How might an internal combustion engine generate substantial external thrust?

This answer mentions the proposed use of an internal combustion engine (ICE) in ULA's ACES upper stage to extract energy from LH2/LOX in multiple forms for multiple uses, which is described further in Evolving to a Depot-Based Space Transportation Architecture linked there as well. For example, as described in the linked paper An Integrated Vehicle Propulsion and Power System for Long Duration Cryogenic Spaceflight an electric generator would convert mechanical energy to electricity, heat would potentially be captured by the Integrated Vehicle Fluids system, and the exhaust would be used for "milli-G level settling thrust" which would keep the cryogenic propellants at one end of their cryostats (continuous, low-level ullage for days or weeks), significantly lowering the heat loading.

In a conventional internal combustion engine most of the expansion happens (by design) inside the engine itself, in a controlled way, in order to extract the power. But the quantity of thrust necessary for something on the order of milli-G level settling is quite substantial, and it seems would have to take place within an externally mounted nozzle rather than inside an engine.

How might an internal combustion engine generate substantial external thrust?

• I'm not sure what you're really asking here. It seems clear from the paper that the exhaust is vented to ambient producing thrust. Are you asking for details on the nozzle(s)? There's a picture of it on page 7. Feb 26 '18 at 0:47
• @OrganicMarble consider quantitatively the "substantial thrust" required to produce "milli-G" acceleration of a spacecraft with several tens of tons of mass, then consider the amount of "thrust" produced by a compact car's tailpipe. The problems are quantitative, and worse because expansion happens (presumably) inside the engine rather than in a nozzle. Making this actually work would require some substantial engineering and careful expansion management.
– uhoh
Feb 26 '18 at 1:09
• @OrganicMarble If the second paragraph is not clear enough about this, what changes would you suggest?
– uhoh
Feb 26 '18 at 1:13
• No, if you are asking for details, it's clear. I just don't know where they would come from. Feb 26 '18 at 1:23
• @SF.: I'm pretty sure that's either a) due to the much wider temperature differences involved or b) a misreporting of efficiency relative to the Carnot maximum in one case, and overall efficiency in the other. Carnot heat engines cannot get more than about 37% efficient in the cases typical for gasoline/diesel/coal engines, because they're limited by the temperature spread between cylinder and outside air. Rocket engines have far higher temperatures. See Wikipedia's discussion of limits. Feb 27 '18 at 0:00

The patent indicates they're aiming for exhaust pressures in the region of 10-20 psi (0.7-1.3 bar), which gives a thruster chamber pressure of 5-10 psi (0.35-0.7 bar), which generates 0.5-2 lbf of force (0.25-1 N).

The patent claims is enough for settling the propellants during coast phases (where settling is only used to keep the propellants bunched up in a known place away from heat sources to minimize boiloff). Settling propellants requires 'less than 1/1000 G' (page 14).

On the Delta cryogenic second stage, the ullage gases are vented overboard and provide enough thrust to keep the propellants settled.
Page 16 refers to 'direct venting' of the exhaust gases, i.e. the IVF exhaust is not burned again in this mode.

For more thrust (e.g. when preparing to start the main engines), the hydrogen-rich exhaust can be combined with oxygen and ignited.

• Thanks, I'll give it a read. Of course F = 1.0 Newton and a = 0.0098 m/s^2 acceleration suggests a mass of only 102 kg, but I'll read the patent before drawing any conclusions.
– uhoh
Feb 26 '18 at 12:40
• So I'm still confused. In the Summary section, I've found "Low thrust for sustained coast settling, (such as in the range between about 0.5. to 2 lbf) may be provided directly by the hydrogen rich ICE exhaust gas fed to the axial thrusters." So are they burning the hydrogen-rich exhaust in the thruster, or just using the thruster as a passive expansion nozzle for the cold gas? Also, I can't find the number 0.001G, nor understand how 1 Newton could accelerate the entire vehicle's mass at that rate. I don't think we're at an answer yet here.
– uhoh
Feb 27 '18 at 10:24
• I think this is as close as we're going to get, since I've started by asking about a quote in a non sci/tech source.
– uhoh
Mar 6 '18 at 15:08