Question General Spaceflight Q&A

[Izack]

From where do the turbopumps in the Space Shuttle Main Engine (or any similar engine) draw their power, and how are they started? Wiki says they are 'powered by liquid oxygen' but that hardly covers the issue.

 

[Fabri91]

Turbopumps in many engines are powered by a gas generator (in essence a secondary combustion chamber which produces gas for the explicit purpose of spinning a turbine, which is then connected to a pump, hence turbopump) or from gas diverted from the main combustion.

It's not fundamentally different from the tubrocharger in a car, which is moved by the exhaust gases produced by the engine.

 

[Urwumpe]

From where do the turbopumps in the Space Shuttle Main Engine (or any similar engine) draw their power, and how are they started? Wiki says they are 'powered by liquid oxygen' but that hardly covers the issue.

The turbines of the turbopump are powered by the exhaust gases of the preburners (which are just gas generators with a slightly different role). These are started by augmented spark igniters (ASI), technically just pretty normal spark plugs (like all cars have, except Diesels) which are augmented by injecting a small amount of hydrogen and oxygen close to the spark.

On the Shuttle, this works like that because the injection pressure in the preburner can slowly be increased, while the main oxidizer valve (MOV) of the engine is closed, preventing oxygen from entering the main combustion chamber. There the pressure in the preburners and the main combustion chamber is slowly "oscillated" upwards, while the pressure waves wander through the pipes of the engine. This required a lot of testing, but actually made it possible to start the SSME electrically and without ignition charges like earlier engines.

In other engines, you can have different start systems. The AJ-26 of the Antares for example uses a small start turbine powered by a solid fuel gas generator at one end of the single shaft, that quickly spins up the pumps for a faster start of the preburner.

 

[Izack]

Ah, thanks. Approaching the whole system at once is somewhat overwhelming. Explanations like this help a lot.

So can they be started solely with pressure from the propellant/oxidiser tank feeds, or is there an external mechanism? I haven't read of one anywhere, but just to be sure...

 

[Fabri91]

I'm not sure what you mean? Urwumpe cited the AJ-26, whose turbopump is started by a small solid gas generator (think srb but just used to produce gases), which should count as "external mechanism".

 

[Izack]

I'm not sure what you mean? Urwumpe cited the AJ-26, whose turbopump is started by a small solid gas generator (think srb but just used to produce gases), which should count as "external mechanism".

Well, that was one example. I'm wondering if such a device is absolutely necessary or not (Urwumpe mentioned that it made for a faster start, which sounds like it might be a convenience not a necessity). And by 'external mechanism' I was thinking of ground infrastructure.

 

[garyw]

Remember that with the shuttle the turbopumps get power from the tail service masts which start the whole sequence off at T-6.6, after that, the actual flow of the liquid oxygen keeps the process going.

 

[Fabri91]

Ah! Makes sense now.
Well, since the turbopump is necessary for normal engine operation and normally is powered only during said normal operation (normal as in "started and working at nominal thrust"), we need some gas source to spin it up when the engine is being ignited. This can be either single-use solid or a gas generator/pre burner that can be activated with just the pressure of the pressurized tanks.

Such a device (or other metod to spin up the turbopump) is necessary because in order to fire up the "rest" of the engine the pressure of the pressurized tanks isn't sufficient, so we need the boost of the (turbo)pumps.

 

[Urwumpe]

It isn't necessary - the SSME doesn't use ANY such devices for spinning the turbines up.

But the SSME is also much more complex because of it. In the SSME, the propellant flows into the engine and at "tank head" (The pressure from the tank) into the preburners, where the ASI ignites the atomized propellants and creates a small initial exhaust stream, that spins the turbine up at low power. When the pressure in the preburner increases because of the combustion, the propellant flow is reduced and the pressure drops again - the engine operation is still very chaotic and instable at low thrust level.

With the spinning turbine, the propellant pressure increases in the pump, with a small delay because of the length of the lines between pump and preburner, more fuel flows into the preburner, the pressure in the preburner increases much more, the turbine gets more power, the pressure at the pump increases again, etc.

After a few such oscillations, the engine has enough fuel-rich flow, that the MCC can be ignited and oxygen is injected into the combustion chamber. The mixture of the exhaust of the preburner and the oxygen is already too cold to ignite automatically, so a third ASI ignites the MCC. Now the engine is really igniting, though still at low power. Now the pressure of the MCC increases quickly with the oscillations in the preburner power, until 65% thrust is reached - the engine is ready for closed loop control.

Now, if you don't want to spend many thousand hours for testing the ignition sequence, and want to use simpler piping and valve actions, you give the turbine shaft a bit of initial speed, so the early low pressure oscillations can be prevented during engine start up and the start-up be made faster. But such a system is either expendable (limited number of restarts) or complex (additional start gas generator)

 

[Izack]

Now, if you don't want to spend many thousand hours for testing the ignition sequence, and want to use simpler piping and valve actions, you give the turbine shaft a bit of initial speed, so the early low pressure oscillations can be prevented during engine start up and the start-up be made faster. But such a system is either expendable (limited number of restarts) or complex (additional start gas generator)

Would a hypothetical spacecraft's APU suffice as the additional gas generator?

 

[Urwumpe]

Would a hypothetical spacecraft's APU suffice as the additional gas generator?

You can use theoretically anything to spin a pump shaft up. Electromotors, V8 engines, etc. It all just also adds mass. The APU would be less interesting there, because of the additional piping, but you could do that, spinning the turbine shaft up with compressed air or hydraulics or... what ever you have.

 

[Kyle]

Hypothetical Q:

What's the highest altitude a space shuttle orbiter, with no payload, and a bare-minimum crew, could attain and not run out of fuel?

 

[Urwumpe]

Hypothetical Q:

What's the highest altitude a space shuttle orbiter, with no payload, and a bare-minimum crew, could attain and not run out of fuel?

Circular orbit or elliptical? While an elliptical orbit isn't that long-lasting, it can be attained for some days. Could be about 150 x 1250 km then.

 

[Fabri91]

What about circular?

 

[Kyle]

Circular orbit or elliptical? While an elliptical orbit isn't that long-lasting, it can be attained for some days. Could be about 150 x 1250 km then.

Let's say a circular orbit.

 

[Urwumpe]

What about circular?

Let's say a circular orbit.

Then it is around 750 km altitude. At 700 km you barely have any OMS fuel for reentry left. You can't feed RCS fuel into the OMS, that only works the other way around, but you could get a few km more by using the aft and forward RCS for translation - but these engines have a much lower specific impulse than the more effective OMS (Note: Of course, this would never be done with a real Shuttle. You then have no safety reserves left, should for example the OMS engines fail).

With meaningful payload, the 600 km orbit of the HST is the maximum possible.

 

[MattBaker]

#491 from 2012:

Hard maximum is about 700 km circular. That is not only getting at the limits of the propellant resources, but also at the limits of the heat shield.

Highest missions should have been STS-31 and -103 with a bit over 600 kilometers.

Bonus fact: The pilots of the Space Shuttles in these missions are still quite famous and influential. Charlie Bolden and Scott Kelly, who is the american part of the yearlong ISS expedition.

 

[Loru]

Since I haven't found it anywhere else - do any of you know, what TKS' reusable heatshield was made off?

 

[Donamy]

There were plans to add OMS packs to make it possible for much higher orbits.

 

[Urwumpe]

There were plans to add OMS packs to make it possible for much higher orbits.

Yes, but these OMS Packs had been cancelled already long before the first ALT Test - just some switches and wiring remained of it.