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Piston Rocket
No pumps, stages or multiple motors. Might achieve single stage to orbit. | |
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.
(???) 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 08 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 11 2008]
[link]
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Very impressive concept. Good video and narration, too. |
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It might need some engineering and experimenting on seals and sizes, but it is very simple in principle. Croissant for elegant idea! |
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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. |
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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. |
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How did you get Lee Marvin to do the narration? |
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//How did you get Lee Marvin to do the narration?//
I second that. |
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Did you make that video, doc? It is very well done. |
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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. |
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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. |
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On the downside, not reusable. |
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[+] for what I think is a new
take on the idea, but... |
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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. |
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The outer shell may burn away
unevenly, causing aerodynamic
instabilities. |
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//Is reaction force going to create
enough force to keep the fuel pressures
high enough to be in their ideal
injection pressures?// |
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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. |
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//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". |
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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. |
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But like all new rocket designs, I
wouldn't wanna be standing next to the
prototype on it's first static test. |
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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. |
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Good idea and very impressive video.
Can I get you to dub my actual life? |
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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? |
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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? |
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I'd also like to use the word "stochastic",
but can't think of a suitable
opportunity. |
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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. |
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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.) |
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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. |
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[+] 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. |
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Also imagine shooting this, bottle-rocket style, down a street. Everytime it would strike something, it would be rewarded with a burst of energy. |
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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. . . |
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I can see some other problems:
Ratio control.
Starting.
Stopping. |
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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. |
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Stopping shouldn't be a problem, finally!
But starting would need some special allowance. |
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+ for the video, don't have any expertise to comment further. |
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I really like the creativity and concept, but I may have found a flaw. Maybe. Let's see if I can write it out. |
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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. |
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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. |
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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? |
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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. |
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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? |
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That's as far as I can crunch that tonight. Sorry. |
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[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... ? |
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A-ha! They call them pulse jets! Just add a little valve. See link. |
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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. |
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I'm pretty far out of my league here though... |
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//The nozzle may affect things, but I'm guessing that the pressure against the top of the chamber is what does the work.// |
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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. |
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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. |
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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. |
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Anyway that's my understanding. I'll try to find that link and put it up. |
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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.) |
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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. |
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The other problem is stability. As discussed before somewhere here, there are 3 basic ways to stabilize a rocket: |
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1. Fins. Forget it. They have to be on the bottom, and we just burned that off. |
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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. |
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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. |
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RCS system on the nose, all the way... your RSO will just keep repeating "Hell, no!" until you shoot him. |
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//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.// |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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). |
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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. |
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[edit: for instance, consider your injection port to be just another throat to just another exhaust... ] |
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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. |
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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. |
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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. |
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//for instance, consider your injection port
to be just another throat to just another
exhaust...// |
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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. |
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Wow, thank you for the "Liquid Fueled Rocket design" link MisterQED. Lots of good stuff. |
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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. |
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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. |
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As said, in flight "steering" (is that word used in this context?) would obviously be a huge challenge with this design. |
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Apart from that - brilliant concept! I don't know nothing about rockets, so I can only appreciate its marvellous simplicity. |
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You could be the man who gets more people interested in rockets, simply by making them more easy to understand... :-) |
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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. |
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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. |
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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. |
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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. |
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Wow, can't wait to see the results of that experiment, doctorremulac3. |
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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. |
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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. |
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I like this idea. It combines the disadvantages of solid rockets with the disadvantages of liquid rockets, and burns itself up to boot. |
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[doctorremulac3] - You mean Don LaFontaine - see link |
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Yup. That's the guy. Mr: "In a world
where..." |
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/The gas from the engine would push in both directions, but only come out one end/ |
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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? |
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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. |
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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. |
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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. |
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//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?// |
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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. |
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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. |
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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. |
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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. |
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It's schematically the same as the thing
in the video but just a little easier to
make. |
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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. |
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Naa, this is just to make sure the
pressure in the combustion chamber
mockup doesn't stifle the movement of
the piston. |
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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. |
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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? |
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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. |
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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. |
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Make a dry ice rocket with hot water in the reservoir. Powerful, safe. |
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Yea, I'm not crazy about burning my
face off. I just wish there where some
safe bi-propellant I could use. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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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. |
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[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. |
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I'm keeping them warm... :-) |
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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. |
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A graphite combustion chamber and nozzle would reduce friction; graphite is a valid material for small rocket motors anyway. |
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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. |
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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! |
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Inconclusive but interesting. I'll post the
video tomorrow. |
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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. |
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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. |
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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. |
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Anyway, I've got a couple of ideas for a
new model. More hi-jinks as they come. |
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//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.// |
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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. |
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[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. |
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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. |
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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. (+) |
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(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]? |
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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. |
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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. |
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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. |
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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. |
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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? |
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Exploding gases push on engine assembly
Engine assembly pushes on contained fuel
Fuel pushes on forward bulkead of fuel tank
Rocket goes forward
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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. |
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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: |
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// segments of heat resistant material, and weld them together with an alloy with a lower melting point. // |
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What happened to that awesome video? If it has to do with national security, I understand. |
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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. |
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