Half a croissant, on a plate, with a sign in front of it saying '50c'
h a l f b a k e r y
"My only concern is that it wouldn't work, which I see as a problem."

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Piston Rocket

No pumps, stages or multiple motors. Might achieve single stage to orbit.
  (+22)(+22)(+22)
(+22)
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The combustion chamber moves within the body of the rocket pushing propellant into itself eliminating the need for pumps, stages and multiple rocket motors.

I linked the video below.

doctorremulac3, Sep 05 2008

(???) Piston rocket http://www.youtube....watch?v=LpGDg-GNYZU
[doctorremulac3, Sep 05 2008]

Artwork for this thing http://www.createth...l.html?entryID=1863
This is at a contest site where I entered this. [doctorremulac3, Sep 05 2008]

The Single Stage to Orbit Manifesto http://vorlon.case....jam64/work/ssto.htm
Great reading if you're into this stuff like I am. [doctorremulac3, Sep 06 2008]

Pulse Jet http://en.wikipedia...ki/Pulse_jet_engine
High frequency piston engine? [mylodon, Sep 07 2008]

Barry Whites' super sexy rocket nozzle design page. http://www.braeunig.us/space/sup1.htm
Just kidding. It's actually the kind of math stuff that used to put me in a coma in school. [doctorremulac3, Sep 07 2008]

Liquid Fueled Rocket design http://gramlich.net...s/rocket/index.html
[MisterQED, Sep 07 2008]

Don LaFontaine http://en.wikipedia...wiki/Don_LaFontaine
[hippo, Sep 10 2008]

The un-compressibility of liquid http://www.youtube....watch?v=X-hBMyaigqI
Breaking things while drunk for science. [doctorremulac3, Sep 10 2008]

[link]






       Very impressive concept. Good video and narration, too.   

       It might need some engineering and experimenting on seals and sizes, but it is very simple in principle. Croissant for elegant idea!
baconbrain, Sep 05 2008
  

       Thank you B!   

       (Check's in the mail) ;)
doctorremulac3, Sep 05 2008
  

       It's a beautiful design, but I have some questions: Is reaction force going to create enough force to keep the fuel pressures high enough to be in their ideal injection pressures? i.e. in a kerosene and oxygen rocket, one bell shape needs 500psi pressures from each of the components for peak efficiency, can you get 500lbs of thrust for each sq.in. of cross sectional area of piston? Second, minor redesign: how many oxidizers can you run thru an Al tube? I would think most would auto-combust, so you may want to add a liner.
MisterQED, Sep 05 2008
  

       A couple of general possibilities: Put a little widger in the combustion chamber to control the shape of the oxidizer tube as it softens and melts. Put spiral threading/rifling in different places and to control movement ratios and slightly affect sealing . . . somehow.
baconbrain, Sep 05 2008
  

       How did you get Lee Marvin to do the narration?
AbsintheWithoutLeave, Sep 05 2008
  

       //How did you get Lee Marvin to do the narration?// I second that.
MisterQED, Sep 05 2008
  

       Did you make that video, doc? It is very well done.
bungston, Sep 05 2008
  

       baked, sort of, but I can't find the reference. Small rocketry mentioned in a book (about Parsons? NASA? "The Right Stuff"? "Rocket Boys"?) in the 1950s-60s timeframe used acceleration to compress propellant. I know my response has the fail, alas.
willard_b_trophy, Sep 05 2008
  

       It's a great idea! However, seems like there might be issues with direct contamination if you put both reactants in the same cylinder. Some sort of flexible liner for the propellant maybe. I love the elegance of a container that gets smaller and lighter as it is used.
WcW, Sep 05 2008
  

       On the downside, not reusable.
WcW, Sep 05 2008
  

       [+] for what I think is a new take on the idea, but...   

       The "parts burn away" bit makes me a bit nervous. I would guess that the central aluminum tube will either not burn off fast enough, thereby causing the combustion reaction to occur lower and lower in the chamber, or it will burn away too fast and no longer deliver anything into the combustion chamber.   

       The outer shell may burn away unevenly, causing aerodynamic instabilities.
James Newton, Sep 05 2008
  

       //Is reaction force going to create enough force to keep the fuel pressures high enough to be in their ideal injection pressures?//   

       Ya know, I'm really not sure, that's why I put it up here. I was hoping somebody might be able to figure that out without me having to actually build one of these. It seems that if a rocket motor can push a several thousand pound rocket to thousands of miles an hour it should be exerting enough force to push the necessary fuel into itself.   

       //How did you get Lee Marvin to do the narration?// LOL. On the first take I sounded like an excited little kid so I tried to sound a little less geeky. It came out sounding like "Barry White's groovy rocket design love bungalow".
doctorremulac3, Sep 05 2008
  

       I was thinking that the ox supply tube would be doused in flowing oxidizer on the inside and fuel on the outside that would cool it until it entered the chamber at which time it would melt away. It could also be made of different material. The theoretical advantage of the aluminum is that it adds mass to the exhaust flow without raising the temperature.   

       But like all new rocket designs, I wouldn't wanna be standing next to the prototype on it's first static test.   

       By the way, the way this thing would be kick started is by putting a plug of solid rocket fuel in the combustion chamber to block the flow of fuel/ox until it burns out of the way and gives the first push to pressurize the system.
doctorremulac3, Sep 05 2008
  

       Good idea and very impressive video. Can I get you to dub my actual life?   

       I also had concerns about the aerodynamic problems of burning away the outer casing. Perhaps concerns could be alleviated by having the outer casing made of successive rings held together by easily-combusted material, so you'd shed rings rather than burning the casing away?   

       One other question. You're in a positive feedback situation, in that increased thrust leads to increased fuel flow which leads to... How do you stabilize against that?   

       I'd also like to use the word "stochastic", but can't think of a suitable opportunity.
MaxwellBuchanan, Sep 05 2008
  

       Thank you Max. Sure, I'll do your voice over work. I'd really like to get the job of that movie trailer guy who just passed. The guy who says: "In a world where..." in all the movie adds.   

       I don't know if this thing would work or not, there's a lot of things that could go boom and I'm an "enthusiast" not a rocket scientist so I'm guessing at all this stuff. I'm entering it in a NASA design contest that I entered a few years back to see if I can get any more insight into whether or not I should try to build one of these. (I won power tools last time I entered.)   

       I put the link up. It has some art I did to the best of my abilities that sort of clarifies the main elements. Hope it's ok to put a link up to another invention page. Don't want to look like I'm advertising for them or whatever. I'll take it down of it's not kosher.
doctorremulac3, Sep 05 2008
  

       [+] Although I think you'll have to do a lot of work sealing a piston which has to seal against an oxidizer, fuel, and air, with all the vibration, etc, that goes on.   

       Also imagine shooting this, bottle-rocket style, down a street. Everytime it would strike something, it would be rewarded with a burst of energy.
mylodon, Sep 05 2008
  

       This just might work for a Single-Stage Orbiter, but it does not solve the problem of having to use a new rocket for every launch. Now, if the engines were subsequently recovered after a flight. . .
Moonguy, Sep 05 2008
  

       I can see some other problems:
Ratio control.
Starting.
Stopping.
  

       I have a feeling that the slight ratio control problem might be overcome with an elegant redesign of the piston and chambers, but it's just a feeling at the moment, and I can't put my finger on the solution.   

       Stopping shouldn't be a problem, finally!
But starting would need some special allowance.
Ling, Sep 05 2008
  

       + for the video, don't have any expertise to comment further.
zeno, Sep 05 2008
  

       I really like the creativity and concept, but I may have found a flaw. Maybe. Let's see if I can write it out.   

       The motor presses up into the rocket body because of internal pressure in the combustion chamber, does it not? The nozzle may affect things, but I'm guessing that the pressure against the top of the chamber is what does the work.   

       In this case, the pressure against the top of the chamber pushes against the fuel tank. The area of the tank piston is GREATER than the area of the top of the combustion chamber. So the push from the combustion chamber is spread out over a larger area than the inside of the top of the chamber, and the psi inside the tank must be LESS than the pressure inside the chamber.   

       That's basic hydraulics, innit? A hundred psi of raging rocketfire against a one-square-inch chamber-top pushes a hundred pounds against a two-square-inch fuel-tank-bottom piston. The pressure in the fuel tank must be fifty psi, right?   

       Now, lead that fifty psi fuel back around to the hundred psi combustion chamber, and ask it to inject itself into the chamber, which is at twice the pressure of the fuel. I don't think it's gonna happen.   

       If, viewed from the top, the rocket motor has less area than the fuel tank, the pressure inside the tank must be less than the pressure in the motor. Mustn't it?   

       That's as far as I can crunch that tonight. Sorry.
baconbrain, Sep 06 2008
  

       [baconbrain] What about a pulsating combustion chamber? Fuel enters when there is no pressure in the combustion chamber, driven there from momentum and gravity. Then there is a burst of energy, the piston thrusts upwards, the chamber shuts off again, and more fuel arrives... ?   

       A-ha! They call them pulse jets! Just add a little valve. See link.   

       Something else I was thinking about, in terms of a continuous feed, is having a very short injection pipe. Enough that the act of burning some fuel, exposes more fuel, which then burns as well. Much like a solid motor on a rocket, because effectively this piston rocket is attempting to behave like a common bottle rocket, but with fluid fuel.   

       I'm pretty far out of my league here though...
mylodon, Sep 07 2008
  

       //The nozzle may affect things, but I'm guessing that the pressure against the top of the chamber is what does the work.//   

       May be true to some extent, but here's what I learned from my web based rocket science education consisting of piecing together tidbits-o-information that may or may not be true.   

       Supposedly a lot of the thrust comes from the gasses expanding and hitting the cone after it leaves the chamber, that's why the shape of the cone is so important. The gas in the combustion chamber is under high pressure but relatively low velocity until it leaves the chamber and hits the bell. That's when it expands, get it's velocity and pushes against the inside of the bell giving the rocket a lot of it's push. That's why the stages in rockets have different shaped cones for different altitudes.   

       I'll try to put up the link of where I read that. Seems kind of counter intuitive I know. I though it was just like the balloon you let go of that flies across the room like we all learned in school. Evidently that expanding high velocity gas in the bell is a big part of it, otherwise why have a bell at all right? They're a lot of weight and a hassle to keep from melting and if it were just the chamber doing the pushing you could just use a hole and be done with it.   

       Anyway that's my understanding. I'll try to find that link and put it up.   

       And thank you for helping with this. I'd like as much input as possible before I waste any time actually trying to build one of these things so see if it flies, goes boom or just sits there. My Sunday project will be writing to a couple of physics professors from MIT, Stanford and a some other places that have been helpful with other questions I've had in the past. I'll post their answers if and when I get them. I've found that physics professors are usually pretty chatty and like to share their knowledge. (Guess that's why they became teachers.)
doctorremulac3, Sep 07 2008
  

       Take one of those push-up frozen desserts on a stick. Hold it by the stick. Without touching any other part, push the goop up out of the tube.   

       The other problem is stability. As discussed before somewhere here, there are 3 basic ways to stabilize a rocket:   

       1. Fins. Forget it. They have to be on the bottom, and we just burned that off.   

       2. Gimballed rocket nozzles. Well, that looks pretty iffy, given that we don't have a firm grip on anything with which to yaw and pitch the nozzle; having it get cockeyed-wedged in the outer casing doesn't sound too joyous.   

       3. Vernier rockets. Best bet, but pretty delicate as the center of mass *and* center of pressure of the rocket are continually moving; and having the verniers at the aft-end is not an option since they would be lost just like the fins.   

       RCS system on the nose, all the way... your RSO will just keep repeating "Hell, no!" until you shoot him.
lurch, Sep 07 2008
  

       //Take one of those push-up frozen desserts on a stick. Hold it by the stick. Without touching any other part, push the goop up out of the tube.//   

       True, but this isn't ice cream, it's liquid that would drain out of the bottom anyway even without pushing against it. If you're pushing that stick with a couple of thousand pounds per square inch you're going to get some increase in how quickly that liquid comes out. How much? I don't know but certainly some. It's also not just the weight of the rocket your pushing against to exude the propellant, it's the air pressure pushing against the top of the rocket. You're basically squeezing the chambers between the force of the thrust on one end and gravity and air pressure on the other, but, yea, I have no idea if there would be enough pressure to give this the flow you would need, that's the big question.   

       I was thinking about the stability. You could have a set of continuous fins that run down the whole body of the rocket and just burn off with the rest of it but that would sort of defeat the purpose of eliminating as much weight as possible.   

       I was thinking you might get initial stability from spinning the fuel in the chambers then giving a gentle spin to the rocket via the shape of the nozzle but then, yea, all your steering would have to come from vernier nozzles on your payload module which you'd have there anyway.   

       Of course as this whole thing is an exercise in saving weight you can't start adding heavy stuff to make up for what you loose in a standard rocket design and, yea, a gimbaled nozzle is out.   

       You could have aileron panels on the nose that swing out into the airflow to push the rocket the other direction without adding too much weight I guess.   

       Of course there's the line from the old song: "Vunce rockets are up, who cares vere zey come down? Zats not mein department!" says Werner von Braun.
doctorremulac3, Sep 07 2008
  

       But [doctorremulac3], you are not addressing the concern that fuel cannot enter the combustion chamber, due to the pressure (which you admit) that exists there. Instead (say the combusion was able to continue without fuel), the fuel would be pushed back up the fuel injectors and into the tank. Because basically the combustion chamber doesn't care whether it is liquid or metal, it is trying to push everything away.   

       Current approaches to solve fuel injection seem to be either pumps (which can be complex) or pressurized fuel systems (which is what Armadillo Aerospace is doing).   

       I don't think the pressure at the top of a bell is greater then the pressure at the top of the combustion chamber where your injection ports are.   

       [edit: for instance, consider your injection port to be just another throat to just another exhaust... ]
mylodon, Sep 07 2008
  

       Well, you've got two opposing forces that coincidentally, happen to be the same force, that is, the pressure in the combustion chamber acting both to exude the fuel into the chamber and push it back at the same time. If it works, it would work on the principal that liquid can't be compressed and the greater pressure is that forcing it out of it's container. So you've got say, 2 square feet of area at the top of the chamber and through that, maybe 2 square inches of holes penetrating the top of that chamber. The force acting against that 2 square feet is going to be greater than the force acting against the 2 square inches. Since it's a liquid and not a gas you're pushing against, and you can't compress a liquid, it has no where to go but into the chamber. Theoretically that is. And remember, the pressure in the chamber is fluid, it's going to push against the area of least resistance. It will push against the fuel coming into the chamber through the holes but if there's something coming through those holes the expanding chamber gas would rather just move out of the way and hit another part of the chamber or go out the back.   

       But again, I can't totally defend a concept that I'm not sure would work. Checking around it's looking more and more like I should make a model to try out the idea and it doesn't necessarily need to be pyrotechnic. I might be able to make one with water and compressed air. Make a tube shaped water rocket that slides snugly within another tube, fill the second tube with colored water and have a one way valve, a piece of tape on a hole in the top of the rocket section. Pump it up. pull the plug and if it pushes against the colored water section enough to push the colored water from the tank section into the rocket section, viola! Proof of concept without the big government grant or blowing my face off.   

       Next weekend's project maybe. I'll have my 5 year old daughter help. She knows only slightly less about rocket science than me and she can draw daisies and smily faces on it. At least it'll look nice.
doctorremulac3, Sep 07 2008
  

       //for instance, consider your injection port to be just another throat to just another exhaust...//   

       But you're right if you put it that way, if the exhaust nozzle and the input holes were the same size the fluid wouldn't go in.
doctorremulac3, Sep 07 2008
  

       Wow, thank you for the "Liquid Fueled Rocket design" link MisterQED. Lots of good stuff.
doctorremulac3, Sep 07 2008
  

       Okay, per coincidence I've just been watching "From the Earth to the Moon" and "In the shadow of the Moon", because I had a free rapidshare account with which I could illegally download all these movies.   

       My English comprehension isn't that well, but I seem to remember the words 'gimbal' and 'stabilisation' very well. Many Apollo astronauts were nervous as hell when they felt the nozzles stabilize right after lift-off, knowing that the tower was only a few inches away.   

       As said, in flight "steering" (is that word used in this context?) would obviously be a huge challenge with this design.   

       Apart from that - brilliant concept! I don't know nothing about rockets, so I can only appreciate its marvellous simplicity.   

       You could be the man who gets more people interested in rockets, simply by making them more easy to understand... :-)   

       Finally - and please don't laugh if this sounds utterly foolish - maybe you have a solution to recovering the rocket engine for re-use: just cover it entirely with the material out of which heat shields are made. Then, once in orbit, you ask an astronaut to do an EVA to give the engine a good kick, so it re-enters the atmosphere and can be re-used. The kick should be rather hard. But with pencil and papers one can calculate how hard, etc.
django, Sep 09 2008
  

       A better idea might be to just make the nozzle out of some kind of ceramic material instead of metal and use the entire unit for the re-entry heat shielding. The thing is cheap enough so that would probably be a better use for it that trying to get it back down to re- use since it's just a chunk of material.   

       Heat shields use ablation, that is: eroding part of what their made out of during re-entry. Just flipping the vehicle around and burning off the remaining piston/nozzle assembly would be a great way to minimize weight. Dual use of one of the components.   

       By the way, I'll post the proof of concept test using a baking soda rocket in the coming days or weeks as I get time. I'll just have a rocket push into a tube full of colored water and see if the rocket ends up filled with colored water from the second chamber. If not, the design's a no go. Should be interesting one way or another.
doctorremulac3, Sep 09 2008
  

       Wow, can't wait to see the results of that experiment, doctorremulac3.   

       Interesting idea too to use the engine as the heat-shield for re-entry. But wouldn't you have to invert the nozzle to make it work? I mean, if you don't, you would have a cone trapping all the heat, instead of it bouncing off.
django, Sep 09 2008
  

       Put the model together but the friction of my home made piston is seems too great for a baking soda rocket to overcome. I had to push it pretty hard. I'm looking for an off the shelf piston / cylinder combo that has better tolerances that I can use. If I need more thrust I can make a model that hooks up to the garden hose or go pyrotechnic and buy a big model rocket engine that can give me about 4 pounds thrust, stick the whole engine in the "combustion chamber" and light it. The gas from the engine would push in both directions, but only come out one end and show if it would drive the water from the adjacent chamber into the combustion chamber. Kind of busy with other stuff so I'll get to this when I can.
doctorremulac3, Sep 09 2008
  

       I like this idea. It combines the disadvantages of solid rockets with the disadvantages of liquid rockets, and burns itself up to boot.
ldischler, Sep 10 2008
  

       [doctorremulac3] - You mean Don LaFontaine - see link
hippo, Sep 10 2008
  

       Yup. That's the guy. Mr: "In a world where..."
doctorremulac3, Sep 10 2008
  

       /The gas from the engine would push in both directions, but only come out one end/   

       In your video, skinny tubes lead from the bottom of the combustion chamber up, around, and into the reservoir above. Why would gases from combustion not travel through these tubes as well? Or maybe there are valves?   

       Maybe the proof of principle can be done in a halfassed manner that minimizes friction. Example reservoir is a plastic bag of colored water glued with a single glue blob inside a plastic 7-11 cup. Reservoir bag opening is taped around aquarium tube. Cup and bag are perched atop rocket, and tube run down the length of rocket, taped, then threaded up through nozzle.   

       Now when rocket fires, it is itself the piston, compressing the bag above it into the cup and pushing colored water out of the bag and down the tube.   

       Test 2: if you have an excess of baking soda in the rocket, putting vinegar in the bag should fuel the rocket and allow a farther flight than the same rocket with water or with tube blocked.
bungston, Sep 10 2008
  

       //In your video, skinny tubes lead from the bottom of the combustion chamber up, around, and into the reservoir above. Why would gases from combustion not travel through these tubes as well? Or maybe there are valves?//   

       That's a good question. The reason would be an un-compressible liquid is in the way. Since liquid can't compress, it's basically like the pressure in the combustion chamber is pushing against a solid surface. It's like the trick of hitting the top of a bottle full of water just right with your hand to bust the bottle. Check out the link.   

       I bought the second round of parts today for the proof of concept test I'll try to get to this weekend. It's a bike pump with a nice, smooth, low friction feel to it. Cost 9 bucks. I'll mount a clear combustion chamber mockup that will have 3 holes in it to the handle. The first hole will be the "nozzle" out of which the water will spray, the second hole will be attached to a garden hose to provide the pressure to the chamber and resultant thrust and the third hole will be for the tube leading from the bike pump output into the chamber.   

       I'll fill the bike pump with colored water instead of air, and turn the water on. Water will fill the chamber and squirt out the "nozzle" pushing the bike pump handle but the same amount of pressure will be pushing against the tube leading from the bike pump output to the chamber.   

       If the thrust moves the pump despite the fact that that pump is pushing into the "combustion chamber", the concept is sound. Not sure if that makes sense.   

       It's schematically the same as the thing in the video but just a little easier to make.   

       I'll post a video.
doctorremulac3, Sep 10 2008
  

       Do you anticipate the pump to move up into the air? The hose might be a deal killer. You could put it on a skateboard and move it horizontally but it would still have to drag the hose.
bungston, Sep 11 2008
  

       Naa, this is just to make sure the pressure in the combustion chamber mockup doesn't stifle the movement of the piston.   

       I'm figuring any pressure will do since, as the combustion chamber pressure increases so does the thrust but so too does the pressure against the piston on the other side of the equation.   

       My question is: if it does work, what kind of flow can I get from the piston area into the combustion chamber? How is the size of the tube from the piston chamber to the "combustion" chamber limited?   

       I think the next step would be to make a pyrotechnic desktop version maybe using kerosene and hydrogen peroxide or liquid oxygen. Unfortunately, there are no oxidizers that aren't incredibly nasty to deal with. We're talking haz- mat suits, license to buy, major undertaking. Fortunately I live in California which has the world's highest concentration of high powered rocketry clubs. I'm hoping to get some interest from that community to help build a prototype.   

       That is if step one works. If it doesn't, oh well. I'll have at least learned something and had some fun in the process.
doctorremulac3, Sep 11 2008
  

       Make a dry ice rocket with hot water in the reservoir. Powerful, safe.
bungston, Sep 11 2008
  

       Yea, I'm not crazy about burning my face off. I just wish there where some safe bi-propellant I could use.   

       Oh, I know! I could have one chamber with vinegar and the other with baking soda mixed with water! That would test all the aspects of the thing including the pistons and the mixing without getting a-blowed up.   

       I just had a pretty important job come up this weekend though. I need to start treating this more like a sideline hobby and less like an obsession. I'm starting to dream about this thing.
doctorremulac3, Sep 11 2008
  

       I am thinking about how to test whether the piston helps - gradual introduction of propellant vs all propellant in reaction chamber at once. Either the vinegar / bicarb rocket or the dry ice / hot water rocket could be used for this and would be easy enough to compare 2 compartment rock vs 1 compartment and consecutive fires.   

       It is hard to gauge visually how far up a rocket goes. My idea is to fire along a parabola in a field and measure distance from fire to landing. A twobyfour cross platform with a dowel at a fixed angle would provide support to ensure same angle of departure each time. One would have a ring on the side of the rocket and slide it over the dowel. Serial fires and multiple measurements would even out noise related to sloppy rocket control.
bungston, Sep 12 2008
  

       I hadn't thought of water and dry ice. Lots of potential energy in that dry ice that gets released pretty quickly when you hit it with hot water. I'd say that's probably the most powerful "fuel" I can buy readily and use safely without burning things.   

       I could loose the garden hose then. Just fill the first chamber with dry ice, give it a little push until the hot water from the second chamber starts squirting in and stand back.   

       Loosing the garden hose would give me a model that could move as well. I don't know about flying, pretty heavy with the pistons, but maybe on a skateboard or something for starters.   

       Thanks B, great idea!
doctorremulac3, Sep 12 2008
  

       Glad you like it. Look at dry ice rockets on youtube. Lots of them. They really fly. Also, in case you missed it, take a look at my earlier anno about how to use a compressible bag for the hot water instead of a piston - lighter than your bike pump.
bungston, Sep 12 2008
  

       Yea, I was checking them out. Good idea about the bag as well, less friction eh? Hopefully I'll have a little time to try some of these ideas this weekend. Results, positive or negative will be posted.
doctorremulac3, Sep 12 2008
  

       Pondering this idea from the safety of my armchair, it occurs to me that the principle here might be used to construct a staged dry ice rocket.   

       Consider the dry ice rocket. I suspect that most of the thrust is produced by pressure built up while the thing is sitting on the ground. Once airborne, I suspect that unused (here unsublimated) CO2 bits and hot water are expelled behind the rocket. I am not sure that adding more hot water at this stage will help, although it might if some sort of screen or fixings were put in the rocket.   

       However, a second stage would do well with the remulac piston. This would be a second rocket, identical to the first. Between the two rockets is the hot water reservoir. When the first rocket goes off, instead of pushing hot water from the reservoir into itself, the hot water is pushed up into the second stage.   

       Used as an airborne rocket, I can imagine the thing tumbling before the second stage goes off, then blasting down into the ground. I am sure that this problem is a possibility with any staged rocket. But the test of principle would be much better done on a skateboard, or a model boat.   

       The first stage should have chunks of dry ice, to slow the reaction and allow the rocketeers time to setit up and run. But the second could have granules or powder to speed the reaction.   

       The second stage is attached with a ring of glue occluding the nozzle (bottle neck). When the pressure in the second stage builds up, it blows off the first stage and the reservoir below it when it takes off.   

       In the skateboard example, the rocket should not be fixed to the skateboard. With one stage piston rocket, the reservoir only should be attached because the rocket must move relative to the board in order to push into the reservoir. With two stages the second stage rocket only should be attached for the above reason and also to facilitate blowing the first stage and reservoir backwards off the board.   

       Another science fair project (and in my mind, I have hopes that some high schoool student somewhere googles for "science fair" and finds the halfbakery!). What is better for propulsion: a punch or a push? Which goes farther: fire all your propellant at once, or sequenced? I predict sequenced but I am not sure - and certainly students who have come of age in the era of shock and awe might predict the punch to be better.
bungston, Sep 13 2008
  

       [doctor] over here, the weekend's almost over. It's Sunday night as I write this. I've bought some extra croissants this morning (literally), just so I can wait until late this night, with a coffee, to see the results of any trials you conducted this weekend.   

       I'm keeping them warm... :-)
django, Sep 14 2008
  

       A monofuel version might also work, e.g. hydrogen peroxide in a single cylinder and a catalyst in the combustion chamber. Not to discourage you from the full version.   

       A graphite combustion chamber and nozzle would reduce friction; graphite is a valid material for small rocket motors anyway.   

       It is true that most of the thrust is generated in the expansion part rather than in the combustion chamber, so in theory the pressure difference will be in the required direction. Not so with a non-pyrotechnic rocket, so don't be discouraged if these don't work.   

       One possible way to control the rate is to use a material with a known burn rate, such as epoxy/potassium nitrate, for the central tube, and have this butt up against a constriction in the hole at the top of the combustion chamber. As it burns away the combustion chamber is allowed to rise. In any case, an elegant part of your design is that this tube, in burning, becomes fuel!
spidermother, Sep 14 2008
  

       Ok, I promised to post.   

       Inconclusive but interesting. I'll post the video tomorrow.   

       First, the hole between the two chambers was too big and it didn't budge. I then made the hole smaller and a couple of times it made a nice smooth motion for a couple of seconds then locked up. Does it mean the concept works? I dunno. I wouldn't bet my life on it yet. Kind of a big leaky spraying mess frankly. But I was really excited, got the video camera and guess what? It didn't do the nice smooth motion again. Under the "cameras rolling curse", I got the thing moving reaaaallly slowly but I had to wiggle the whole thing to overcome the piston getting hung up I guess. Interestingly enough, pushing it as hard as I could wouldn't make it budge at all, not surprising since I think it's about 75 psi. But I swear, it did move twice really smoothy without my touching it.   

       An engineer friend of mine said I built the model wrong to demonstrate the idea. I think I'll try some different elements in the next model.   

       Long story short, I'm not calling the Smithsonian and telling them to clear a space next the Goddard exhibit for my wax likeness just yet. The only way I'm getting into the Smithsonian at this point is with a ticket.   

       Anyway, I've got a couple of ideas for a new model. More hi-jinks as they come.
doctorremulac3, Sep 14 2008
  

       //It is true that most of the thrust is generated in the expansion part rather than in the combustion chamber, so in theory the pressure difference will be in the required direction. Not so with a non-pyrotechnic rocket, so don't be discouraged if these don't work.//   

       You know? That's a good point! Thanks S! You made my weekend. No matter where this leads I really am having a lot of fun with this and I hope it's interesting to you guys as well.   

       More as it comes.
doctorremulac3, Sep 14 2008
  

       [Max] My fluid dynamics knowledge is elementary, but I think you might get a small positive feedback effect as thrust is directly proportional to fuel flow rate while fuel flow rate increases faster than linear as the Reynold's number increases and flow becomes non-laminar. This might be offset as the reaction in the combustion chamber becomes saturated, leading to a steady state, but I'm only guessing here.   

       As for fins, they could be mounted on rods running parallel to the rocket body, attached to the rocket near the top and to the fins at the bottom.   

       If nothing else, this idea might allow amateur rocketry to include liquid fuelled rockets without much more complexity or difficulty than current solid fuelled rockets. (+)   

       (edit) Oh, and this non-laminar effect is a stochastic process. Also, if all rates were linear, there would neither be positive nor negative feedback so the rate would be quasi-stable and unpredictable (stochastic). Happy, [Max]?
spidermother, Sep 14 2008
  

       I have my doubts, but I hope this works, it could be a real breakthrough. Perhaps the piston motion could be used to eject the payload into orbit.
Spacecoyote, Sep 16 2008
  

       My prediction of fuel flow rate was based on flow through a tube; flow through a hole is proportinal to the square root of pressure, so if you have constricted injector nozzles, there's your negative feedback and therefore stable and predictable burn rate.   

       Your throttling down effect after air resistance and weight decrease will be very small - the only difference will be the slightly greater proportion of thrust going into accelerating the piston itself. Otherwise the system does not 'know' whether it is reacting against gravitational weight, air resistance or inertia. In fact, greater acceleration will increase the pressure of the fuel and oxidiser at the bottom of their respective chambers and increase their flow rate. In any case, it's best (as I understand) for a rocket motor to operate at its maximum design thrust throughout.   

       I like the flow of propellant through the cooling jacket - two functions in one. Very neat. You even have it running counter to the flow of hot gases.   

       Not sure about the oxidiser tube's burning's adding mass without increasing the temperature of the exhaust. I thought it would add heat, and that that would be a good thing.   

       I like this idea a lot.
spidermother, Sep 16 2008
  

       Surely the reason why there is a favourable non-symmetry to the pressure equation is because one end of the rocket is open, and the other is closed?   

       Think of it this way:   

       Exploding gases push on engine assembly
Engine assembly pushes on contained fuel
Fuel pushes on forward bulkead of fuel tank
Rocket goes forward
  

       I'm assuming here that friction between the sliding engine assembly and the interior rocket body is insignificant. Perhaps it can slide up on rails to keep it aligned.   

       I have a picture in my head of tailfins that are cast as part of the engine assembly, and trail out a long way behind. If the expended shell sections can really burn away cleanly, then the fins can extend beyond the radius of the rocket body = if not, perhaps making them longer works, or using [rcarty]'s idea:   

       // segments of heat resistant material, and weld them together with an alloy with a lower melting point. //
BunsenHoneydew, Oct 07 2008
  

       What happened to that awesome video? If it has to do with national security, I understand.
bungston, Mar 24 2010
  

       I thought rockets were staged to eliminate the redundant weight of an empty fuel tank? But I'm not a rocket scientist so I don't really know.
simonj, Mar 25 2010
  
      
[annotate]
  


 

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