Why can't rocket engines be throttled / restarted?

[Comrade Jenkens]

First I know that modern ones do have some limited throttle capability and can restart a few times but what I want to know is why it is so difficult for them to do this?

[Motokid600]

I'm not sure id like to know as well. My guess is carbon build up.

[andrew123]

I am rather ignorant to the scientific details. I can tell you it is possible but impractical due to all of the needed equipment.

[Camacha]

I think it all might stem from problems with the combustion chamber - engines have to be started carefully and gradually to avoid catastrophic peak loads leading to uncontained engine failure. It is not true for all engines though - the hypergolic thrusters on the MSL skycrane could be throttled back all the way to 3%; very useful when attempting advanced hover. However, this is fairly rare, although numbers do seem to vary from model to model.

I can also imagine that it sometimes is the simple matter of lighting the thing - on the ground you can easily ignite the engine, but in space there is no launch platform or 'spark' to get things going. Again, not an issue when going the hypergolic route, otherwise slightly problematic.

Edited March 12, 2014 by Camacha

[Accelerando]

Some engines also can't restart because they keep cool by allowing some metal inside their engine nozzle to be burned away by the exhaust.

[Idobox]

There are also several concepts where the turbo-pump is powered by the combustion of the fuel. If you stop it, you need a second power source to restart the pump, a little bit like the battery that starts your car. So unless you really need it, it's much simpler not to have the capacity.

[Nibb31]

The biggest issue with restarting in microgravity is that fuel is not settled in the tanks unless you are already accelerating. The fuel floating around in the tank would cause cavitation in the turbopumps, and a rather catastrophic failure.

One solution is to use solid ullage motors to provide a small forward motion, which is enough to settle the fuel at the bottom of the tanks. Of course, solids are one-shot, which means that you only get a limited number of restarts.

[Skyler4856]

The biggest issue with restarting in microgravity is that fuel is not settled in the tanks unless you are already accelerating. The fuel floating around in the tank would cause cavitation in the turbopumps, and a rather catastrophic failure.

One solution is to use solid ullage motors to provide a small forward motion, which is enough to settle the fuel at the bottom of the tanks. Of course, solids are one-shot, which means that you only get a limited number of restarts.

Most modern tanks AFAIK use helium to keep themselves pressurized

[Camacha]

Depending on the fuel the solution might be simple - a bladder that contains the fuel inside the tank could ensure that, at no time, there is anything else than fuel to be pumped out. You could even pump helium between the bladder and the tank to force it towards the pumps a little more.

The biggest problem I see is making a bladder that resists the fuel well enough to be safe.

Most modern tanks AFAIK use helium to keep themselves pressurized

Pressure is not a problem, gravity is. If you have a nice loose helium/fuel-mixture floating around in your tank, it is hard to get your engine started because the fuel will not drain where you want it to go.

[sal_vager]

A bladder adds more mass that has to be justified, it's easier to design your flight profile with limited/no restarts in mind.

[Camacha]

A bladder adds more mass that has to be justified, it's easier to design your flight profile with limited/no restarts in mind.

That depends on the design - although a rupture would not be preferable, there is a real tank outside the bladder to contain fuel in the case of a problem. Or you could go with a tough bladder and no external tank at all. If you do not need it, sure, don't do it.

[Elthy]

You can find some facts about engine restarts on this blog (its from an engineer involved in the developement of the J2-X engine) in the most recent posts. But i would recommend to read the whole blog if you are interested in rocket engines anyway...

[DerekL1963]

First I know that modern ones do have some limited throttle capability and can restart a few times but what I want to know is why it is so difficult for them to do this?

It's not difficult to throttle or restart (in flight) - if the engine is designed for it. Most engines aren't so designed because there's very little advantage to doing so.

[nhnifong]

Pressure is not a problem, gravity is. If you have a nice loose helium/fuel-mixture floating around in your tank, it is hard to get your engine started because the fuel will not drain where you want it to go.

This can be partially mitigated with baffles and vanes to draw the fuel towards one end by capillary action.

For example

[cantab]

Some rockets do use pressurised tanks, often with helium but a few fuels can use their own vapour pressure. The advantage is that there's no need for fuel pumps, the drawback is that the tanks are heavier.

[Aghanim]

Or by using cold gas thruster instead of solid state ullage thruster

[Nuke]

pressure fed hydrazine thrusters could also be used for ullage, and give you a bit more power than cold gas.

the pressure fed tanks usually keep the fuel in a flexible bladder inside a tank which is pressurized with helium or some other inert gas. to start it you just open a valve. once the hydrazine reaches the catalyst in the combustion chamber, it pretty much ignites instantly. You can start it as many times as you want. using a servo valve you could achieve some degree of throttle. you could also use a solenoid valve and cycle it on and off real fast with a pwm signal to control thrust.

[Kerbin Dallas Multipass]

The biggest issue with restarting in microgravity is that fuel is not settled in the tanks unless you are already accelerating. The fuel floating around in the tank would cause cavitation in the turbopumps, and a rather catastrophic failure.

One solution is to use solid ullage motors to provide a small forward motion, which is enough to settle the fuel at the bottom of the tanks. Of course, solids are one-shot, which means that you only get a limited number of restarts.

Exactly that.

This is one of the reasons why I believe we will see more hybrid engines in the future http://en.wikipedia.org/wiki/Hybrid_rocket

[Deathsoul097]

Why not use a small Hydrazine thruster? It would allow you to restart as many times as you have the fuel for, unlike solid fuel ullage motors. You could also use that Hydrazine as an RCS propellant if needed.

[Space_Kraken]

Hydrazine is some nasty stuff. It's highly toxic and has a lot of drawbacks. Just check out this wikipedia article.

[cantab]

Hydrazine, cold gas, whatever, it's going to be more complex, expensive, and heavy than a solid ullage motor.

As Derek mentioned, some rocket engines have restricted control because they only NEED restricted control. If the task does call for an engine that can be widely throttled and restarted, such an engine will be made. For example, the Lunar Module's Descent Propulsion System was gimballed and could throttle from 10-60% thrust, while the Ascent Propulsion System was nongimballed and non-throttlable.

[Aghanim]

Hydrazine, cold gas, whatever, it's going to be more complex, expensive, and heavy than a solid ullage motor.

Cold gas thruster isn't that complex/expensive, its basically just a nozzle connected to a compressed gas source

But if we really wanted interplanetary restartable engine anyway, we could use NERVA/VASMIR/ion thruster anyway

[Alchemist]

There are 2 main problems about engine ignition.

1) Ignition itself - you need something to start the fire:

* Single-use ignition system (often electric discharge) - is destroyed by the high temperatures when engines ignite

* External ignition system - often used for stages that are ignited on the launchpad (for example, R-7 rockets)

* Hypergolics injection - the reusable system (injects self-igniting compounds to ignite the fuel)

* Hypergolic fuels - do not require ignition system.

2) ensuring the fuel gets to the engine. Besides pump starting problem (for example, LK had limited ignitions because the pumps were started by solid charges), you have to ensure that the fuel will be in the lower part of the tang and the pump won't be pumping gas instead of liquid, which is a problem in 0 g:

* Having some force accelerating the rocket:

** Ignition on the pad (first stages, of course)

** Hot staging - igniting the next stages before the previous cuts off (most Russian rockets, first used on R-9 ICBM and Luna/Vostok launch vehicles).

** Solid ullage motors. Especially actual for staging in the upper atmosphere (Saturn V), because you need to overcome the drag. Can have charges for multiple ignitions.

** Auxiliary propulsion units (S-IVB, Blok D and many other upper stages). Often work also as attitude control RCS before ignition. Usually, pressure-fed hypergolic RCS - the optimal multi-ignition solution for heavy stages.

* Pressure fed tanks - there's no gas directly over the liquid (it's separated by the membrane). Also, no turbopump required, which make that perfect for RCS and other small thrusters. However lower ISP than with turbopumps and not compatible with cryogenic fuels, therefore using that for the main engines (all 3 engines of Apollo) is a major trade-off.

* Special combined solutions. For example, on LK part of the tanks was separated by special membrane ensuring the gas won't get under it in 0 g, but allowing fuel flow during burns.

Having re-ignitable engines imposes some disadvantages comparing to simpler single-ignition solutions (additional hardware or lower ISP), so when it is possible, the engineers try to use the simplest things (and they can be made even lighter by making other elements designed for single use). The same with wide range throttling - it require more complex heavier hardware, so if your rocket doesn't really need it, it's better to avoid this.