Altman, a re t i red founder of UTC Chem-ical Systems Di v., who has been experi-menting with hybrids for four decades.

Potential problems with wax includeslumping in storage and cracking at largesizes. But cracking in a hybrid is not thecatastrophe it is with a solid.

“The downfall of hybrids has been theburn rate,” said Gre g o ry G. Zilliac, NASAAmes Re s e a rch Center chief test engineerfor the Ames Hybrid Combustion Facil-i t y, part of a joint re s e a rch program be-t ween St a n f o rd and Ames. A 220,000-p l u s - l b f. motor built by the defunct hybridcompany Amroc had 15 ports. “Ve ry fewm u l t i - p o rt motors have flown successful-l y, and they have had problems in gro u n d

demonstrations. With the high G-loadsof flight, many are concerned that the we b swill tear out and it will puff chunks offuel.”

With wax requiring only 1-2 ports, themotor diameter is smaller, the weight islower, and the internal fuel shape is sim-pler and more robust. Wax is cheaper thanHTPB. St a n d a rd hybrids are big, heavy,l ow - p e rformance motors, but a wax hy-brid should be close to a standard solid.

Nice theory, but what is the proof? St a n-f o rd, with the support of the Defense Ad-vanced Re s e a rch Projects Agency (Da r p a )and NASA Ames, has made more than 300firings of wax hybrid motors ranging inscale from 2.0-7.5 in. dia. and pro d u c i n g50-3,500 lbf. thrust, using both nitro u soxide and oxygen as ox i d i zers. They flewa 2.0-in. 5.5-ft.-long rocket to 6,000 ft. in1999 and plan to send a 12-ft.-long, 7-in.-dia. 165-lb. rocket to 80,000 ft. in Ap r i lor Ma y. Fo rty tests of the 7.5-in. motorsh a ve been conducted at the purpose-builtAmes facility.

These tests show that wax’s desirablecharacteristics are so far notaffected by scale, said Br i a nJ. Cantwell, chairman of theSt a n f o rd Ae ro / A s t ro De p t .who was Karabeyo g l u’s the-sis adviser. He wants furt h e rtests at scales of 2 ft., 4 ft.and 6 ft. dia. to check thingslike pre s s u re fluctuationsand fuel structural charac-teristics. Altman would lat-er like 8-10-ft. and 14-15-ft. versions. Zilliac wouldlike to see a 2-ft.-dia. mo-tor tested at Stennis Sp a c eCenter in two years, and at h i rd-generation soundingrocket with 150-mi. altitudec a p a b i l i t y.

If tests continue to gowell, an operational hybrid

52 AVIATION WEEK & SPACE TECHNOLOGY/FEBRUARY 3, 2003 www.AviationNow.com/awst

COVER STORY

Ideal Hybrid FuelIs . . . Wax?MICHAEL A. DORNHEIM/LOS ANGELES

A graduate student at St a n f o rd Un i-versity has found what promises tobe a better hybrid rocket fuel than

the usual ru b b e r i zed polymers like hy-d rox y l - t e r m i n a t e d polybutadiene (HTPB).The new fuel is a broad class of paraffins,i m p recisely known as wax, and may makethe hybrid rocket a more viable contender.

Wa x’s main virtue is that it burns thre etimes faster, producing more thrust for thesame surface area. Hybrids have adva n-tages over conventional solid and liquidrockets of safety, low cost, throttleability,and simplicity, but with fuels like HTPBthe burn rate is so slow that it must belaced with holes for more surface are a ,making it prone to internal breakup.

With wax’s higher burnrate, fewer holes, or ports,a re needed. A single portmay be enough and nom o re than two would bere q u i red, said Arif Karabe-yoglu, the graduate studentwho is now a re s e a rch as-sociate at Stanford’s Dept.of Ae ronautics and Astro-nautics. This compare swith a typical minimum offour ports for smallerHTPB grains and up to 32for larger grains, said Da v i d

ca504“ It might be that at least at that sizethey’re ready for industry to develop intoc o m m e rcial launchers,” Lyles said. “If that’sthe case we may not need to be doing alot of technology work on hybrids. We

can pick them up and use them when weget ready to develop the next-generationlaunch system.”

While liquid oxygen was the ox i d i ze rused on the SOAREX-2 flight and on the

Tests show the burning rateof paraff i n is about thr e etimes higher than standardHTPB hybrid fuel over awide range of conditions.

motor could be a reality in 5-10 ye a r s ,C a n t well said. Altman has formed a com-pany called Space Propulsion Group tod e velop paraffin hybrids, and is part ofthe Stanford team.

The discove ry of wax started withK a r a b e yoglu and Altman attending a1995 propulsion conference where U.S.Air Fo rce re s e a rchers discussed their testsof fro zen pentane as a hybrid fuel. T h epentane burned about three times fasterthan expected, which the Air Fo rce re-s e a rchers attributed to a lower heat of va-porization. Altman and Karabeyoglu re c-o g n i zed the importance of the highburning rate, but knew that cryo g e n i cpentane was not practical. The burn rate,also known as the re g ression rate, is thespeed at which the fuel surface is turnedinto combustible material.

K a r a b e yoglu calculated that pentane’slower heat of vaporization could only ac-count for a small fraction of the burn rate,and re a l i zed that another phenomenonmust be invo l ved. He found the answe rin Alex Craik’s 1960s theory of thin-filmi n s t a b i l i t y, which describes how gas flow-ing over a thin film of liquid can cre a t eunstable waves in the liquid. The impor-tant phenomenon in fro zen pentane is thata thin, low-viscosity film forms unstablewaves, and tiny droplets are produced atthe tips of the waves and then en-trained and combusted in the ox-i d i zer flow (see drawing at right) .

It is this atomization effect thatis key to the burn rate, not the va-porization caused by combustionheat. This conclusion was sup-p o rted by the Air Fo rc e’s re s u l t swith fro zen alcohol, which pro-

duces a more viscous film that is not pro n eto Craik instability. The alcohol burnedat the usual low rate expected from heatof vaporization alone.

Similar results are had with polyethyl-ene and HTPB fuel. When burned, theyhave a liquid film, but it is too viscous top ro d u c t i vely atomize. HTPB burns muchfaster in conventional solid rockets, whereit is intimately mixed with ammoniumperchlorate oxidizer.

THE SEARCH WAS ON for a practical ro o m -temperature fuel with the right thin-filmcharacteristics. After several weeks in thec h e m i s t ry library, Karabeyoglu found thatcheap, plentiful wax might be the ticket.Mo re technically, paraffins with carbonnumbers averaging 32 that are refined toremove oils. Wax for hurricane candles isthe right stuff.

A dye gives the proper opacity to con-t rol melting, creating a 0.004-in.-thick liq-uid layer for atomization. Wax is a goodi n s u l a t o r. Other additives are for stre n g t h —they double shear strength and incre a s etoughness sixfold. The additives total toabout 1% of the wax and make it appearblack. The fuel is called “SP-1.” Cantwe l lsaid aluminum could be added to improvep e rformance in some cases, such as withh yd rogen peroxide ox i d i ze r. Paraffin is notfar re m oved from kerosene, and all the re-

s e a rchers agreed that the wax hybrid is es-sentially an ox y g e n / k e rosene engine.

C a n t well sees one possible early appli-cation as a two-stage upper stage for Da r p a’sRascal program, which uses an aircraft asa first stage to place small payloads into or-bit (AW & S T Fe b. 25, 2002, p. 19). Ot h-er upper stage applications are envisioned,including for the space shuttle payload bay,as well as the high-impulse main motorsfor interplanetary spacecraft. A hybrid mainmotor would reduce the size of the liquidfuel tank, making fuel handling easier.

Hybrids could be used as first-stageboosters, a job now done by strap-on solidsor liquids. Altman is studying the ultimatee x p ression of this—replacing the solidrocket boosters (SRBs) on the space shut-tle. He estimates that the 12-ft.-dia. 150-ft.-tall SRBs could be replaced with waxhybrids that are 14-15-ft.-dia. and 160-170-ft. long and increase total liftoff we i g h tby less than 5%. With the ability to thro t-tle and shut off a hybrid, they can be ru nup and checked on the pad before liftoff,like the liquid main engines are now, andbe shut off early for inflight emergencies.They are sized to give more impulse thanthe SRBs, as a rew a rd for the trouble ofimplementing new boosters. The incre a s ein performance would translate into moreweight to orbit and greater abort options.

Altman has been discussing thisscheme with NASA’s Ma r s h a l lSpace Flight Center. Because waxis safer to fabricate and store thansolid rocket propellant, costsshould be lowe r.

Wax and liquid oxygen (LOX )h a ve an average specific densityof 1.1, similar to HTPB/LOX andlighter than the 1.8 of HTPB sol-id rocket propellant infused withox i d i zer and aluminum. Bu tw a x / LOX vacuum specific im-pulse is about 30 sec. higher thant o d a y’s SRBs—about 295-300

www.AviationNow.com/awst AVIATION WEEK & SPACE TECHNOLOGY/FEBRUARY 3, 2003 53

four ground tests of the bigger motor atStennis, other ox i d i zers have been triedas well. Beginning in the 1980s, a com-m e rcial startup called American Ro c k e tCompany (Amro c ) s p e n t some $20 mil-lion on hybrid technology, including mo-tors that used nitrous oxide and hyd ro g e np e roxide as well as LOX .

O ver time the company conductedsome 300 hybrid motor tests using a va-riety of fuels, ox i d i zers and configurations.One motor, the DM-1, generated thrustl e vels above 220,000 lb. in tests at Ed-wards AFB, Calif., that also demonstrat-ed thrust vector control. But the compa-n y’s planned hybrid commercial spacelaunch vehicle never got off the ground.

A m ro ce ventually failed. But LockheedMartin continued to work on the hybridtechnology it developed for them, ac-c o rding to Tassin. Sp a c e De v, a start u pbased in Pow a y, Calif., also acquired tech-nical rights, proprietary data and patentsf rom Amroc and is continuing to deve l-op commercial hybrid products.

The company has put together a con-cept for a series of orbital “Maneuveringand Transfer Ve h i c l e s” (MTVs) that wouldtransfer payloads between Earth orbits us-ing hybrid propulsion systems. The Na-tional Reconnaissance Office and the Cal-ifornia Te c h n o l o g y, Trade and Commerc eAgency h a ve supported some of the workwith grants and motor contracts, and the

Air Fo rce Re s e a rch Laboratory has award-ed SpaceDev a Phase I contract to devel-op a hybrid propulsion module that canbe qualified to fly in the space shuttle toboost payloads to higher orbits.

While Lockheed Ma rtin has studied us-ing hybrids to replace existing ro c k e t s ,Tassin concedes it is more likely the tech-nology will have to find new applicationslike the Hybrid Sounding Rocket.

“ One of the things you must ove rc o m e ,of course, is that a lot of the existing sys-tems already have rockets that supportthem, so you really need a new program or. . . the ability that your non-recurring costswould be more than offset by the adva n-tages of changing. So , [ we think] you re-

P a r a f fin has a high burn ratebecause its low molten viscosityp romotes unstable waves thatatomize the fuel. More viscousfuels rely on heat of vaporization.

sec. versus 267 sec.NASA has con-

cepts for a flybackSRB, and Zi l l i a csaid there are plansto launch a foldingoblique wing d e-sign next year witha Stanford hybrid.

In such a largem o t o r, the fuel hasto be strong enoughto support its ow nweight under gasf l ow and accelera-tion without shear-ing out, and nots l u m p, or cre e p.The shear stre n g t hof paraffin with ad-d i t i ves is 2-3 timesg reater than HTPB,and while Altmanhas not performed structural calculations,he does not expect this to be a pro b l e m .Tests of SP-1 fuel show that slumpingshould not be a problem if temperatureis maintained below 40-45C (104-113F),C a n t well said, though Altman is morec o m f o rtable with 35C. The melt temper-a t u re is 70C. But they are cautious aboutthese conclusions and look forw a rd to larg-er scale tests for pro o f.

An SRB-size hybrid would pro b a b l yhave the liquid oxygen injected by a tur-bopump because a helium pressurant sys-tem would get too large, Altman said. An-other idea is to burn a small amount ofh yd rogen with the oxygen to heat the he-lium. This technique was planned by Am-roc and can cut the amount of helium inh a l f, Altman said. He consulted with Am-roc for five years (AW&ST Mar. 1, 1993,p. 51). Small rockets work best with coldhelium pressurant, he said.

All the motors fired so far have a singlecylindrical port. They are made by fill-ing a tube completely with molten wax,capping the ends, laying the tube on itsside and spinning it about the longitudi-nal axis at about 2,500 rpm. The waxshrinks 17% as it solidifies, and the cen-trifugal force forms the central port witha glass-smooth finish, Cantwell said. Ot h-er casting techniques will likely be need-ed for large motors, but have not beenw o rked out yet. Ve ry big motors may havethe wax inserted via thick disks that arestacked, Cantwell said.

One factor in solid rocket design is the“b/a ratio,” where “b” is the grain outsidediameter and “a” is the diameter of thep o rt. When a rocket ignites, the case pre s-s u re makes it expand, creating high ten-sile stress trying to rip cracks in the port

s u rface. Higher b/a ratios create highers t resses on the port. Wax is more prone tocracking than ru b b e ry HTPB. But becausewax is stiffer, it helps to resist and reducethe outward deflection, while HTPB justtransmits the loads like jelly. The 7.5-in.motors have been successfully fired atAmes with b/a=2.5 and a chamber pre s-s u re of 960 psi.; a 600 psi. motor withb/a=3 works as we l l, Cantwell said. Ex-pected b/a is below 3. “Cracks are an is-sue with larger motors,” and will bewatched as the tests grow to larger scale.

If micro-cracking becomes a problem,it might be fixed by annealing the grainin a 45C oven. A large crack in a con-ventional solid motor can be catastroph-ic because it increases the burning are a ,but in a hybrid the ox i d i zer can’t get dow nthe crack. Se veral of the 7.5-in. motorsh a ve been fired with cracks with im-measurable effect.

The theoretical vacuum specific im-pulse (Is p) for wax with oxygen is 369 sec.,assuming 500 psi. chamber pressure anda 70:1 expansion nozzle, Zilliac said.HTPB with oxygen is about 360 sec. Inpractice, Altman hopes to achieve 92%efficiency with wax, or 339 sec. The meas-u red efficiency so far is about 85-90%,and it should improve with size. Becausecombustion may not be complete withinthe wax port itself, a post-combustionchamber 10-15% of the length of the grainis added to let the unreacted gases com-bine. Amroc achieved 92%, Altman said.

Tests at Ames with the 7.5-in.-dia. 43-

in.-long motorsh a ve used port size sf rom under 3 in. toabout 5 in., andgaseous ox y g e n( G OX) flow ratesof 2-6 kg./sec. (4.4-13.2 lb./sec.). Mo s ttests are with GOX ,but LOX and ni-t rous oxide havealso been used. T h eG OX/wax mass ra-tio for best Is p i s2.5, but the testshave been run overa wide range, up toa ratio of 4. Hy b r i dcombustion natu-rally shifts tow a rdox i d i zer-rich as themotor burns, butwax does this less

than HTPB, which is good.The Ames motors produce about 2,500

lbf. and take 10 sec. to burn empty. Onehas been shut off at 5 sec., then re s t a rt e d .One also has been throttled back to 50%,then pushed back to maximum. Mo t o r sa re ignited with an initial GOX / m e t h a n eflow and a spark plug. “In 40 runs it hasalways ignited,” Zilliac said. “It’s not afraidto ignite.”

Hybrids have their own design method-ology that is different from other ro c k e tmotors, Cantwell noted. The length of themotor has to be matched to the ox y g e nflow rate. If it is too long, too much fuelis produced and the motor will run fuel-rich, and too short results in oxygen-richconditions.

Since the amount of available ox y g e nd e c reases along the length, one would ex-pect more burning up front, but there ares e l f - regulating effects. The gas flow speedis higher tow a rd the end because it hasbeen heated and fuel mass has been added,and the higher speed creates more heattransfer and fuel entrainment. Si m i l a r l y,a less-burned area will be narrowe r, in-c reasing gas velocity and burn rate untilthe diameter is more even. Tests of the 7.5-in. motors running for 10 sec. showed onlya few percent variation in burn rate alongthe bore, Cantwell said. Even when thro t-tled, the burning is even, Karabeyoglu said.Nonetheless, the Lockheed Ma rtin Hy-brid Sounding Rocket has a second LOXinjector halfway down the bore to miti-gate typical hybrid low - f requency com-bustion instability at startup.

The California Rocket Society tested aform of wax in 1938, but it was not suc-cessful, Cantwell said. Now wax has an-other chance. c

COVER STORY

54 AVIATION WEEK & SPACE TECHNOLOGY/FEBRUARY 3, 2003 www.AviationNow.com/awst

NASA Ames Research Center has testedforty 7.5-in.-dia. wax motors in its HybridCombustion Facility. Stanford and Ameshave a joint hybrid r e s e a rch pro g r a m .