Rocket Motor that Eats Itself

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Wow. Now that's different! When will AT, CTI and Loki start making them? :wink:
 
What would that do to the CP and CG? Seems like that would push the CP above the CG - no?

The opposite. If the aft end is getting lighter, then the CG moves forward. CP only changes if the shape of the airframe changes.
 
As I think about this there must be some mechanism and restraint to feed the solid fuel and hold the pressure in the combustion area. This must add weight and could offset any possible advantage of eliminating the case.
 
I would worry about the fuel being structurally strong enough to support all the mass above it.
 
I would worry about the fuel being structurally strong enough to support all the mass above it.

Conceivably, the fuel could be solid plastic or rubber, so I that don't think that is a problem. The oxidizer is crystalline powder, so that might be a concern. The design might be constructed that the fuel would take the load. As it is now AP composite propellants are bonded to the case and carry some kind of load.
 
I would suspect military and space administrations will use it first. It will be years till amatures get it. Then agin, we might be the better place to test it.
 
I've had a few discussions on burning boosters instead of throwing them away, but this is a very different level.
 
The opposite. If the aft end is getting lighter, then the CG moves forward. CP only changes if the shape of the airframe changes.

Its not a question of the aft getting lighter, that happens in all SRBs.

This rocket is getting SHORTER! (the air frame is changing)

So assuming the solid fuel is evenly dense, and there are fins on the engine-pod-crawler-thingy, then the CoM and CoP are both moving up during flight but are staying proportional to the length of the rocket.

By the time substantial changes occur, you're up out of the atmosphere.
 
Researchers have tested a solid propellant rocket motor that eats itself. Evidently, the solid fuel rod is fed into the combustion chamber. It sounds like this would eliminate carrying a lot of the case weight.

https://www.bbc.com/news/uk-scotland-44226893
Beat me to it. I had planned to post this today:

Engineers aim for the stars with new rocket engine
May 24, 2018

https://phys.org/news/2018-05-aim-stars-rocket.html

A 'self-eating' rocket engine which could place small satellites in orbit more easily and more affordably is under development at universities in Scotland and Ukraine.

In a paper published in the Journal of Spacecraft and Rockets, engineers from the University of Glasgow and Oles Honchar Dnipro National University in Ukraine discuss how they have built, fired, and for the first time throttled up and down an 'autophage' engine which could change how small satellites are sent into space.

Today, most rockets use tanks to store their propellant as they climb, and the weight of the tanks is usually many times greater than the weight of the useful payload. This reduces the efficiency of the launch vehicle, and also contributes to the problem of space debris.

However, a launch vehicle powered by an autophage engine would consume its own structure during ascent, so more cargo capacity could be freed-up and less debris would enter orbit.

The autophage engine consumes a propellant rod which has solid fuel on the outside and oxidiser on the inside. The solid fuel is a strong plastic, such as polyethylene, so the rod is effectively a pipe full of powdered oxidiser. By driving the rod into a hot engine, the fuel and oxidiser can be vaporised into gases that flow into the combustion chamber. This produces thrust, as well as the heat required to vaporise the next section of propellant.

Simply by varying the speed at which the rod is driven into the engine, the researchers have shown that the engine can be throttled – a rare capability in a solid motor. Currently, the team have sustained rocket operations for 60 seconds at a time in their lab tests.

Dr. Patrick Harkness, senior lecturer at the University of Glasgow's School of Engineering, leads Glasgow's contribution to the work.

Dr. Harkness said: "Over the last decade, Glasgow has become a centre of excellence for the UK space industry, particularly in small satellites known as 'CubeSats', which provide researchers with affordable access to space-based experiments. There's also potential for the UK's planned spaceport to be based in Scotland.

"However, launch vehicles tend to be large because you need a large amount of propellant to reach space. If you try to scale down, the volume of propellant falls more quickly than the mass of the structure, so there is a limit to how small you can go. You will be left with a vehicle that is smaller but, proportionately, too heavy to reach an orbital speed.

"A rocket powered by an autophage engine would be different. The propellant rod itself would make up the body of the rocket, and as the vehicle climbed the engine would work its way up, consuming the body from base to tip.

"That would mean that the rocket structure would actually be consumed as fuel, so we wouldn't face the same problems of excessive structural mass. We could size the launch vehicles to match our small satellites, and offer more rapid and more targeted access to space.

"While we're still at an early stage of development, we have an effective engine testbed in the laboratory in Dnipro, and we are working with our colleagues there to improve it still further. The next step is to secure further funding to investigate how the engine could be incorporated into a launch vehicle.

The paper, titled 'Autophage Engines: Toward a Throttleable Solid Motor', is published in Journal of Spacecraft and Rockets.


This link provides the abstract shown below and links to test data and videos ("Available files"):

https://researchdata.gla.ac.uk/605/

This paper describes the instrumented test firing of a rocket that seeks to combine the throttleability of a liquid-fueled engine with the simplicity of a solid motor. The concept is that a differentiated fuel and oxidizer rod is forced into a vaporization unit where its constituents transition into separate propellant gases, which are then mixed in a combustion chamber. The vaporization unit is heated by the combustion, and the throttle setting is adjusted by changing the force used to drive the solid propellant rod into the vaporizer, which naturally influences the propellant feed rate. In experiments using a solid propellant rod consisting of polypropylene fuel and a 1:1.5 mixture of NH4ClO4 and NH4NO3 oxidizer, operations have been sustained for around 60 s. During testing, using propellant feed forces of between 250 and 900 N, propellant feed rates of between 100 and 300 mm/min have been achieved, which are in turn correlated to chamber pressures of between approximately 300 and 700 kPa. These correlated cycles of control input (the feed force), throttle response (the propellant feed rate), and implied thrust (the chamber pressure) demonstrate, for the first time, a simple solid rocket that can be throttled in real time.
 
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