Thursday, May 31, 2018

Genuinely New Ideas in Rocketry Are Rare

Vanishingly rare.  I don't know if this is the first time it has ever been discussed, but according to the phys.org website, a collaboration between two universities is researching a booster that literally consumes itself on the ascent to orbit. 
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.
'Autophage' translates as 'self-eating', and the concept is that instead of fuel tanks and the superstructure that the booster provides, a vehicle powered by an autophage engine would consume its own structure during ascent.

A typical booster, like the Falcon 9 boosters that SpaceX recovers and reuses, contains liquid fuel and oxidizer tanks along with plumbing to connect those tanks to the booster's nine engines   Big boosters tend to be liquid fueled, with notable exceptions for the large solid rocket boosters used in the Space Shuttle program, and large vehicles like the Delta IV Heavy.  Typically, the booster's weight is several times the weight that can be delivered to orbit.  For every pound of weight added to payload, the vehicle has to carry more fuel, and the fuel to lift that fuel. If the booster's weight was usable fuel that would be an incredible advance. 

The autophage engine concept allows weight from the plumbing tanks and other essentials to go into larger cargo capacity.  If the same technology were to be in upper stages, less debris would survive to make it into space.  No boosters (or less junk) to recover or dump in the ocean; and less debris from upper stages.  More efficient from all perspectives.
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.
I'm having a difficult time visualizing how this could work.  The vehicle has a "combustion chamber", or chambers, so some structure needs to last the entire time it's burning to hold the combustion chamber and engine nozzle(s) in place.  If some control or telemetry to the ground is involved, that will have to be in the booster just as it is now, and will get thrown away, too.  If the autophage motor is just burning along the length of the body, it's not that different from any solid rocket motor; those tend to burn from the center outward along the entire length of the body.  Having the ability to throttle the engine up and back is unusual.  Solid motors are typically said to burn until they burn out.

The researchers describe it in starkly different terms than I do and don't address my thoughts at all.  Dr. Patrick Harkness, senior lecturer at the University of Glasgow's School of Engineering, puts it this way:
"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."

A small autophage motor at the University of Glascow.  University of Glascow photograph.


17 comments:

  1. Sounds great. But like so many things in life the devil is in the details. Thing that work small scale/in the lab frequently cannot be
    scaled up to real world utility. Time will tell if this idea pans out.

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  2. The propellant pipe/stick is structurally the main cylindrical body of the rocket. The engine reaction chamber chews its way up the propellant stick/rocket body from the bottom. When the fuel is all consumed the reaction chamber is next to the payload at the top. The structural material that is saved is 200 feet of body tube and tanks.

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  3. I'm not sure how you melt a plastic fuel while at the same time it's strong enough for you to press upward on it at a couple G's and also plug a hole in a combustion chamber holding rocket engine exhaust pressure.

    Maybe there needs to be a sleeve shape on top of the engine that extends up onto the non-melted part of the propellant stick for structural purposes. Then the melting fuel can be a liquid pool above the engine which doesn't need bending strength, but can transmit acceleration force through fluid pressure.

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    1. I'm not sure how you melt a plastic fuel while at the same time it's strong enough for you to press upward on it at a couple G's and also plug a hole in a combustion chamber holding rocket engine exhaust pressure.

      That's the big one. I think the center of gravity has to move up the disappearing booster and change the handling characteristics of the thing. To throttle the engine, some sort of feed mechanism forces the rod into the engine. A motor?

      I just don't see how they make it out of the lab and into the field.

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    2. If the cup/tank shape on top of the combustion chamber was the entire height of the fuel, then clearly it would work, that's the solid fuel rocket answer. How much of this tank wall height can be omitted? Combustion heat may not progress up through 200 feet of plastic fuel, a thermal insulator, all that fast relative to burn time of several minutes.

      Forces from gravity plus rocket acceleration forces fuel into engine; combustion chamber pressure forces fuel out. Which number is larger? I imagine worm screws digging vertically into the fuel plastic around the outside edge to control the motion.

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  4. SiG...The Delta IV Heavy is liquid fueled. It burns LH2 and LOX.

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    1. Oops. I thought the strap-ons were solid, but a quick look at the link shows that not the case.

      Time to fire the editor!

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    2. Fire Judy (again).
      (Reference Dave B’s blog)

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    3. The problem with firing her is that you'd have to start paying her first, right?

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  5. The solid fuel would act as a pillar or column. As long as there is thrust and acceleration at the bottom and a payload at the top, the fuel is essentially a compression loaded pillar. The engine thrust also creates a thrust that can be used to inch the engine up the fuel stack. Throttling would only requires a means of controlling the speed at which the engine can advance up the fuel stack.

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  6. This sort of thing has been done in model rocketry for at least 25 years. Is it still a model when it's 40 feet long and goes up over 100,000 feet?

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    1. Is it still a model when it's 40 feet long and goes up over 100,000 feet?

      Considering how short that is of orbit, I'd say so. Maybe that's just me.

      It's in that awkward range. High enough to get sucked into a jet engine, too low to consider space. AFAIK, whenever they say someone got into space, they specify over 62 miles and 100k feet is less than 20 miles.

      I've seen some high power rocketry and been to one local meet, but never heard of a consumable stage. Can you point me to anyplace?

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    2. I really wish I could. I visited a dedicated model rocketry shop in Ohio about 25 years ago. I didn't drive, and I was never sure where it was. It's just somewhere (north?) west of Columbus, out on the plains.

      Anyways, they had NASA contacts for high altitude shots, their own tracking radar (shared with a small airfield), and this giant (20') solid-plastic stage with a metallic rod core they were really proud of. What they were telling me didn't make much sense at the time, but I now recognize it from the description above. I remember it as being something like a combination of thermite and a special sort of plastic, but it was a long time ago.

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  7. REALLY old idea.
    https://patents.google.com/patent/US3250216A/en

    It really bothers me when credit is given to people who recycle ideas, with zero credit to the originators of the idea.

    This happens so much in tech, especially when it’s younger people doing the recycling.
    “Hey look how much smarter these kids are than those old grey beards”
    Sheesh.

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    1. Thanks for that Marc. I was telling my wife last night as I posted it that someone will probably tell us Tsiolkovsky or Goddard or someone back then wrote about it.

      Not quite that far back.

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  8. Don’t thank me too much, while I have a few decades of experience in aerospace, and have heard of this before long ago, this link was the result of googling “autophagus rocket” and that patent was in the top ten....wonder why the reporter didn’t do the same.

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  9. I can see an engine assembly that crawls up a structural fuel and oxidizer rod, but to me the control problems get... rather interesting - your total weight and center of gravity change even more with this design than with a 'normal' rocket, and you end up with a structure much, much smaller than what you launched.
    I think it is an idea worth pursuing, but doubt it will be feasible for a long time.
    It seems to me an idea that would be easier to develop with an unguided rocket, for example something like the old Soviet Katuysha (sp?) rocket systems - the accuracy error induced there wouldn't really matter, so the designers could focus on the engine.

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