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SS1 Rocket Fuel

Posted by: skyhigh - Thu Nov 25, 2004 4:24 pm
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SS1 Rocket Fuel 
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Post SS1 Rocket Fuel   Posted on: Thu Nov 25, 2004 4:24 pm
I remember hearing that Space Ship One used effectively a combination of rubber and laughing gas for its rocket fuel. They were looking for a very inexpensive fuel (apparently its old technology). Anyone have any details on this? How efficient is it and how much cheaper is it compared to other options? Its probably a gross simplification to say its laughing gas and rubber, but how much of a simplification is this? Just curious.


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Post Re: SS1 Rocket Fuel   Posted on: Fri Nov 26, 2004 5:59 pm
skyhigh wrote:
I remember hearing that Space Ship One used effectively a combination of rubber and laughing gas for its rocket fuel. They were looking for a very inexpensive fuel (apparently its old technology). Anyone have any details on this? How efficient is it and how much cheaper is it compared to other options? Its probably a gross simplification to say its laughing gas and rubber, but how much of a simplification is this? Just curious.


Nope, it's not an oversimplification at all, it really does run on laughing gas and rubber. And it's not really old technology, it was tried and tested back in the 50's, but I don't think a large scale motor was ever flown until about 10 years ago.

http://en.wikipedia.org/wiki/Hybrid_rocket covers a lot of information, although here is a quick summary and some things it missed:

- Hybrid engines can run on either N20 or LOX, the latter being more efficient but more dangerous also. SpaceShipOne used N20, which is laughing gas, as you find it in a dentists.
- Hybrid motors generally have an ISP around 230 - above solid rockets, but below liquid rockets. The monoprops used by Armadillo are probably in a similar sort of category, although I get the impression monoprops are more difficult to produce.
- They are very cheap to make, and very safe, which is their main selling point. Unfortunately, their lack of efficiency means that you will never be able to have a single-stage-to-orbit rocket powered by such a motor, it's simply totally impracticle.
- For fuel they generally either use polyethene (ie, plastic) or HTPB, which is the primary constituent of tyre rubber.

Anything else?

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Post Thanks   Posted on: Sun Nov 28, 2004 5:09 pm
Thank you very much for the reply.

So here is my next question. You wrote:

" They are very cheap to make, and very safe, which is their main selling point. Unfortunately, their lack of efficiency means that you will never be able to have a single-stage-to-orbit rocket powered by such a motor, it's simply totally impracticle."

First, how cheap is cheap. I've heard it costs NASA about $10,000 to put one pound into space. Of course they go further into space than does SS1. It seems to me once your fixed cost is a sunk cost (ie. making the rocket ship) you then only have to cover your variable costs which would mostly be the cost of fuel as well as servicing the rocket.

The second question I have concerns how much further SpaceShipOne can get into space. I'm sure I'm oversimplifying this, but I can't see how. I don't know the specs of SS1 or White Knight. But we know that SS1 can reach an altitude of 65 miles above the earth to the point that they are now weightless. I would think the hard part (or a major hard part) would more or less have been accomplished. Next, if they built a White Knight that was bigger (say twice as big), then built SpaceShipTwo that had the same seating, but twice the rocket fuel, I would think they could get very far into space. So I would think that if you had a rocket ship (say SS2) that was twice as big, you should be able to get atleast 130 miles into space (2(X)65)? I say atleast 130 miles, because I would imagine there must be efficiency once you enter zero gravity. I would think once you break earth's gravity, that the next use of fuel would propel much further. I would guess they could go several hundred miles into space. Am I wrong in this regard? And if I'm not wrong, then isn't it only a matter of time before the Bigelow prize is won? I mean for $50 million prize money, plus the $10 million they already won from X-Prize, I would think they'd just have to double the size of their machinery.


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Post A quick calculation   Posted on: Sun Nov 28, 2004 8:42 pm
Just thinking about this abit. They say they are thinking of offering rides into space on SS1 for between $100K - $200K. For the sake of argument, lets say it costs $200,000 and lets use an average weight for a passenger of 150 pounds. This would mean SS1 is getting 1 pound into space for about $1300. Of course they aren't going as far as NASA does but there is a profit provision in that price.

Another thing I was thinking. I wonder if they would even need to spend more money building another larger ship. Would it be feasible to create a "strap-on" second stage rocket to SS1. White Knight would take it up. Then drop, then the second stage would launch. After it got up high enough it would detach, and then SS1 would take over for another push. I suppose it would be important to make this second stage be reusable, with its own gliding wings, or perhaps a parachute.


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Post    Posted on: Sun Nov 28, 2004 8:58 pm
I dont think its a question of just increasing engine size or strapping on additional rockets, its got to come back again.

To get SS1 to orbit would mean that it has to travel at something like 17,000mph and the main problem is loosing that velocity in the upper atmosphere when it comes back to earth. The reentry temperature would melt SS1 and the stresses would probably rip it apart. Feathering the wing would probably reduce SS1 to a pile of junk.

By the time you've added a heat shield, reinforced the structure and beefed up the engine to propell all that weight and extra propellant you end up with something that bears very little resemblance to SS1. Its prbably easier to start with a new design.

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Post    Posted on: Sun Nov 28, 2004 10:04 pm
Andy Hill's right-- the possible SS3 (orbital) designs that we saw on black sky really looked nothing like SS1, though they used a similar principal. similarly, even the possible SS2 had some major changes- like a second motor- and it's only designed to carry two more people maybe 10 miles further into space.

SS1's motor may be rubber and laughing gas, but it's still one of the more expensive engines proposed by an xprize team- comes in a bit over $100,000 if i remember right, and it's not reusable. it's more efficient than armadillo's mixed monoprop- 200 something isp as opposed to 180- and i think it's a little safer, but it costs at least one order of magnitude more to run for a single flight. the reason it's this expensive is because the "rubber" has to be made to a very specific grain for it to work perfectly, said grain is a trade secret of spacedev. i imagine that you can use alot of different types of rubber for a hybrid motor, but i also imagine that melting down a few tires, putting that rubber in a motor mold, and running N2O through the resulting solid would not make a valid engine.

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Post Re: Thanks   Posted on: Mon Nov 29, 2004 10:03 am
skyhigh wrote:
I would think once you break earth's gravity, that the next use of fuel would propel much further. I would guess they could go several hundred miles into space. Am I wrong in this regard?

Yes, you are. At 100km, you have not "broken" earth's gravity at all. Not by a longshot. In principle, you won't ever break free of earts gravity, no matter what distance you travel from it, but even in practice, the "limit" lays much further away. See the escape velocity thread for a more complete explanation.

The fact that you experience weightlessness is due to the fact that you are in free fall from the moment the engines are turned off to the moment the craft starts to decellerate in the atmosphere. Weightlessness is nothing more than the sensation that you and everything around you are accelerated by the earth in the same way. That means that you can float free with respect to your surroundings (SS1, the shuttle, ISS, the vomit comet), but it does not mean that you are not affected by gravity.

edit: What you have 'broken' is another important thing: the atmosphere. At 100km, you have left most of it, meaning you won't be hindered by air resistance anymore.


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