How low can the throttle go? (in vacuum)

[Pipcard]

The Apollo LEM had a minimum thrust level of 10-13%, but can a rocket engine go lower than that?

 

[Loru]

Theoretically yes but most rocket engines use [ame="http://en.wikipedia.org/wiki/Choked_flow"]choked flow[/ame] and below certain level of pressure inside combustion chamber engine efficency drops dramatically.

 

[Quick_Nick]

Found info on the CECE, a variant of the RL10:
"In 2009 NASA reported successfully throttling from 104 percent thrust to eight percent thrust, a record for an engine of this type."

You can also look up the document "CECE: Expanding the Envelope of Deep Throttling Technology in Liquid Oxygen/Liquid Hydrogen Rocket Engines for NASA Exploration Missions" where it is described as "~7% power".

 

[MaverickSawyer]

There was the Liquid Engine Testbed that made it down to 5%, but I don't know if that was in vacuum or at atmospheric pressure.

 

[Pipcard]

Found info on the CECE, a variant of the RL10:
"In 2009 NASA reported successfully throttling from 104 percent thrust to eight percent thrust, a record for an engine of this type."

You can also look up the document "CECE: Expanding the Envelope of Deep Throttling Technology in Liquid Oxygen/Liquid Hydrogen Rocket Engines for NASA Exploration Missions" where it is described as "~7% power".

What about hypergolic engines?

 

[Urwumpe]

What about hypergolic engines?

The main limit is not the propellants that you use, but injector and combustion chamber.

If you reduce the mass flow into the combustion chamber (=reducing thrust), you need injectors, that either adapt (e.g. Pintle Injectors) or can be turned off (for example having separate rings of injectors), so that the fuel is properly atomized and mixed.

Also, you need ways to make sure that the combustion remains stable even if you lower the chamber pressure, without pressure oscillations. Ideally, the combustion gases flow from one end to the throat in a straight line, or at least you have designed recirculation. There are ways to ensure this.

Next problem you can get is chamber cooling: Many engines need a film of cool gases at the chamber walls for not melting. Reducing the mass flow also for this injectors means, that this cooling film can fail and no longer protect the chamber walls, even if the engine produces less heat.

Finally, the temperature in the chamber has to stay high enough (Remember, fire triangle: heat, fuel, oxygen), that combustion does not cease. Usually, that's not a problem, but can further reduce thrust independent of the mass flow ("the relation becomes non-linear at the extremes")

So, for hypergolics, the same problems remain, without exception, but also without new problems. Usually, you can expect the hypergolics to ignite even if the heat is low, but the chamber cooling is often a bigger problem since hypergolics are less good coolants than for example hydrogen.

 

[kuddel]

Just out of curiosity: Would pulsing the (hypergolic) engine be an option?
The average thrust would be a function of the pulse width.
/Kuddel

 

[Urwumpe]

Just out of curiosity: Would pulsing the (hypergolic) engine be an option?
The average thrust would be a function of the pulse width.
/Kuddel

Works for smaller engines, but for bigger chambers, this leads to combustion instabilities, which are very hard to fix.

Remember, that there is some distance between valve and injector, and the bigger this distance gets and the more difference there is between first and last injector pair to receive propellants, the harder it is to ensure that the propellants are properly injected and especially not reversing flow, with hot exhaust gases damaging the injectors.

 

[Hlynkacg]

The Apollo LEM had a minimum thrust level of 10-13%, but can a rocket engine go lower than that?

It depends on the engine's injector design, and the minimum flow rate at which stable mixing and combustion can be maintained.