I know the upper stage of the Falcon 9 can be restarted once, thus allowing an additional thrust for some circumstances that might require it. However, I don't really understand the mechanism that limits the number of restarts. What limits the number of restarts that a liquid fueled rocket can perform?

  • $\begingroup$ Attitude thrusters can restart thousands of times. Please specify what engines you have in mind. $\endgroup$ Jul 22, 2015 at 16:46
  • $\begingroup$ Yes, but the main propulsion systems seem to have a limited number of restarts... $\endgroup$
    – PearsonArtPhoto
    Jul 22, 2015 at 16:47
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    $\begingroup$ Related: space.stackexchange.com/questions/2446/… (looks like a dupe thereof) $\endgroup$ Jul 22, 2015 at 16:47
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    $\begingroup$ Is this really a duplicate? The other question is about the difficulty of restarting in zero gee and addresses propellant placement in the tanks. This asks what is the *limiting factor*(s) on the number of restarts. I would say the answer to this question is really that the amount of zip fuel / purge gas / battery power for restarts is limited. $\endgroup$ Jul 22, 2015 at 18:05
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    $\begingroup$ I asked a similar but more generalized question: Practical limits to a restartable rocket engine. I'd still be interested in the current record holder for number of restarts while in space. $\endgroup$ Jul 24, 2015 at 12:28

2 Answers 2


Per this old usenet thread, there are a lot of design choices, but the main problems are in getting the fuel to the engines while in free-fall, and igniting non-hypergolic propellants.

To get the fuel into the pumps / engines, you can use:

  • bladder pressurization (which is a materials problem with cryo fuels)
  • piston pressurization (difficult to do on a large scale)
  • small solid-fuel ullage rockets (which you'd have a limited number of)
  • small liquid-fuel ullage rockets (which has the same fuel supply issues, but on a smaller and more easily solvable scale)
  • boiloff venting of cryo propellants for ullage!
  • a mesh or membrane system to hold sufficient amounts of fuel at the pump inlet via capillary action

To ignite non-hypergols, you can use:

  • hypergolic chemical igniters (which you'd have a limited number of)
  • electric spark igniters (which might dirty up or wear down)
  • solid fuel pyrotechnics (which you'd have a limited number of, and in turn require electric igniters)
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    $\begingroup$ You omitted membranes/capillary action mechanisms. At least membranes were used operationally. $\endgroup$ Jul 22, 2015 at 16:50
  • $\begingroup$ Does this really answer the question - defining the limiting factor on the number of starts? $\endgroup$ Jul 23, 2015 at 13:36
  • $\begingroup$ I don't believe there's a simple answer due to the variety of different engine/system designs; this does define some possible limiting factors. $\endgroup$ Jul 23, 2015 at 13:58
  • $\begingroup$ With cryofuel boil-off and spark igniters you have a restartability limited only by the ability to maintain cryofuel at all. The wear of the spark igniters is very minor comparing to the actual needs of restarting the engines - you'd use up all your fuel on repeated restarts before the igniters would become a factor. $\endgroup$
    – SF.
    Feb 28, 2016 at 22:32

There is no SENSIBLE upper limit in terms of the likely requirements in the next few decades. Increasing restart capability adds complexity and a certain amount of mass but you can soon get to a system which is essentially reliability limited. Tens to hundreds of restarts would readily be achievable with existing technology.

As one only example

You can use cold gas thrusters to achieve ullage location of main tank fuel and if desired replenish the gas reservoirs from main fuel or oxidiser once the contents are stable.

Anywhere outside a permanent shadow (which are rather hard to find usually) you can use photovoltaic panels to recharge electrical system energy stores. Existing battery technologies have calendar and cycle lifetimes well in excess of what is sensibly needed.

Lack of hypergolicity is a challenge but, if the need exists, small igniters running on main fuel mixes heated to combustion point electrically are an entirely realistic solution.

It's likely that dozens of restarts could be achieved by such systems, and unlikely that more than that number would be required in any likely application short term. Extension to say hundreds of restarts is "just a matter of engineering" with increased need for maintaining reliability as restarts increased.

  • $\begingroup$ If nothing else, the amount of propellant onboard would be a limiting factor. $\endgroup$ Jul 23, 2015 at 13:36
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    $\begingroup$ @OrganicMarble Indeed. That's why the word SENSIBLE is in bold and capitals and 4th word in as any sooner would have made for awkward English :-). eg SENSIBLE restrictions on ... . ie YES sooner or later you'll run out of fuel, meteorites will riddle your tanks neutrons will decay (OK, not that), nobody will care etc. But if you want 10's of restarts now it's "not that hard" - just a matter of engineering. 100's gets annoying as there will be extra wear & tear at start up and it will affect mass ratio overall etc. But it's essentially not a problem - just an Engineering opportunity :-) $\endgroup$ Jul 23, 2015 at 14:47

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