when using propellants containing 40-50% boron, irregular combustion took place• The surface regression was non-uniform both temporally and spatially. The solid propellant combustion exhibited a sort of periodic behavior (Fig. 6) with a characteristic time scale of about 100-300 ms, during which large variations in the regression rate occurred. The period included three major stages: (a) slow, quiescent, nonluminous deflagration including penetration of glowing particles under the condensed surface: (b) ejection of a segment or a layer of condensed material to the gas phase: and (c) a high flux of burning and TUNNEL
l
~ ~
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SLOW PROPAGATION AND QUIESCENT COMBUSTION. FORMATION OF "TUNNELS" INSIDE THE SPECIMEN
COMBSI~L~ON -
EJECTION OF A SEGMENT OF PROPELLANT
AGGLOMERATES
~
--
LUX AND
OF BURNING AGGLOMERATES PARTICLES
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t.L
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~0.5
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300
APPARENT AGGLOMERATE SIZE (/~m)
Fig. 7. Cumulative volume fraction of agglomerates ejected during the combustion of a fuel-rich propellant containing 40% boron vs apparent agglomerate size[12]. non-burning particles• The boron particles, originally of sub-micron size, formed agglomerates of typical dimensions between 60 and 150 #m. The ejected agglomerates size distribution was of the log-normal type (Fig. 7). It should be noted that the agglomeration phenomenon is well known from solid rocket metallized propelants. It has also been demonstrated that during the combustion of solid propellants with high aluminum content (e.g. 20%), irregular phenomena such as flaking of the burning surface take place, especially at low pressures• As the burning rate and combustion product characteristics in the gas generator affect the instantaneous fuel-air ratio, the particle dispersion process, and the necessary particle burning time in the second combustor, it is anticipated that the irregularities of propellant regression rate and the relatively large boron agglomerates, which are ejected from the condensed surface to the gas stream in the gas generator, may cause undesirable thrust modulations and poor overall combustion efficiencies, especially in small size motors.
-9.3. Solid fuel ramjets The solid fuel ramjet is the simplest air breathing propulsion device. It consists of a cylindrical combustion chamber in which a fuel grain, often with a cylindrical bore, is placed, where the incoming air flows through its port. A gas phase diffusion flame between the volatile fuel decomposition products and the air takes place within the boundary layer above the condensed surface. Boron or other metal particles, which are incorporated in the polymeric HC fuel matrix, leave the condensed surface after being exposed to the gas as a result of the fuel regression. Ignition and combustion of the particles take place during their motion in the gas stream• A schematic diagram of a boroncontaining SFRJ combustor is shown in Fig. 8 (see also [ 13,14]). The theoretical energetic performance of the SFRJ is superior to that of the solid DR; however,
Boron-fueled propulsion REATTACHMENT ZONE
EDGE OF DIFFUSION BOUNDARY LAYER FLAME
185 AFT MIXING CHAMBER
SOLID
PARTZCLES
Fig. 8. Schematic diagram of a boron-containing SFRJ combustor. it exhibits lower thrust at low flight speeds and may suffer from low combustion efficiencies and difficulties in the flame stabilization. The main flame-holding mechanism in the SFRJ combustor is associated with the sudden expansion geometry, which generates a hot fuel-rich flow recirculation zone immediately downstream of the inlet step. Comprehensive theoretical studies on boron partitle behavior in the flowfield of an SFRJ (see details in[13,14]) lead to the conclusion that the specific conditions within the SFRJ combustor are inherently unfavorable for the peculiar requirements for ignition and combustion of boron particles. Boron particles which emerge from the condensed surface have to pass in the vicinity of the hot gas phase reaction zone in order to heat-up and attain sufficiently high temperatures which allow ignition. The physical ignition zone in respect to the particle ejection point stretches between the surface and the combustor centerline, downstream of the ejection location[13] (Fig. 9). The larger the particles, the smaller the ignition zone. However, in order for the particles to burn extensively, they have to reach the high oxygen concentration zone, further from the surface along the combustor hi (J tt. (l¢
core. Satisfaction of both ignition and combustion requirements corresponds to a very limited particle ejection velocity range--a parameter which it is very hard to control directly. Hence, in a typical SFRJ combustor, although most boron particles are likely to attain ignition conditions, the overall degree of boron combustion is expected to be very poor, as indicated by Fig. 10 (see details in[13-16]). Furthermore, high speed photographs of the combustion processes of boron-containing fuels in two-dimensional SFRJ combustors [17,18] reveal that the condensed material ejected from the fuel surface is often neither in the form of individual boron aprticles, nor in the form of boron agglomerates. Typically, fragments and flakes of material leave the surface sporadically and may later disintegrate in the gas stream to form smaller pieces and agglomerates, which are much larger than the original boron particles. This fact worsens the situation, since the combustion time of large agglomerates may be much longer than the residence time in the combustion chamber.
INCREASING
LIJ Z I00 n..
PARTICLE SIZE
gO
z
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LI.
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PAR EJECTION POINT
f - CONDENSEO FUEL SUR7:ACE
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DIAMETER 40/J.m ~
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t/) IE
PARTICLE
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0
5
I0
15
20
25
EJECTION V E L O C I T Y [ m / s I AXIAL
DISTANCE
Fig. 9. Particle ignition zone within the SFRJ flowfield with respect to the ejection point.
Fig. 10. Fraction of boron mass burned inside a 1-m-long combustor as a function of the particle ejection velocity [13-151.
186
ALON GANY and YAAKOV M. TIMNAT
A promising solution to overcome the poor combustion efficiency of boron in SFRJ combustors has resulted from a theoretical investigation by Natan and Gany [15,16]. The idea is to introduce an aft-mixing-chamber with bypass air. In this way the combustor is divided into two main parts, each of them fulfilling a different function: (a) the main combustor, where the solid fuel is placed, provides the fuel ingredients to the flow and is the major site of boron particle ignition. (b) The aft-mixing-chamber, whose main role is to enable efficient combustion of the already-ignited boron particles. Using such an arrangement, the theoretical predictions reveal good combustion efficiency and overall performance. Recent experimental data[19] support this approach. One may also consider the enhancement of ignitability and combustion properties of boron containing fuels by the inclusion of certain additives in the solid fuel formulation. Note, however, that in this way the theoretical energetic performance is always lowered. Hence, one should use only minimal amounts of additives, which ensure that the improvement in combustion efficiency is larger than the reduction in theoretical heat of combustion. Different ingredients yielding energy release at the fuel surface or energetic interactions with the boron can promote boron ignition because of preheating of the particles prior to their ejection into the gas stream. Certain polymeric materials and energetic binders can be useful[20,21]. Fluorocarbon polymers such as polytetrafluoroethylene (PTFE, Teflon) or a copolymer of vinylidene fluoride with perfluoropropylene (Viton A) act like oxidizers in their vigorous exothermic reaction with boron yielding gaseous BF 3. Such reactions can initiate boron particle heating while in the condensed phase, enhance material ejection from the surface, and increase the burning rate. The combination of boron and energetic binders, in particular copolymers of azidomethyl oxatane, e.g. 3,3 bis azidomethyl oxatane (BAMO, (CsH8N60),) and nitrate esters, e.g. 3 nitratomethyl 3 methyl oxatane (NMMO, (CsH9NO4),), was shown to produce energetic surface reactions, lower surface temperatures (of the order of 500 K vs 700 K of HTPB binders), higher regression rates, and better dispersion of the boron particles to the gas phase. Metal powders may be other boron combustion promoters[17,18]: magnesium which is easy to ignite improves the general ignition characteristics of the fuel and generates intense heating in the vicinity of the boron particles, thus supporting their ignition and combustion. Another metal, titanium, may be added because of its exothermic condensed phase reaction with boron to yield TiB2, which can cause preheating of the boron in its preignition stage. Experiments with fuels containing mixtures of B and Ti particles [18] reveal surface reactions appearing as a greenyellow glow beneath part of the large surface flakes. The promising potential of boron-titanium interactions for enhancing boron ignition has also led to
the idea of using titanium-coated boron particles [22]. Another interesting concept, which has recently been suggested by Gany[14], proposes the use of composite particles of conglomerates consisting of a mixture of smaller boron and titanium particles. The internal heat source resulting from the exothermic gasless reaction within the composite particle thus augments the external heating. Analysis accounting for borontitanium sintering processes predicts pronounced shortening of the overall ignition time. 3. CONCLUDING REMARKS
The use of boron in ramjet-type propulsion systems may offer remarkable energetic advantages, particularly for volume limited systems, along with high specific thrust levels. On the other hand, the achievable specific impulse (on fuel mass basis) of practical boron-containing compositions demonstrates a little gain over that of HC fuels, attributed in part to incomplete combustion. The most severe problem in using boron is the inherent difficulty concerning its ignition and sustained combustion resulting from the formation of an oxide layer around the particles. In addition, boron particle agglomeration may take place in all three operation modes considered, i.e. liquid fuel ramjet, solid ductedrocket and solid fuel ramjet, leading to long required burning times compared to the residence time within the combustor. LFRJ employing a boron slurry has very high theoretical energetic performance as well as convenient thrust control capacity. The solid DR, which exhibits somewhat lower energetic performance, may provide better ignition and flame-holding characteristics along with a higher thrust at low speeds and simpler operation. The SFRJ is the simplest engine demonstrating theoretical energetic performance similar to that of LFRJ. Nevertheless, its flowfield characteristics lead to inherent difficulties to obtain efficient combustion of boron. The investigation revealed that this problem can be overcome by using an aft-mixing chamber with bypas air or by incorporating certain additives such as energetic binders or metal powders in the fuel grain, which can cause enhanced heating of the boron by exothermic chemical interactions. REFERENCES
1. A. Gany and D. W. Netzer, Fuel performance evaluation for the solid fueled ramjet. Int. J. Turbo Jet Engines 2, 157-168 (1985). 2. A. Gany, Theoretical considerations of the specific impulse of ramjet engines. Proceedings of the 15th Congress of the International Council of Aeronautical Sciences,
London, England. Paper ICAS-86-3.9.5. (1986). 3. G. M. Faeth, Status of boron combustion research. AFOSR
Specialists Meeting on Boron Combustion
(1984). 4. M. L. Pinns, W. T. Olson and H. C. Barnet, NACA research on slurry fuels. NACA Report 1388 (1958).
Boron-fueled propulsion 5. I. Peleg and Y. M. Timnat, Investigation of slurry fuel performance for use in ramjet propulsors. Israel J. Technol. 20, 206-213 (1982). 6. S. Gordon and B. McBride, Computer program for calculation of complex chemical equilibrium compositions, rocket performance, incident and reflected shocks, and Chapman-Jouget detonations. NASA SP-273, CFC-71 (1971). 7. F. Takahashi, F. L. Dryer and F. A. Williams, Combustion behavior of free boron slurry droplets. 21st Symposium (International) on Combustion, pp. 19831991. The Combustion Institute (1986). 8. J. T. Holl, S. R. Turns, A. S. P. Solomon and G. M. Faeth, Ignition and combustion of boron slurry agglomerates. Combustion Sci. Technol. 45, 147-166 (1986). 9. J. J. Lipinski, E. G. Coleman and B. R. Heath, Boron slurry fuel atomization evaluation. AIAA Paper 85-1184 (1985). I0. I. Peleg and Y. M. Timnat, Combustion of aluminum and boron slurry fuels in a dump combustor. 19th Symposium (International) on Combustion, pp. 557-563. The Combustion Institute (1982). 11. R. Strecker and H. L. Besser, Overview of boron ducted rocket development during the last two decades. 2nd International Symposium on Special Topics in Chemical Propulsion: Combustion of Boron-Based Solid Propellants and Solid Fuels, Lampoldshausen, Germany (1991). 12. D. Laredo and A. Gany, Combustion phenomena of highly metallized solid propellants. Acta Astronautica 10, 437-441 (1983). 13. B. Natan and A. Gany, Ignition and combustion of boron particles in the flowfield of a solid fuel ramjet. J. Propulsion Power 7, 37-43 (1991). 14. A. Gany, Combustion of boron-containing fuels in solid fuel ramjets. 2nd International Symposium on Special Topics in Chemical Propulsion: Combustion o f Boron-
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Based Solid Propellants and Solid Fuels, Lampoldshausen, Germany (1991). B. Natan and A. Gany, Effect of bypass air on the combustion of boron particles in a solid fuel ramjet. AIAA Paper 89-2886 (1989). B. Natan and A. Gany, Combustion characteristics of a boron-fueled solid fuel ramjet with aft-burner. International Symposium on Air-Breathing Engines, Athens, Greece, pp. 140-148, ISABE Paper 89-7013, (1989). A. Gany and D. W. Netzer, Combustion studies of metallized fuels for solid fuel ramjets. J. Propulsion Power 2, 423-427 (1986). A. Karadimitris, C. K., Scott II, D. W. Netzer and A. Gany, Regression and combustion characteristics of boron-containing fuels for solid fuel ramjets. J. Propulsion Power 7, 341-347 (1991). Also 26th J A N N A F Combustion Meeting, Jet Propulsion Laboratory, Pasadena, Calif. CPIA Publication 529, Vol. 1, pp. 355-369 (1989). B. Natan and D. W. Netzer, Experimental investigation of the effect of bypass air on boron combustion in a solid fuel ramjet. 2nd International Symposium on Special Topics in Chemical Propulsion: Combustion of BoronBased Solid Propellants and Solid Fuels, Lampoldshausen, Germany (1991). D. M. Chen, W. H. Hsieh, T. S. Snyder, V. Yang, T. A. Litzinger and K. K. Kuo, Combustion behavior and thermophysical properties of metal-based solid fuels. J. Propulsion Power 7, 250-257 (1991). W. H. Hsieh, A. Peretz, I. T. Huang and K. K. Kuo, Combustion behavior of boron-based BAMO/NMMO fuel rich solid propellants. J. Propulsion Power 7, 497-504 (1991). H. F. Calcote, R. J. Gill, C. H. Berman and W. Felder, Production of pure titanium coated boron powders. 2nd International Symposium on Special Topics in Chemical Propulsion: Combustion of Boron-Based Solid Propellants and Solid Fuels, Lampoldshausen, Germany (1991).