Thinking about ram-rockets

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Zippiot
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Thinking about ram-rockets

Post by Zippiot » Mon Nov 10, 2008 8:11 am

Took a good long look at my propane torch today and wondered if it was the key to making a hobby ramjet-boosted rocket. Even if the ramjet doesnt boost the rocket any higher would be cool to have a giant controlled torch shoot out the back.
I stuck this in the ramjet forum b/c I fear the ramjet's functionality over the rocket's.

Have the rocket shoot down the ramjet uninterupted would be a necessity, gotta keep rocket thrust up!!
things considered:
*Safety- The liquid fuel cant set the rocket ablaze or endanger those on the ground
*Ejection- With almost no real way of knowing how long the ramjet would burn I think I figured out an ejection method that is safe
*Ignition- The ramjet's fuel would be ignited by the rocket and not rely on any special timing, this is all built into the design
*Fueling- Much safety has been built into the general design so the rocket isnt spraying flaming fuel as it plummets to earth :(


Basic design:
Rocket nozzle points down the intake of the ramjet, creating a venturi effect [ramjet is ducted]. So the rocket exhaust helps get things going, it also ignites the fuel. Maybe a nozzle like Image could be used to incorporate a spray to the fuel without messing with the rockets exhaust. The flameholder would be a finger style with a good sized hole down the middle to not interfere with the rocket.
Fuel is shot in by compressed air, it can only work when the rocket is pointing straight up; if the rocket were to nose over the fuel tank would shoot out its compressed air and instantly stop spitting fuel [for safety].
Ejection would be achieved by a type of Mercury switch. Maybe a 4 inch pvc pipe with a bit of saltwater in it. 2 contacts in the top are activated when the rocket noses over and set off the charge, proabably a compressed air charge though it adds weight it keeps any fire away form the fuel tanks.
Ignition of the rocket starts the fuel flow. Either by a "force of gravity" switch or the ignition charge wires are also in contact with a solenoid that will open the fuel lines.
I think the rocket itself can be kept pretty small, dont want it to be bogged down by a larger body of heavy materials.


If the ramjet doesnt light hopefully it makes a giant "propane torch" look and doesnt interfere with the rocket.

Got a 5 minute google sktchup pic I just drew, hopefully more to come!
Of course they are not finished, class tomorrow gotta sleep......
Image
Image
Image
Image
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larry cottrill
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Re: Thinking about ram-rockets

Post by larry cottrill » Mon Nov 10, 2008 2:10 pm

Zippiot wrote:Fuel is shot in by compressed air, it can only work when the rocket is pointing straight up; if the rocket were to nose over the fuel tank would shoot out its compressed air and instantly stop spitting fuel [for safety].
Sorry, sir, it won't work like that. You cannot rely on gravitational forces helping you in that way in a flight vehicle -- the inertial forces will win almost every time. As long as your rocket is accelerating, the liquid will "bottom out" in the tank regardless of direction of motion. The other inertial force that could affect it is any lateral force caused by sharp turns (either from intentional maneuvering, or from a catastrophic event such as combustion chamber burn-through).

If you think gravity gives you something you can hang your hat on, just take an hour of flying instruction "under the hood" with a good instrument instructor. You will be amazed at the meaninglessness of your sense of "up and down".

L Cottrill

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Re: Thinking about ram-rockets

Post by Zippiot » Mon Nov 10, 2008 3:51 pm

Hmm, gotta rethink it then.
Would the compressed air bottom out in a tank too at these accelerations?
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Re: Thinking about ram-rockets

Post by tufty » Mon Nov 10, 2008 4:15 pm

Zippiot wrote:Hmm, gotta rethink it then.
Would the compressed air bottom out in a tank too at these accelerations?
Yep.

Any acceleration at all will change how a material M surrounded by a fluid behaves. Vector sum of forces, innit.

Zippiot
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Re: Thinking about ram-rockets

Post by Zippiot » Mon Nov 10, 2008 6:59 pm

Im concerned about fuel shut off if the thing noses over, any ideas?
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Re: Thinking about ram-rockets

Post by larry cottrill » Tue Nov 11, 2008 12:31 pm

I can think of two things that would work, and there are probably more. Unfortunately, these would be heavy solutions for a small rocket. They are: a small electric gyro; or, a rate-of-climb indicator with a circuit that is 'armed' by the initial acceleration. And, of course, you'd need an electric shutoff valve or servo-driven valve for the fuel cutoff.

But, I am no rocket hobbyist. I'm pretty sure that experienced rocketeers will have better answers.

L Cottrill

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Re: Thinking about ram-rockets

Post by Zippiot » Wed Nov 12, 2008 12:26 am

I wonder if at apogee, all the forces so go poof for a split second or so right? Doin some more research
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Re: Thinking about ram-rockets

Post by Najm » Fri Dec 05, 2008 2:14 pm

Just wondering...
Why would the rocket reach apogee while the ramjet is producing thrust?

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Re: Thinking about ram-rockets

Post by Zippiot » Fri Dec 05, 2008 11:34 pm

Producing thrust =/= net thrust, especially in something I build. So even if the ramjet makes thrust its probably not enough to keep it moving skyward. This and all projects are on hold while I'm at school cant burn down the new house!!
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Re: Thinking about ram-rockets

Post by Najm » Sun Dec 07, 2008 2:12 pm

okay....
but testing would be a good idea...

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