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mugaliens
2008-Oct-10, 09:27 PM
I've had this idea at the back of my head for some time of a variable rocket, one in which both the propulsion fuel as well as the nozzle changes as the fuel is expended and the rocket's propulsion needs change. More like an multiple-staged rocket where each stage is perfectly suited for that phase of flight, runs dry, and is jettisoned.

Is there a way of using part of the mass of the expended stage as fuel/reaction mass for the next stage?

stutefish
2008-Oct-10, 10:44 PM
I've had this idea at the back of my head for some time of a variable rocket, one in which both the propulsion fuel as well as the nozzle changes as the fuel is expended and the rocket's propulsion needs change. More like an multiple-staged rocket where each stage is perfectly suited for that phase of flight, runs dry, and is jettisoned.
I'd say the Rocket-Assisted Projectile is an extreme case of this: A solid rocket motor is included with the payload of an artillery shell. The first "stage"--i.e., the powder charge that launches it from the gun, and the gun itself--is promptly burned up and left behind, while the second--rocket--stage continues to provide thrust for the payload.

ETA: Hold on... are you seriously arguing that previous attempts at multistage rockets have resulted in stages that were not optimized for their specific flight profile?

Larry Jacks
2008-Oct-11, 01:09 AM
Variable nozzles are used on many military aircraft (particularly high speed jet fighters) but I don't find many actual applications of variable nozzles on rockets. They're apparently too heavy.

I seem to recall reading some proposals for variable nozzle ideas but I can't think of any that were actually built. Here's a brief description of a patent for variable expansion ratio reaction engine (http://www.freepatentsonline.com/4707981.html). This link (http://pdf.aiaa.org/preview/1996/PV1996_3221.pdf) provides the first page of a technical article on advanced rocket engine nozzles. If you want the rest of the article, you'll have to find it at a library or buy it.

The aerospike (http://en.wikipedia.org/wiki/Aerospike_engine) engine design is supposed to be self-optimizing. It was being developed for the X-33 and was reportedly tested on an SR-71 about 10 years ago (test payload on an SR-71 flown by NASA). The program was canceled before getting to flight hardware. I seem to recall some advanced model rocketeers actually flew one with somewhat mixed results.

Most liquid fuel rocket motor designs are bi-propellant (fuel and oxidizer). The Russians built a tri-propellant engine called the RD-701 (http://www.astronautix.com/engines/rd701.htm) on the MAKS spaceplane.

The RD-701 was developed for the 22 tonne MAKS spaceplane, which was to have been launched at an altitude of 8 kilometres from the back of a behemoth An-225 Mriya transport. It was planned for 15 re-uses and featured both first and second stage engine characteristics in one reusable package. The tripropellant engine used dense kerosene and liquid oxygen for initial operations, then switched modes to a more modest thrust and higher specific impulse using low density liquid hydrogen and liquid oxygen. This reduced the huge hydrogen tank otherwise required. The state pulled out of the venture in cutbacks following perestroika but it was an extremely effective engine.

This is as close to a variable rocket engine that I know of. This patent description (http://www.patentstorm.us/patents/6619031/description.html) mentions tri-propellant rockets a bit and could be a beginning point for additional research if you wish. While it might be possible to change propellants in a solid fuel rocket, I know of none that do nor of any advantage from doing so.

Ara Pacis
2008-Oct-11, 02:33 AM
I've read that solid rockets can be made variable through grain size, composition, and position in the matrix, and the hole shape and size.

PraedSt
2008-Oct-11, 02:03 PM
I always thought the aerospike was a pretty nifty idea.
I'm probably wrong of course, judging by the slow pace of development...

Jerry
2008-Oct-11, 04:21 PM
Is there a way of using part of the mass of the expended stage as fuel/reaction mass for the next stage?
I know of two current applications - cases that use this concept.

cjameshuff
2008-Oct-11, 04:54 PM
Is there a way of using part of the mass of the expended stage as fuel/reaction mass for the next stage?

Hm. Use aluminum-lithium and thermoplastic polymer fuel tanks, and just oxidizer for the second stage. Melt the first stage tanks down and cast them into hybrid rocket motors or fuel pellets for the second stage. Only feasible if the rocket's large enough and has enough time that it doesn't require too much processing machinery...maybe a tank with accordion-style sides which can fold down directly into a usable fuel grain. Not so much optimizing each stage for its region of operation, just reusing structure as fuel in later stages...

Ara Pacis
2008-Oct-12, 12:10 AM
I've been wondering if a solid-fuel or hybrid-fuel rocket booster could be so designed that after the solid fuel burns out, the case becomes the bell extension for a higher ISP liquid-fueled/nuclear-fueled rocket at vacuum. IIRC, in space, you want longer bells/skirts, right?

PraedSt
2008-Oct-12, 05:06 AM
IIRC, in space, you want longer bells/skirts, right?

Right. And narrower I think.

Larry Jacks
2008-Oct-12, 05:16 PM
I've been wondering if a solid-fuel or hybrid-fuel rocket booster could be so designed that after the solid fuel burns out, the case becomes the bell extension for a higher ISP liquid-fueled/nuclear-fueled rocket at vacuum. IIRC, in space, you want longer bells/skirts, right?

I think the problem with your idea is that rocket engines designed for low altitude have smaller expansion ratio nozzles. Just using the casing itself probably wouldn't increase the expansion ratio of the attached nozzle. You'd also have to contend with the dead weight of the empty casing and quite likely a decrease in performance due to drag inside the empty casing. The exhaust flow from the upper stage would be flowing (probably at supersonic speeds) inside the empty casing which would probably cause friction losses. Between that and the extra weight, you'd almost certainly be better off just jettisoning the empty casing and building a bigger nozzle on the upper stage.

Ara Pacis
2008-Oct-12, 10:21 PM
Well, you'd jettison the nozzle that was attached to the lower casing and the case itself would be designed with the proper cross-section to act as the bell extension to the "upper" engine. Note, this is merely a minor adaptation to a higher altitude engine. There would probably be other engines, SRBs operating in tandem at lower altitude that would be dumped en route.

Larry Jacks
2008-Oct-13, 12:56 AM
I still expect that an analysis will show your idea is far heavier than simply putting the correct size nozzle on the upper stage. An upper stage nozzle isn't that large or heavy because, while the expansion ratio is higher, the thrust is lower. If you have not seen it, go to the SpaceX website and watch the video of their successful Falcon 1 launch. You can clearly see that the upper stage nozzle (sized for high altitude operations) is quite small.

Ara Pacis
2008-Oct-13, 04:00 AM
The question was about variability. Performance advantage is a separate issue.

Larry Jacks
2008-Oct-13, 01:09 PM
It seems to me the only reason to strive for variability is to achieve a performance advantage.

Ara Pacis
2008-Oct-13, 02:56 PM
Striving and achieving are two different things.