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Hypergolic propellant

The hypergolic fuel    hydrazine   being loaded onto the

MESSENGER   space probe. Note the safety suit the tech-

nician is wearing.

A  hypergolic propellant   combination used in a  rocket

engine   is one whose components   spontaneously ignite

when they come into contact with each other.

The two propellant components usually consist of a fuel

and an   oxidizer. Although commonly used hypergolic

propellants are difficult to handle because of their ex-

treme toxicity and/or corrosiveness, they can be stored as

liquids at room temperature and hypergolic engines are

easy to ignite reliably and repeatedly.

In contemporary usage, the terms “hypergol” or “hy-

pergolic propellant” usually mean the most common

such propellant combination,  dinitrogen tetroxide  plushydrazine and/or its relatives  monomethylhydrazine and

unsymmetrical dimethylhydrazine.

1 History

Soviet rocket engine researcher Valentin Glushko exper-

imented with hypergolic fuel as early as 1931. It was

initially used for “chemical ignition” of engines, start-

ing kerosene/nitric acid engines with an initial charge of

phosphorus dissolved in carbon disulfide.

Starting in 1935, Prof. O. Lutz of the German Aeronau-

tical Institute experimented with over 1000 self-igniting

propellants. He assisted the Walter Company with the

development of C-Stoff which ignited with concentrated

hydrogen peroxide. BMW developed engines burning a

hypergolic mix of nitric acid with various combinations

of amines, xylidines and anilines.[1]

Hypergolic propellants were discovered independently,

for the third time, in the U.S. by  GALCIT   and Navy

Annapolis researchers in 1940. They developed en-

gines powered by aniline and nitric acid.[2] Robert God-

dard,   Reaction Motors   and   Curtiss-Wright   worked on

aniline/nitric acid engines in the early 1940s, for small

missiles and jet assisted take-off (JATO).[3]

The first-ever hypergolic-propellant rocket engine used for hu-

man flight, the Walter 109-509A of 1942–45.

In Germany from the mid-1930s through World War II,

rocket propellants were broadly classed as monergols, hy-

pergols, non-hypergols and lithergols. The ending ergol  is

a combination of Greek ergon or work, and Latin oleum

or oil, later influenced by the chemical suffix   -ol   from

alcohol.[Note 1] Monergols were  monopropellants, while

non-hypergols were  bipropellants which required exter-

nal ignition, and lithergols were solid/liquid hybrids. Hy-pergolic propellants (or at least hypergolic ignition) were

far less prone to   hard starts  than electric or pyrotech-

1

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2   3 HYPERGOLIC COMBINATIONS 

nic ignition. The “hypergole” terminology was coined by

Dr. Wolfgang Nöggerath, at the Technical University of

Brunswick, Germany.[4] The only rocket-powered fighter

ever deployed, the Messerschmitt Me 163B  Komet , de-

pended on its  methanol/hydrazine fueled,  high test per-

oxide consuming HWK 109-509A rocket motor, using its

hypergolic propellants for its fast climb and quick-hittingtactics, at the cost of having a supremely volatile power

system capable of causing a massive explosion, with any

degree of inattention at any time. Other proposed combat

rocket fighters like the Heinkel Julia and reconnaissance

aircraft like the  DFS 228   were meant to use the Wal-

ter 509 series of rocket motors, but besides the Me 163,

only the Bachem Ba 349  Natter  vertical launch expend-

able fighter was ever flight-tested with the Walter rocket

propulsion system as its primary sustaining thrust system

for military-purpose aircraft.

The earliest ballistic missiles, such as the Soviet R-7 that

launched Sputnik 1 and the U.S. Atlas and Titan-1, usedkerosene and liquid oxygen. Although they are preferred

in space launchers, the difficulties of storing a  cryogen

like liquid oxygen in a missile that had to be kept launch

ready for months or years at a time led to a switch to hy-

pergolic propellants in the U.S. Titan II and in most So-

viet ICBMs such as the R-36. But the difficulties of such

corrosive and toxic materials, including leaks and explo-

sions in Titan-II silos, led to their near universal replace-

ment with solid-fuel boosters, first in Western submarine-

launched ballistic missiles   and then in land-based U.S.

and Soviet ICBMs.[5]

The trend among western space launch agencies is awayfrom large hypergolic rocket engines and toward hydro-

gen/oxygen engines with higher performance.   Ariane 1

through 4, with their hypergolic first and second stages

(and optional hypergolic boosters on the Ariane 3 and 4)

have been retired and replaced with the Ariane 5, which

uses a first stage fueled by liquid hydrogen and liquid oxy-

gen. The Titan II, III and IV, with their hypergolic first

and second stages, have also been retired. Hypergolic

rockets are still widely used in upper stages when mul-

tiple burn-coast periods are required.

2 Characteristics

2.1 Advantages

Hypergolic rockets are usually simple and reliable be-

cause they need no ignition system. Although larger hy-

pergolic engines in some launch vehicles use turbopumps,

most hypergolic engines are pressure fed. A gas, usu-

ally helium, is fed to the propellant tanks under pressure

through a series of check and safety valves. The propel-

lants in turn flow through control valves into the com-

bustion chamber; there, their instant contact ignition pre-vents a mixture of unreacted propellants from accumu-

lating and then igniting in a potentially catastrophic hard

Hypergolic propellant tanks of the Orbital Maneuvering System

of Space Shuttle Endeavour

start.

The most common hypergolic fuels,   hydrazine,monomethylhydrazine   and   unsymmetrical dimethyl-

hydrazine, and oxidizer,   nitrogen tetroxide, are all

liquid at ordinary temperatures and pressures. They are

therefore sometimes called storable liquid propellants.

They are suitable for use in spacecraft missions lasting

many years. The   cryogenity   of   liquid hydrogen   and

liquid oxygen limits their practical use to space launch

vehicles where they need to be stored only briefly.

Because hypergolic rockets do not need an ignition sys-

tem, they can fire any number of times by simply open-

ing and closing the propellant valves until the propellants

are exhausted and are therefore uniquely suited for space-craft maneuvering and well suited, though not uniquely

so, as upper stages of such space launchers as the Delta II

and Ariane 5, which must perform more than one burn.

Restartable cryogenic (oxygen/hydrogen) rocket engines

nevertheless exist, notably the RL-10 on the Centaur and

the J-2 on the Saturn V.

2.2 Disadvantages

Relative to their mass, traditional hypergolic propellants

are less energetic than such cryogenic propellant com-

binations as liquid hydrogen / liquid oxygen or liquidmethane / liquid oxygen. A launch vehicle that uses hy-

pergolic propellant must therefore carry a greater mass of

fuel than one that uses these cryogenic fuels.

The corrosivity, toxicity, and carcinogeneity of traditional

hypergolics necessitate expensive safety precautions.

3 Hypergolic combinations

3.1 Common

•  Aerozine 50 +  nitrogen tetroxide (N2O4) – widely

used in historical American rockets, including the

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3

Titan 2; all engines in the Apollo Lunar Module; and

the Service Propulsion System in the Apollo Service

Module.  Aerozine 50 is a mixture of 50% UDMH

and 50% straight hydrazine (N2H4).[6]

•   Unsymmetrical dimethylhydrazine  (UDMH) + ni-

trogen tetroxide (N2O4) – frequently used by theRussians, such as in the Proton (rocket family) and

supplied by them to France for the Ariane 1 first and

second stages (replaced with UH 25); ISRO PSLV

second stage.

•   UH 25 is a mixture of 25% hydrazine hydrate and

75% UDMH.

•   Monomethylhydrazine   (MMH) + nitrogen tetrox-

ide (NTO) – smaller engines and reaction control

thrusters:   Apollo Command Module reaction con-

trol system; Space Shuttle OMS and RCS;[7] Ariane

5   EPS;[8] Draco   thrusters used by the  SpaceX

Dragon spacecraft.[9]

The corrosiveness of nitrogen tetroxide can be reduced by

adding several percent nitric oxide (NO), forming mixed

oxides of nitrogen (MON).[10]

3.2 Less common and obsolete

•   Hydrazine   +   nitric acid  (toxic but stable),[11] also

known as "Devil’s venom", as used in the Soviet R-

16 rocket of the Nedelin catastrophe.

•   Aniline +  nitric acid (unstable, explosive), used inthe WAC Corporal

•   Aniline + hydrogen peroxide (dust-sensitive, explo-

sive)

•  Furfuryl alcohol  +  IRFNA (or white fuming nitric

acid)

•   Turpentine +  IRFNA (flown in French Diamant A

first-stage)

•   UDMH + IRFNA – MGM-52 Lance missile system

•   T-Stoff   (stabilised >80% peroxide) +   C-

Stoff   (methanol/hydrazine/water/catalyst)–

Messerschmitt Me 163   World War II German

rocket fighter aircraft, for its   Walter 109-509A

engine

•   Kerosene   + (high-test peroxide   + catalyst) –

Gamma, with the peroxide first decomposed by a

catalyst. Cold hydrogen peroxide and  kerosene are

not hypergolic, but concentrated hydrogen peroxide

(referred to as high-test peroxide or HTP) run over a

catalyst produces free oxygen and steam at over 700

°C (1,300 °F) which is hypergolic with kerosene.[12]

•   Tetramethylethylenediamine   +   IRFNA   – A lesstoxic and non-mutagenic alternative to  Hydrazine

and its derivatives.

4 Related technology

Although not hypergolic in the strict sense (but rather

pyrophoric),  triethylborane, which ignites spontaneously

in the presence of air, was used for engine starts in the

SR-71 Blackbird, the F-1  engines used in the Saturn V

rocket, and the Merlin engines used in the SpaceX Falcon

9 rockets.

5 Notes

[1] "-ergol”, Oxford English Dictionary

6 References

Citations

[1] O. Lutz, in History of German Guided Missiles Develop-

ment, 1957

[2] Sutton, George P., History of Liquid Propellant Rocket

Engines

[3] The Papers of Robert H. Goddard

[4] Botho Stüwe, Peene Münde West, Weltbildverlag ISBN

3-8289-0294-4, 1998 page 220, German

[5] Clark (1972), p.214

[6] Clark (1972), p.45

[7] T.A. Heppenheimer, Development of the Shuttle, 1972–

1981. Smithsonian Institution Press, 2002.   ISBN 1-

58834-009-0.

[8]   “Space Launch Report: Ariane 5 Data Sheet”.

[9]   “SpaceX Updates — December 10, 2007”.   SpaceX.

2007-12-10. Retrieved 2010-02-03.

[10]   “ROCKET PROPELLANTS”. Retrieved 4 January

2014.

[11] Brown, Charles D. (2003). Elements of spacecraft design.

AIAA. p. 211. ISBN 978-1-56347-524-5.

[12]   “High Test Peroxide” (pdf). Retrieved July 2014.

Bibliography

•   Clark, John (1972).   Ignition! An Informal History

of Liquid Rocket Propellants . New Brunswick, New

Jersey: Rutgers University Press. p. 14.   ISBN 0-

8135-0725-1.

•   Modern Engineering for Design of Liquid-Propellant 

Rocket Engines , Huzel & Huang, pub. AIAA, 1992.

ISBN 1-56347-013-6.

•   History of Liquid Propellant Rocket Engines , G. Sut-

ton, pub. AIAA 2005. ISBN 1-56347-649-5.

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4   7 EXTERNAL LINKS 

7 External links

•   “Hypergolic Reaction”.   The Periodic Table of 

Videos .  University of Nottingham. 2009.

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5

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