XCOR Aerospace and ULA Announce Successful Hydrogen Piston Pump Tests

[mmeijeri]

So what are the drawbacks of piston pumps? Weren't they replaced by turbopumps in the olden days because they are heavier? Are the newer ones much more competitive or is cost a bigger factor than it used to be?

[FinalFrontier]

Neat! I see this as having a role in future commercial resupply activites.

[yinzer]

So what are the drawbacks of piston pumps? Weren't they replaced by turbopumps in the olden days because they are heavier? Are the newer ones much more competitive or is cost a bigger factor than it used to be?

It's probably the same as with airplane engines.  Turbines scale up well, weigh less, are more reliable, and more efficient when operating at their design condition.  Piston engines scale down well, cost less, and are more efficient over a wide range of operating conditions.

I think there are two main issues scaling down turbines - the clearance between the tip and the housing can only be so small and this gap and associated losses becomes proportionally larger as the turbine shrinks, and a lot of a turbine's performance is related to tip speed which requires progressively higher rpm at smaller sizes.

[mmeijeri]

Isn't a higher rpm actually easier at smaller sizes? At least that's the case for hard disks I think.

[jongoff]

Since we're on topic, does anyone know what the specific impulse is for XCOR's pumped engines 4K14 and 5K18?

No, XCOR has been pretty consistent about not publishing their engine Isp data.  They usually cite ITAR as the reason, though that's kind of weird since almost everyone else in the industry cites Isp values.  They may just have an overly paranoid ITAR lawyer.

~Jon

[Namechange User]

They may just have an overly paranoid ITAR lawyer.

~Jon

Nah, they don't exist and are stuff of legend and folklore. 

[Patchouli]

Isn't a higher rpm actually easier at smaller sizes? At least that's the case for hard disks I think.

Thermodynamics also gets harder on a smaller scale ie keeping stuff want to stay hot at a high temp and the stuff you want to keep cool at a low temp.
Turbines don't always scale down well because they require high temps for efficiency but the bearings and seals need to be kept relatively cool.
I'm talking the power head vs the pump part.

But I really suspect the piston pump may not be for an engine at all but instead for a fuel depot.
I could also see it being used with an accumulator on thrusters or OMS engines vs main engines as a piston pump can start and stop easier.

[Robotbeat]

Isn't a higher rpm actually easier at smaller sizes? At least that's the case for hard disks I think.

Thermodynamics also gets harder on a smaller scale ie keeping stuff want to stay hot at a high temp and the stuff you want to keep cool at a low temp.

There are certain things that work better at a smaller scale. If you scale up an internal combustion (piston) engine by 2 in every direction, you will have a hard time keeping the rpms up as high (because greater inertia... more mass has to travel more distance in the same time, thus the inertia of the piston increases faster than the mass of the engine at the same rpms, thus reaching material limits). And since power is proportional to volume x rpms (and compression ratio), and (if operating at material limits) volume (and pressure) is proportional to mass, then larger engines of that type will have lower specific power than smaller engines.

Think of those high-performance crotch-rocket motorcycle engines going up to 15,000 rpm and compare to the huge diesel engines found in ocean-going ships (100 rpm). Although the power is far greater in the bigger engines, the power-per-kg is _far_ greater in the smaller engines due to the higher rpm (and also the heavier diesel engine, but that's a separate issue).

[Patchouli]

There are certain things that work better at a smaller scale. If you scale up an internal combustion (piston) engine by 2 in every direction, you will have a hard time keeping the rpms up as high (because greater inertia... more mass has to travel more distance in the same time, thus the inertia of the piston increases faster than the mass of the engine at the same rpms, thus reaching material limits). And since power is proportional to volume x rpms (and compression ratio), and (if operating at material limits) volume (and pressure) is proportional to mass, then larger engines of that type will have lower specific power than smaller engines.

Think of those high-performance crotch-rocket motorcycle engines going up to 15,000 rpm and compare to the huge diesel engines found in ocean-going ships (100 rpm). Although the power is far greater in the bigger engines, the power-per-kg is _far_ greater in the smaller engines due to the higher rpm (and also the heavier diesel engine, but that's a separate issue).

True smaller engines can have a higher power to weight.
But that big diesel engine in the ship has a much higher Carnot efficiency then the motorcycle engine.

Some large diesels such as those found in ships can exceed a Carnot efficiency of 50% while most small engines struggle to achieve 22%.

This is partly due to the laws of squares there just is less surface area for the for a given volume of gas to leak heat into the head and cylinder walls so that heat can do work instead.

Turbines also are subject to similar rules though much worse then a piston engine.

[butters]

So this is essentially a Sterling engine running on nozzle heat?

[mlorrey]

Isn't a higher rpm actually easier at smaller sizes? At least that's the case for hard disks I think.

Thermodynamics also gets harder on a smaller scale ie keeping stuff want to stay hot at a high temp and the stuff you want to keep cool at a low temp.

There are certain things that work better at a smaller scale. If you scale up an internal combustion (piston) engine by 2 in every direction, you will have a hard time keeping the rpms up as high (because greater inertia... more mass has to travel more distance in the same time, thus the inertia of the piston increases faster than the mass of the engine at the same rpms, thus reaching material limits). And since power is proportional to volume x rpms (and compression ratio), and (if operating at material limits) volume (and pressure) is proportional to mass, then larger engines of that type will have lower specific power than smaller engines.

Think of those high-performance crotch-rocket motorcycle engines going up to 15,000 rpm and compare to the huge diesel engines found in ocean-going ships (100 rpm). Although the power is far greater in the bigger engines, the power-per-kg is _far_ greater in the smaller engines due to the higher rpm (and also the heavier diesel engine, but that's a separate issue).

This is why you pile the pistons on. My grandfather, back in the 30's when he was a student at WPI, built a 36 cylinder engine out of three 12 cylinder blocks. Built a hellacious drag racer with it. Had a very short drive shaft, and the driver had to sit in the back seat of the original car cabin. Couldn't turn for anything.

Sadly, the Depression forced him out of school and he became an executive in the publishing industry. A loss for Detroit.

[Swatch]

Isn't a higher rpm actually easier at smaller sizes? At least that's the case for hard disks I think.

Easier to spin fast, but there's still a penalty to be had just rotating at those RPM's... primarily with friction losses but also structural since your centripetal force goes up with the square of velocity and is inversely proportional to your radius (think figure skater).

[docmordrid]

Turbochargers and turbopumps share a lot of features.  Those in automobiles turn at ~100,000 RPM with a turbine diameter of ~76mm (Garrett GT3076R) and with high reliability. 

[mlorrey]

Turbochargers and turbopumps share a lot of features.  Those in automobiles turn at ~100,000 RPM with a turbine diameter of ~76mm (Garrett GT3076R) and with high reliability. 

I wonder if one could mock up a cheap turbopump with a couple turbochargers... I imagine that turbopumping liquids would require a different turbine wheel than one uses to turbocharge air...

[strangequark]

I wonder if one could mock up a cheap turbopump with a couple turbochargers... I imagine that turbopumping liquids would require a different turbine wheel than one uses to turbocharge air...

The turbine itself would still see a gas. It's the compressor you'd have to worry about.

[FutureSpaceTourist]

Here's an update on the XCOR & ULA project: http://www.spacenews.com/article/launch-report/36548xcor-ula-engine-project-taking-page-from-auto-industry

[uranium]

More progress:

“The test marks the first time liquid hydrogen and liquid oxygen have been supplied to a rocket engine with a piston pump,” says Jeff Greason, XCOR’s chief executive. “It is also the first time an American LH2 engine of this size has successfully fired liquid hydrogen and liquid oxygen together in pump-fed mode. We are happy to be making solid progress on the engines. This will also bring us to a new phase in our plans for orbital flight.”

Full text at http://www.xcor.com/blog/lh2progress/

[Planetaryduality]

Does anyone know the performance specifications on the piston fed engine? Better/comparable/worse than a similarly sized expander cycle?

[dcporter]

(Who owns what in this kind of relationship?)

[Moe Grills]

More progress:

“The test marks the first time liquid hydrogen and liquid oxygen have been supplied to a rocket engine with a piston pump,” says Jeff Greason, XCOR’s chief executive. “It is also the first time an American LH2 engine of this size has successfully fired liquid hydrogen and liquid oxygen together in pump-fed mode. We are happy to be making solid progress on the engines. This will also bring us to a new phase in our plans for orbital flight.”

Full text at http://www.xcor.com/blog/lh2progress/

Liquid hydrogen with LOX?  I can understand the use of kerosene and LOX..... Jeff is really pushing the propulsion envelope for commercial spaceflight; makes me nervous.  LH2 with LOX require special handling
and care. Look how long it took the Soviets/Russians to use the propellant on Briz upperstages, etc.