Starting a turbopump

[EnderKid2]

I've been watching Tim's Firefly Aerospace tour and they were talking about using nitrogen to spin up the turbopumps. I know Rocketlab uses electric motors to spin their pumps, so I was wondering if there might be utility in an electric starter motor for turbopumps, like a car's internal combusion engine starter. Perhaps it would be useful for situations where you have a turbopump-fed engine and want an arbitrarily high amount of restarts.

[Meecrob]

This excerpt is from blog.nasa.gov on the RS-25:

"The most complex part of the engine is its four turbopumps which are responsible for accelerating fuel and oxidizer to those insanely high flow rates. The high pressure fuel turbopump main shaft rotates at 37,000 rpms compared to about 3,000 rpm for a car engine at 60 mph."

https://blogs.nasa.gov/Rocketology/2015/08/13/rs-25-engines-meeting-the-need-for-speed/

 

[EnderKid2]

Yes, yes, but in that vid Firefly said they only needed to spin up the pumps to low velocity before injecting fuel. Perhaps their tap-off cycle makes this easier. This starter motor thing would likely only be practical for small engines though.

[Meecrob]

True, but car starters only spin the engine up to a fraction of operating RPM to start as well (those old timey videos of people hand cranking their cars would be a lot more interesting if they had to get to 1000 RPM!!). I just wanted to illustrate there is an order of magnitude difference in operating regimes of the engines.  And its a pretty cool article. Having said that, I'm thinking the main obstacle here is the fact that (as you say) this would only be practical on smaller engines, and thus they currently cannot lift enough fuel to utilize practically unlimited restarts. Let's keep our eyes out when they get orbital refueling down, because that will eliminate this constraint.

[DDE]

16 hours ago, EnderKid2 said:

Perhaps it would be useful for situations where you have a turbopump-fed engine and want an arbitrarily high amount of restarts.

The intersection between large restart counts and engines powerful enough to merit turbopumps is pretty small.

Quote

The Agena upper stage, developed by Lockheed, had an auxiliary or integrated secondary propulsion system for roll control during the firing of the main Agena engine and also for some attitude control during the subsequent coast period. Initially this secondary sytem had small thrusters, its own small high-pressure propellant tanks with positive expulsion features, and a pressurized inert gas tank of its own. Later the propellants were obtained directly from the main engine's propellant tanks, which were at a much lower tank pressure and were pumped by a small set of gear pumps driven by an electric motor through magnetic (nonleak) couplings. This eliminated the separate heavy pressurized tank system.