Autophage Engines: Toward a Throttleable Solid Motor

[Asteroza]

Joint work between Scotland and Ukraine on a throttleable solid, effectively a rod (wire?) of propellant fed into a chamber.

No clear shutdown/restart tests yet (currently uses ground propane to get it to self-ignition temps), but interesting.

https://arc.aiaa.org/doi/10.2514/1.A34153



https://www.theregister.co.uk/2018/05/25/boffins_unveil_a_rocket_that_will_eat_itself/

Kind reminds me of those plasma thrusters that rely on vaporizing teflon blocks that are fed into the chamber...

[A_M_Swallow]

A lot of propane was used to warn the engine. Was this sufficient to lift the rocket off the launch tower?

[Asteroza]

A lot of propane was used to warn the engine. Was this sufficient to lift the rocket off the launch tower?

Considering the engine appears to be upside down in a test rig, I think it isn't going anywhere fast...

[Crispy]

Not made quite clear in the OP is that the propellant rod would form the actual body of the rocket. The combustion chamber would "eat" its way up the stage until making contact with the interstage/payload. Very clever!

[sanman]

What would be the best application for an engine like this? Some mission where you badly want to avoid boil-off? A return mission from the surface of Venus or Mercury perhaps?

[speedevil]

What would be the best application for an engine like this? Some mission where you badly want to avoid boil-off? A return mission from the surface of Venus or Mercury perhaps?

There are storable propellants which are credibleish at moderately elevated temperatures, comparable with what solids might be able to be stored at.
For example, RFNA/FFA - both components have boiling points of over 100C. Moderately elevating the pressure takes these even higher.

For mercury, in some cases, it may even be plausible to do cryogens stored long-term as most of the sky is still black.

[sanman]

So is this another solution looking for a problem? Or do they have some particular use in mind for it?

[jbenton]

So is this another solution looking for a problem? Or do they have some particular use in mind for it?

https://www.theregister.co.uk/2018/05/25/boffins_unveil_a_rocket_that_will_eat_itself/

The article says that they want to use it to launch cubesats. Apparently, the self-eating feature makes it easier to scale it down. They also want to build a low-key spaceport in Scotland from which to fly them.

[jbenton]

If the fuel has no casing, then how does it maintain it's length? Would the stage have a thin cylindrical exterior (basically a fairing) that would attach the engine section to the part they is mated to the second stage (and/or the payload)?

[sanman]

Gee, could this lead to technology improvements in the fireworks market? 

If this is easier to scale down, then could it be used beneficially in retropropulsion/braking solid motors like those in the Soyuz or the New Shepard capsules?

[speedevil]

If the fuel has no casing, then how does it maintain it's length? Would the stage have a thin cylindrical exterior (basically a fairing) that would attach the engine section to the part they is mated to the second stage (and/or the payload)?

It's basically a solid motor with a solid fuel tank.
You would need at least a high power press, and an aero framework round it at least. The press can be entirely tensile which helps somewhat in terms of lightness.

The press needs to be high power - of the same sort of power as turbopumps for an equivalent thrust liquid engine, because it's doing the same thing - forcing an unpressurised fuel into a pressurised combustion chamber.

[sanman]

It's basically a solid motor with a solid fuel tank.
You would need at least a high power press, and an aero framework round it at least. The press can be entirely tensile which helps somewhat in terms of lightness.

The press needs to be high power - of the same sort of power as turbopumps for an equivalent thrust liquid engine, because it's doing the same thing - forcing an unpressurised fuel into a pressurised combustion chamber.

Turbo-pumps can be powered by the fuel's combustion. How do they harness the solid fuel's combustion to power the press? Turbine again -ie. "turbo-press"?

[speedevil]

The press needs to be high power - of the same sort of power as turbopumps for an equivalent thrust liquid engine, because it's doing the same thing - forcing an unpressurised fuel into a pressurised combustion chamber.

Turbo-pumps can be powered by the fuel's combustion. How do they harness the solid fuel's combustion to power the press? Turbine again -ie. "turbo-press"?

In principle you could run a turbine from a gas generator operated at a lower temperature, or tap some of the exhaust gas and run it through a cooling system similarly, or try to make a very high temperature turbine to operate in the complex flow conditions of solid exhaust going through it.

It may be considerably easier and lighter to just to use an electric motor and batteries.
In principle, you can sidestep some of this if you operate it discontinuously - allow the combustion chamber pressure to  decay before replacing the fuel in a sealed container, or only advance the fuel during cyclic periods of low pressure. Both greatly impact efficiency and have other problems.

[jbenton]

Gee, could this lead to technology improvements in the fireworks market? 

If this is easier to scale down, then could it be used beneficially in retropropulsion/braking solid motors like those in the Soyuz or the New Shepard capsules?

I imagine that if it were used in such an application, then the throttleability of the design would also be useful.

[jbenton]

If the fuel has no casing, then how does it maintain it's length? Would the stage have a thin cylindrical exterior (basically a fairing) that would attach the engine section to the part they is mated to the second stage (and/or the payload)?

It's basically a solid motor with a solid fuel tank.
You would need at least a high power press, and an aero framework round it at least. The press can be entirely tensile which helps somewhat in terms of lightness.

The press needs to be high power - of the same sort of power as turbopumps for an equivalent thrust liquid engine, because it's doing the same thing - forcing an unpressurised fuel into a pressurised combustion chamber.

How do you mean "entirely tensile"?

[Stan-1967]

The way I read the article, the engine can be thought of as an expander cycle for a solid rocket.  The heat of combustion will need to be harnessed to affect a phase change in the solid fuel into a gas or liquid, where it then is fed into the combustions chamber.  Because the area of the heat exchange surface scales linear to the radius of the fuel rod, vs the volume of fuel scale to the square, it favors smaller engines ( similar to the expander cycle scaling issues).  I also think the engine could operate as self pressurizing if as the engine climbs up the fuel rod, it also seals off the heat exchanger & combustions chamber.  I think of a sliding "O" ring going up the fuel rod, with the pressurized engine section on one side & the bulk fuel rod on the other side. ( possible failure mode)

I visualize the engine segment crawling up a shaft of propellent until it reaches a staging point, or the PAF if on a second stage.  I would think engine shutoff would also be able to be engineered so that fuel residuals are near zero.

[sanman]

In principle you could run a turbine from a gas generator operated at a lower temperature, or tap some of the exhaust gas and run it through a cooling system similarly, or try to make a very high temperature turbine to operate in the complex flow conditions of solid exhaust going through it.

It may be considerably easier and lighter to just to use an electric motor and batteries.
In principle, you can sidestep some of this if you operate it discontinuously - allow the combustion chamber pressure to  decay before replacing the fuel in a sealed container, or only advance the fuel during cyclic periods of low pressure. Both greatly impact efficiency and have other problems.

Since in the end you're only trying to achieve the same motion as a linear piston, maybe then the turbine thing is inefficient/over-complicated. Maybe turbines are best for transferring power from fluids onto other fluids, while a linear piston might work best to apply power from a fluid onto a linear solid fuel element.

The way I read the article, the engine can be thought of as an expander cycle for a solid rocket.  The heat of combustion will need to be harnessed to affect a phase change in the solid fuel into a gas or liquid, where it then is fed into the combustions chamber.  Because the area of the heat exchange surface scales linear to the radius of the fuel rod, vs the volume of fuel scale to the square, it favors smaller engines ( similar to the expander cycle scaling issues).  I also think the engine could operate as self pressurizing if as the engine climbs up the fuel rod, it also seals off the heat exchanger & combustions chamber.  I think of a sliding "O" ring going up the fuel rod, with the pressurized engine section on one side & the bulk fuel rod on the other side. ( possible failure mode)

I visualize the engine segment crawling up a shaft of propellent until it reaches a staging point, or the PAF if on a second stage.  I would think engine shutoff would also be able to be engineered so that fuel residuals are near zero.

But won't this affect the center of mass of your whole assembly during flight, since the engine will likely be a significant contributor of mass?

This whole idea seems to be a different take on the variable geometry vehicle - like when SpaceShipOne/Two feathers its tail.

[speedevil]

The press needs to be high power - of the same sort of power as turbopumps for an equivalent thrust liquid engine, because it's doing the same thing - forcing an unpressurised fuel into a pressurised combustion chamber.

How do you mean "entirely tensile"?

It can be a winch, with ropes or cables to the far end of the fuel grain, for structural efficiency.

[sanman]

It can be a winch, with ropes or cables to the far end of the fuel grain, for structural efficiency.

Wouldn't these then transmit vibrations very efficiently, to create larger forces in that regard? But maybe those forces would be easier to model, since they're traveling linearly.

[Asteroza]

Looks like a bit of news...


https://www.gla.ac.uk/news/headline_756078_en.html