A fair amount of "what-if" discussion surrounds rocket engine failure at critical points in various crewed missions -- lunar ascent and trans-Earth injection on lunar landing missions, retrorocket failure on LEO missions.

A popular approach to managing these risks is to use a single engine made reliable through simplicity: hypergolic propellants and pressure feed. This has been quite successful.

Including uncrewed missions, when is the last time a mission-critical, pressure-fed, hypergolic engine failed to ignite on command in space? Some technicalities:

  • I would not count a stuck circuit that was solved via something like a circuit breaker reset in short order, i.e. in less than an hour with no screwdriver-level intervention
  • I would not count a single failing attitude thruster that didn't prevent a desired attitude change via other available thrusters
  • I would count something like a single STS OMS engine failing a planned burn in a way that required using the other OMS instead
  • 1
    $\begingroup$ The shuttle OMS engines were built in the 70s, flew on dozens of missions each, and have been cannibalized to use on the Orion service module. I do not believe they ever suffered an ignition failure throughout the program. I'd say those, at least, are pretty reliable. $\endgroup$ Jan 20, 2017 at 0:21
  • $\begingroup$ The most mission-impacting hypergolic failure I can think of from the STS program was an RCS jet leak on the first Shuttle Mir mission. As Murphy would have it, it was a jet that faced Mir, and the Russians freaked out about contamination of the station. It nearly led to loss of mission objectives until cooler heads prevailed. $\endgroup$ Jan 20, 2017 at 0:23
  • $\begingroup$ The Apollo missions used such an engine for the service module and the descent and ascent stage of the lunar module. The only misson when one of these engines could not be used was Apollo 13. But as we all know, this was caused by the burst oxygen tank and not by the engine itself. But the engine of the lunar module did work even while being docked to command and service module and helped to rescue the astronauts. It was never planed before to use the engine this way. $\endgroup$
    – Uwe
    Jan 20, 2017 at 15:44
  • 2
    $\begingroup$ Yeah, I wouldn't count A13 by my criteria: the SPS engine worked fine each time it was fired, and there was no attempt to fire it after the accident. However, it's incorrect to say using the LM engine for maneuvering wasn't considered. The LM lifeboat role was planned for. $\endgroup$ Jan 20, 2017 at 22:45
  • $\begingroup$ Yep, they are very reliable when new or recently serviced. After 12 years in outer space pretty much nothing is reliable. Valves get stuck, pressurizant leaks, hypergols corrode their containers, greases dry up..These engines are still more reliable than most of the competition though. $\endgroup$
    – SF.
    May 25, 2017 at 9:48

3 Answers 3


Generally, well-designed pressure-fed hypergolic engines are very reliable. However, failures of some designs have certainly occurred. Here are some recent examples:

The Juno probe, currently in orbit around Jupiter, had to significantly change its mission plans due to concerns about potential catastrophic (as in explosive) failures due to stuck helium check valves in its main engine, a Moog LEROS bipropellant hypergolic thruster. These concerns arose after malfunctions of the same engine on the MUOS 5 and Intelsat 33e geostationary satellites.

Another example is the failure of the 11D458M pressure-fed vernier engine on the third stage of the Proton-M/Briz-M rocket intended to launch the MexSat-1 telecom satellite in 2015.


Yamal 601 launched this year looks as if it may be a candidate.

Its not clear from the references though some sources seem to have a reason to claim there was a problem with the main 400N thruster as it transferred to complete is GTO transfer on 10N engines.


Gunters "During orbit raising to geostationary orbit, a possible irregularity occured with the apogee engine during the first burb, so further orbit raising is performed with the attitude control thrusters as a reserve propulsion system"

SFN "Gazprom Space Systems said the problem was presumably caused by a deviation in the thrust vector from Yamal 601’s primary orbit-raising engine"


The AJ10 is a pressure-fed hypergolic engines that has been used in multiple programs.

An AJ10 engine was first fired in flight during the third Vanguard launch, on 17 March 1958, which successfully placed the Vanguard 1 satellite into orbit.

The AJ10-137 engine was used in the Apollo Service Module's service propulsion system.

The AJ10-190 engine was used on the Space Shuttle Orbital Maneuvering System (OMS) for orbital insertion, on-orbit maneuvers, and de-orbiting. Following the retirement of the Shuttle, these engines will be repurposed for use on the Orion spacecraft's Service Module.

credit: https://en.wikipedia.org/wiki/AJ10

Likewise, the Descent Propulsion System (DPS) or Lunar Module Descent Engine (LMDE) has a long history of reliable performance. The LMDE is a variable-throttle hypergolic rocket engine developed by Space Technology Laboratories (TRW) for use in the Apollo Lunar Module Descent Stage.

After the Apollo program, the DPS was further developed into the TRW TR-201 engine. This engine was used in the second stage, referred to as Delta-P, of the Delta launch vehicle (Delta 1000, Delta 2000, Delta 3000 series) for 77 successful launches between 1972–1988.


  • $\begingroup$ Interesting information as far as it goes, but it doesn't answer the question in the body of my post: when have such engines failed? See also space.stackexchange.com/q/27449/195 regarding the SPS AJ10's history. $\endgroup$ Aug 15, 2018 at 1:11

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