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I just wrote the following paragraph as an edit to this answer:

Using LH2 for regular burns to maintain ISS altitude would require a whole new set of engines that may not yet exist anywhere. While LH2 is one of the main workhorses getting big things into space from a launchpad (e.g. Saturn V except 1st stage, Space Shuttle) it's not usually kept stored for long periods, and engines designed for long term usage and multiple starts over time without maintenance would be a technical challenge. Engines that use cryogenic fuels are substantially more complex to build, operate, and maintain, as they have components that require pre-ignition cool downs.

So you can see that I have a hunch that the answers for "Have reliable, restartable LH2/LOX ever existed? - ever used?" are probably "No" and "No", that LH2/LOX is only fueled at launch time and ignited once, but I really don't know.

Has there ever been an effort to build or test a restartable LH2/LOX engine? Even one restart? Here restart means in space, not in a test rig or launch pad on Earth.

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    $\begingroup$ Um, your hunch is pretty wrong. The Saturn 5's 3rd stage J2 engines were restartable and demonstrated this capability many times. Also Centaur RL10s. $\endgroup$ – Organic Marble Aug 8 '16 at 3:10
  • $\begingroup$ OK that's great! Can you formulate that as an answer? Thanks! $\endgroup$ – uhoh Aug 8 '16 at 3:17
  • $\begingroup$ OK but it's kind of a trivial look-at-wikipedia answer... $\endgroup$ – Organic Marble Aug 8 '16 at 3:20
  • $\begingroup$ Are any Vernier thrusters hydrogen-fueled? Their restartability is quite exceptional, easily going into tens of thousands firings. $\endgroup$ – SF. Aug 8 '16 at 5:52
  • $\begingroup$ A search for questions tagged restartable-engine has 3 results that answer this. A search for 'restart engine' also brings them up. $\endgroup$ – kim holder Aug 8 '16 at 14:11
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Both the J-2 engine used on the Saturn V and the RL-10 used on the Centaur and DCSS upper stages are hydrogen-fueled and restartable.

The Saturn V third stage burned once to finish insertion into low Earth orbit, once to send the Apollo spacecraft on its way to the moon, and once for its final disposition, either to hit the moon or to go into solar orbit.

Likewise, the Centaur upper stage has been used frequently to insert satellites into geosynchronous orbits and many interplanetary missions. On multi-satellite missions, Centaur RL-10s have been started more than 7 times, and Rocketdyne is investigating a further development of the RL-10 called CECE which is intended to have 50-start capability.

As discussed in this Q&A, hydrogen engines are generally easier to restart than kerosene; they can be started with an electrical spark igniter instead of a more-involved liquid-hypergolic or solid ignition charge.

Japan's LE-5, Russia's RD-0146, and China's YF-75D are all upper stage, restartable hydrogen engines in the same general class as the RL-10.

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  • $\begingroup$ I'm curious - were the centaur engines simply kept cold somehow, or were there several 'cool-downs' - the background, or angle of my question (and the quote) is coming from maintaining altitude of the ISS using say weekly or monthly burns over years. I'm not sure if I should edit and add that to improve the question quality, or if that would be generating a "moving target". $\endgroup$ – uhoh Aug 8 '16 at 3:27
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    $\begingroup$ As I mentioned in another comment, boiloff (for Centaur and similar hydrogen stages) amounts to a few percent a day, and all the required starts are accomplished within a timeframe of hours to days. You'd need active cooling systems to keep hydrogen fuel around on a space station for a mission of months or years; this is why ISS is boosted with storable hypergolics rather than hydrogen. $\endgroup$ – Russell Borogove Aug 8 '16 at 3:31
  • $\begingroup$ @uhoh there's a pretty detailed description of the J2 restart sequence here: nasa.gov/centers/marshall/pdf/499245main_J2_Engine_fs.pdf tl,dr: they did a five minute chilldown prior to ignition. $\endgroup$ – Organic Marble Aug 8 '16 at 3:35
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There may be others, but at least the Saturn 5's J2 engines have a operational history of inflight restarts (from wikipedia)

Unlike most liquid-fuelled rocket engines in service at the time, the J-2 was designed to be restarted once after shutdown when flown on the Saturn V S-IVB third stage. The first burn, lasting about two minutes, placed the Apollo spacecraft into a low Earth parking orbit. After the crew verified that the spacecraft was operating nominally, the J-2 was re-ignited for translunar injection, a 6.5 minute burn which accelerated the vehicle to a course for the Moon.

as did the RL10 engines on various Centaur upper stages

The RL10 is capable of multiple restarts in space. In fact, the engine was started seven times in one mission.

from here.

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