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Development of low cost fuels for hybrid rocket engine

Akiyo Aoki 1 and Apollo B. Fukuchi

2

IHI AEROSPACE Co., Ltd., 900, Fujiki, Tomioka-shi, Gunma, 370-2398, Japan

Hybrid rocket engines have advantages of environmental friendliness, reliability, and

safety because of its chemical and mechanical simplicity. The authors have studied many

kinds of hybrid rocket engine, and one of our main targets is to develop a reusable engine for

reusable vehicle. Supposing that the all ingredients are made in Japan, our estimation

implies that the propellant cost of reusable hybrid rocket engine with N2O/HTPB accounts

for 2/3 of total vehicle recurring cost. Therefore it’s very effective to reduce the propellant

cost to realize low cost reusable vehicle. The authors chose low cost plastics or low cost liquid

polymer as low cost fuel instead of Solid propellant grade HTPB, usual used for hybrid

motor fuel. Our purpose is to develop the low cost fuel which is applicable to flight engine. In

this paper, we experimented two ways of fuel production. One is to mix low cost plastic

particles with binder, and the other is to use low cost polymer as a substitute for Solid

propellant grade HTPB.

The candidates of low cost plastics were polystyrene, polyethylene and polypropylene,

and those of low cost binder were HTPE, industrial popular use HTPB, thermal plastic

elastomer. We examined the property of the fuel process and we could produce 6 kinds of

low cost fuel. The hot firing tests with subscale engine with GOX were performed and

revealed that the ignition and combustion of all these fuels were stable. The regression rates,

c* efficiencies were obtained. The costs of 6 fuels were estimated.

Nomenclature

Gox = GOX flux

oxm& = GOX mass flow rate

Ap = fuel port section area

B = blowing factor

Koxe = Kox is the oxidizer concentration, subscript e indicates the outer edge of the boundary layer

hfl = stagnation enthalpy at the flame temperature

hw = enthalpy at the wall in the gas phase

ΔHv,eff = the total heat of gasification

ρf = fuel density

r = regression rate of fuel

G = the local mass flux due to both oxidizer injection and all upstream fuel addition

x = the distance along the CP grain shape from the entrance

I. Back ground

YBRID rocket engines inherently combine the safety features of a liquid propulsion system (throttle, shutdown,

re - start), while deriving the cost and operational benefits of a solid propulsion system1. In addition, hybrid has

environmental friendliness because the propellant is non toxic and the exhaust gas is HCl free. CO2 emission in the

life cycle of hybrid propellant could be reduced comparing with solid propellant and LOX/LH2. Therefore, hybrid is

one of very hopeful engines in the next generation.

Recent successful application of hybrid rocket engine to a manned flight demonstration by SpaceShipOne may

suggest that advantages have been overlooked in some potential applications, and hybrid rocket engines may be

1 Staff, Technologies Department Office, Technologies Department Dept. 2 Chief, Technologies Department Office, Technologies Department Dept.

H

46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit25 - 28 July 2010, Nashville, TN

AIAA 2010-6638

Pending final paper processing, any questions regarding copyright status and permission to reprint or reuse this paper must be directed to AIAA.

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getting renewed interest1. We could say space tour by reusable hybrid engine is in demand because hundreds people

have already booked the ticket of SpaceShipTwo flight, the succeeding spacecraft of SpaceShipOne.

IHI AEROSPACE CO., LTD (IA) has studied various hybrid rocket engines for about 30 years. One of IA’s main

purposes is to develop a reusable hybrid engine for reusable vehicle. We think issues of our reusable engine are to

develop large scale engine and to reduce the reccuring cost. First, we will work on reduce cost.

II. Purpose

Our purpose is to develop low cost fuel which performance is comparable to HTPB. Supposing that the all

ingredients are made in Japan, our estimation implies that the propellant cost of sub orbital reusable hybrid rocket

engine with N2O / HTPB accounts for 2 / 3 of total vehicle recurring cost. Therefore it’s very effective to reduce the

propellant cost. Solid propellant grade HTPB is narrow distribution and expensive comparing with industrial general

use materials in Japan. Therefore fuel cost can be reduced by using low cost plastics and liquid polymers, which are

industrial general use materials with wide distribution.

III. Fuel Development

A. Selection of the candidates

To develop a new fuel with suitable property for reusable hybrid rocket engine grain, we tried two approaches as

follows (Fig.1).

1. To mix low cost plastic particles to reduce fraction of expensive HTPB for solid motor propellant.

2. To use substitutable low cost binder for HTPB for solid motor propellant.

First, we tried to produce the low cost fuel with plastic particles mixing with binder. Wide distributed popular

plastics tend to be low cost, so we investigated distribution property of plastics and selected following as candidates

of fuel ingredient.

・Polyvinyl chloride (PVC)

・Polyurethane (PU)

・Polymethylmethacrylate (PMMA)

・Polystyrene (PS)

・Polyethylene(PE)

・Polypropylene(PP) We narrowed down the 6 candidates considering with required fuel property, cost, performance, etc. The required

fuel properties are

high density – to improve density specific impulse,

low melting point – to improve regression rate,

high specific impulse – to improve engine performance,

high optimum O/F value – to achieve low cost propellant because fuels are expensive than LOX.

Table 1 shows our trade-off result of six candidates and we selected PS, PE and PP considering the properties

totally.

Next, we tried to produce the low cost fuel with low cost binder. We investigated distribution property of binders

which have elasticity to obtain mechanical property enough to flight, and selected following as candidates of fuel

ingredient.

・Hydroxyl terminated polyether (HTPE)

・Industrial popular use hydroxyl terminated polybutadiene (low cost HTPB)

・thermal plastic elastomer (TH) Table 2 shows the values of density, melting point, specific impulse and optimum O/F value. These propellant

costs are about 15% to 40% of LOX/HTPB.

B. Examination of the fuel process property

We examined the fuel process property of 3 kinds of low cost plastic particles with binder by changing the

composition of 3 plastic particles. Examined the fuel process properties were slurry fluidity and adhesion property

between particles and binder. If slurry fluidity is low, mixing and molding of grain would be difficult. And if

adhesion is insufficiency, particles would come off the burned surface during combustion, then local erosion or

combustion pressure pulse could occur and they would cause combustion instability.

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The binder was HTPB for solid motor propellant, which was known component of polymer and curing agent. To

increase the amount of plastics to reduce the fuel cost, fine and coarse particles were prepared each plastic (Table3).

We tested 2 compositions about each plastic to examine the effect of the ratio of coarse and fine particle on the

slurry fluidity. Table 4 shows the compositions of HTPB for solid motor propellant and coarse and fine particles of

plastic. We determined mass fraction of HTPB for solid motor propellant considering with its volumetric fraction of

composite propellant. No.1 is mixture of equal parts of coarse and fine. No.2 is 3:7, fine particles more than coarse.

Examination results are following:

PS – Both compositions PS No. 1and No. 2 were excellent.

PE – We mixed composition PE No.1 and it showed very low fluidity. We didn’t try PE No.2 because its fluidity

would be lower than PE No.1 as fine particles increased. After fuel thermal curing, the adhesion property between

the coarse particles and binder was very poor.

PP – We mixed composition PP No.1 and it showed very low fluidity. We didn’t try PP No.2 because its fluidity

would be lower than PP No.1 as fine particles increased. After fuel thermal curing, the adhesion property between

coarse particles and binder was good.

From these results (Fig.2), finally we selected PS as plastic mixing with binder because of excellent fluidity and

adhesion property.

About each low cost liquid polymer, we confirmed curing condition. The composition of polymer and curing agent

and catalytic agent was determined. The catalytic agent ratio influenced on mechanical property of HTPE, so we

made some HTPE fuel changing amount of catalytic agent and selected one composition considering with the

hardness, tensile strength of fuel.

Finally we produced 6 kinds of low cost sub-scale fuel grains (Table 5). The fuel grain design is center

perforated, and its outer diameter is 80 mm, inner diameter is 20 mm and length is 140 mm. Figure 3 shows a top

view of a fuel grain.

IV. Hot firing test and results

We examined hot firing tests to measure regression rate and confirm ignition and combustion stability of new

fuels. With above sub - scale fuel grains, the hot firing tests with sub - scale engine were performed at IA Tomioka

test stand.

A. Test conditions and equipment

The test conditions are shown in Table 6.

We use GOX as oxidizer to simplified comparison to combustion properties (regression rate, combustion

stability, etc.) of low cost fuels.

GOX flux is defined as

p

ox

oxA

mG

&=:

oxm& : GOX mass flow rate

Ap: fuel port section area

We determined GOX mass flow rate to achieve the same level of designed engine GOX flux.

GOX supply duration was 3sec to take time profile of regression rate. To obtain regression rate vs. GOX flux

profile, same fuel grains are tested 3 times, and regression rates were measured after each hot firing test.

We used 6 kinds of low cost fuel, HTPB+PS, HTPB+PS (fine increased), HTPE, low cost HTPB+PS, HTPE+PS,

TH and HTPB as a reference. Table 5 shows the composition of fuels.

Combustion chamber pressure was 3MPa. Purge gas was GN2.

Figure 4 shows the test equipment.Mass flow rate of GOX is controlled by an orifice. The flow is arranged by

straightening vane.

We use solid propellant for ignition. First, ignitor heats assistant fuel, then GOX supply starts and engine is

ignited.

B. Test results

The ignition and combustion of all tests were very stable except a few cases with pressure pulse. Figure 5 shows

a hot firing test and figure 6 shows an example of chamber pressure profile. Table 7 shows summery of test results.

GOX flux decreased with increasing fuel port area, resulting decrease of regression rate. Figure 7 shows regression

rate vs. GOX flux, and experimental equation of regression rate (following as rb) with GOX flux (following as Gox).

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Regression rate of TH is about 1.4 times as high as HTPB and those of the others are a little less than HTPB. After

the all hot firing tests, the surfaces of fuel grain were covered with dimples, not only fuels using plastic particles but

also binder only. There were some large cavities on burned surface of HTPB+PS (fine increased), HTPE+PS. It is

possible that the cavities formed by local defection by fuel process in the fuel or local distraction.

HTPB+PS, HTPB+PS (fine increased) and low cost HTPB+PS showed low frequency oscillation in a short time

of last phase of the test. There are two possible reasons for low frequency oscillation. First reason is local

combustion instability at crack of fuel by mixing polystyrene. Polystyrene could come off binder because they are

not chemical bond, then crack between polystyrene and binder could occur. At the crack, heating rate would be high

then regression rate and mass flow rate would be high locally. It is possible that perturbation occur on the fuel

surface by mass flow rate increase. Second reason is coupling of recirculation occurrence and acoustic instability by

test configuration. Recirculation zone occur and exhausted periodic, acoustic instability and recirculation zone

occurrence are coupling, then low frequency oscillation created7.

All of the c* efficiencies were nearly 100%. The reason of high c* efficiency could be using GOX and the

volume of aft chamber is enough flammable gas mixing.

The authors selected HTPE and TH as hybrid fuel from their stable combustion and good mechanical property.

V. Discussion

Simplified regression rate expression for hybrid combustion neglecting radiant heat transfer is described as

follow5,

2.08.023.0 −∝ xGBrfρ (1)

where

( ) ( )[ ]FO

HhhKFOKB

effvwflOXeOXe

/

// ,∆−++= (2)

・ρf: fuel density

・r: regression rate of fuel

・B: blowing number

・G: local mass flux due to both oxidizer injection and all upstream fuel addition

・x: distance along the CP grain shape from the entrance

・Koxe: oxidizer concentration, subscript e indicates the outer edge of the boundary layer

・hfl: stagnation enthalpy at the flame temperature

・hw: the enthalpy at the wall in the gas phase

・ΔHv,eff: total heat of gasification. About HTPB, HTPB+PS, HTPE, HTPE+PS, TH, we calculated their regression rate by O/F profile from hot

firing test results (regression rate profile along the CP grain shape and GOX average mass flow rate), then compared

hot firing test result of regression rate profile （Fig.8）. In this calculation, we supposed as follows: GOX and fuel gas are mixed instantly and completely

GOX average mass flow rate is constant along the CP grain shape

Koxe is 1

Enthalpies are calculated with cp (by SP273) and adiabatic flame temperature or wall temperature

Wall temperature is assumed as fuel pyrolysis start temperature measured by TD-DTA

Table 8 shows density, pyrolysis start temperature, heat of gasification of each fuel.

Regression rate is unaffected by thermal radiation in region that G is 179-455kg/sec/m2, turbulent heat transfer is

dominant5.

HTPB, HTPE, PS, ES calculations are almost correspondent to test results about trend (HTPE increased along

the CP grain shape) and value.

Test result of TH regression rate is not correspondent to calculated value because TH regression couldn’t be

represented Eq.1. We think flux effect on TH is large because the n index of TH is high, such as paraffin-based fuel

combustion6 (Fig.9).

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Figure 1. Image of two approaches to reduce of fuel cost.

Table 1. Trade-off of plastics mixing with binder.

Candidates Density

[g/cm3]

Melting point or

softening point

[℃]

Max

vacuum

specific

impulse

[sec]※

Optimum

O/F

[-]

Rating

HTPB 0.95 ○ 241 ○ 319 ○ 2.2 ○ Reference

PVC 1.38 ◎ 310 △ 280 △ 1.1 △ △

PU 1 ○ 222 ○ 309 ○ 1.6 △ △

PMMA 1.19 ◎ 130 ◎ 292 △ 1.5 △ ○

PS 1.12 ◎ 240 ○ 314 ○ 2.1 ○ ◎

PE 0.94 ○ 130 ◎ 322 ◎ 2.5 ◎ ◎

PP 0.9 △ 180 ◎ 329 ◎ 2.4 ◎ ◎

※calculated value by SP273:3MPa, ε=9

◎: excellent ○: good △: not good, but acceptable

Table 2. Properties of liquid polymer candidates.

Candidates Density

[g/cm3]

Melting point or

softening point

[℃]

Max

vacuum

specific

impulse

[sec]※

Optimum

O/F

[-]

Low cost HTPB 0.88 - 319 2.2

HTPE 1.00 - 322 1.6

TH 0.89 70 316 2.1

※calculated value by SP273:3MPa, ε=9

Low cost plastic particles

Binder

HTPB LowCostbinder

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Table 3. Selected plastic particles sizes.

Materials Coarse[mm]

(average major axis)

Fine[μm]

(average particle diameter)

PS

3

250

PE

4

20

PP

4

4

Table 4. Composition of examination of fuel process, binder and plastic particles (wt.%).

No. HTPB for solid

motor propellant

Plastic

coarse

Plastic

fine

1 30 35 35

2 30 21 49

a) PS: Excellent. b) PE: Poor fluidity & adhesion. c) PP: Poor fluidity.

Figure 2. Fuel mixing results.

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Table 5. Test fuels.

Type Name Composition Cost Max vacuum specific impulse [sec]

HTPB+PS HTPB 30%+PS coarse 35% +PS fine 35%

0.38 315.4

HTPB+PS (fine increased)

HTPB 30%+PS coarse 21% +PS fine 49%

0.38 315.4

Low cost HTPB +PS

Low cost HTPB 30% +PS coarse 35%+PS fine 35%

0.19 315.4

Low cost Plastic Particles + binder

HTPE+PS HTPE 30%+PS coarse 35% +PS fine 35%

0.10 315.9

HTPE HTPE 100% 0.11 321.5 Low cost binder TH Thermal plastic elastomer 100% 0.10 315.6 Reference HTPB HTPB 100% 1 318.9

Figure 3. Top view of sub-scale grain (PS).

Table 6. Test conditions.

Oxidizer GOX GOX mass flow rate 80 [g/sec]

Fuel Table 1 Burn time 3 [sec] x 3 times

Combustion pressure 3 [MPa] (target) O/F 2.2 (target)

Figure 4. Tes t equipment.

80mm

20mm

Restrictor

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Figure 5. Hot firing test.

Figure 6. Example of chamber pressure profile.

Chamber

Nozzle

Water ejector

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Table 7. Summery of test results.

Figure 7. Regression rate vs. GOX flux.

Fuel No.GOX supply

duration[sec]

Average stablechamber pressure

[MPaA]

AverageGOX massflow rate[g/sec]

AverageO/F[-]

AverageGOX flux

[kg/s/m2]

Averageregression

rate[mm/sec]

Average c*efficiency

[-]

1 3.07 3.60 76.8 1.42 156.1 1.19 1.0362 2.96 3.43 76.4 1.41 99.2 0.94 0.9893 3.00 3.41 78.4 1.56 75.9 0.74 0.9671 3.04 3.59 77.0 1.41 162.1 1.10 1.0172 3.04 3.49 77.3 1.66 107.2 0.81 1.0133 3.02 3.22 77.7 1.81 82.7 0.61 0.9621 2.99 3.65 77.8 1.39 166.0 1.09 1.0262 3.02 3.49 78.3 1.61 109.8 0.84 0.9933 3.07 3.49 80.9 1.79 85.5 0.68 0.9451 3.01 3.26 78.2 1.79 184.3 1.00 1.0072 2.99 2.74 82.3 1.91 127.8 0.83 0.9873 3.03 2.43 82.4 1.96 95.0 0.70 0.9661 3.13 3.34 74.2 1.60 154.9 1.11 0.9422 3.01 3.16 76.0 1.96 103.5 0.85 0.9643 3.11 2.90 76.1 2.29 80.9 0.49 0.9791 3.24 3.40 77.1 1.58 169.9 1.11 0.9562 3.09 2.94 73.8 1.73 105.1 0.79 0.9323 3.02 2.89 76.0 1.85 82.1 0.67 0.9521 3.09 3.62 75.5 1.21 146.4 1.66 0.9612 3.10 3.31 75.6 1.60 84.7 1.07 0.9183 3.10 3.03 75.6 1.88 64.3 0.58 0.948

Low cost HTPB+PS

HTPE+PS

TH

HTPB

HTPB+PS

HTPB+PS(fine increased)

HTPE

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0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 50 100 150 200 250 300 350 400 450 500

Distance from entrance [mm]

Regr

ess

ion r

ate

[m

m/se

c]

HTPE HTPE cal.

HTPB HTPB cal.

PS PScal.

ES ES cal.

TH TH cal.

Figure 8. Regression rate vs. distance from entrance (test and calculated result).

Table 8. Density, pyrolysis start temperature, heat of gasification of each fuel.

Figure 9. Schematic of the regression mechanism model6.

VI. Conclusion

We examined fuel production property by mixing low cost plastic particles or using low cost binder.

We confirmed grain production and combustion characteristics of 6 kinds of low cost fuel.

These hot firing test results show that HTPE and TH are more suitable for hybrid fuel than others.

Propellant cost can be reduced with above 2 fuels comparing with HTPB.

FuelDensity

[kg/m3]

Pyrolysis start temperature[K]

Heat of gasification[J/kg]

HTPB 920 681.1 391447PS 990 666.7 455224ES 1030 666.7 385953TH 890 505 708493

HTPE 800 603.9 252907

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References

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to Swirling-Oxidizer-Flow-Type hybrid rocket engines-”, Proceedings of the Conference on Aerospace Propulsion, Vol.47th,

B06, 2007

5M. Chiaverini, “Review of Solid-Fuel Regression Rate Behavior in Classical and Nonclassical Hybrid Rocket Motors”,

Fundamentals of Hybrid Rocket Combustion and Propulsion, pp.37-125, AIAA, 2007 6I. Nakagawa, “A Study on the Regression Rate of Paraffin-based Hybrid Rocket Fuels”, AIAA 2009-4935 7C. Carmicino, “On the Role of Vortex Shedding in Hybrid Rockets Combustion Instability”, AIAA 2008-5016