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Crazy Idea

Posted by: Texan - Sun Jun 27, 2004 9:28 pm
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Post    Posted on: Mon Jun 28, 2004 2:41 pm
Texan wrote:
Sorry Dr Keith... I knew their was a chance you were not swiss... but I felt like saying swiss. :oops: Where are you from?

Australia, but I have spent less than half my life there, so the idea of drawing conclusions about the nature of people based on where I come from is probably not the best way of going about it. However, I understand that sometimes it's all you've got to go on. Perhaps if you had said "Is Dr Keith always so rude?" then we wouldn't have had much to argue about (well, a bit). :wink:

Texan wrote:
Is sending installable heat shields into orbit such a bad idea, just stack them in the cargo bay of one spaceship instead of other cargo, then put them on your other spaceships before re-entry.

It just seems overly complicated to use a (disposable?) launch vehicle to run up a bunch of (disposable?) heat shields in anticipation of using them on orbital RLVs. Not to mention an additionally dangerous procedure for the RLV crew (you have to dock with your cargo boat, get out the shield, install it correctly, with noone around to say "oh look, you forgot to screw that bit down" and make sure you don't flame yourself on re-entry).

The best option in the short term is to make better and lighter heat-shields while you still need them ... and have them pre-installed on the ground where they can be checked out for safety ad nauseum before you launch.

P.S. in that "other" thread you mention ... it's not so much a flame war as a one-sided and light toasting ... it's just Franklin (in his current incarnation as greenmonster - apt choice of name if you ask me), he has been kicked off here one or two times, you get used to him eventually. I have truly flamed him in the past, but since Sigurd informed me of his rules I restrict myself to the "low" setting as much as possible.

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Post Thermal Protection & Mass Ratio   Posted on: Mon Jun 28, 2004 3:32 pm
Hey Dr. Keith -- *I* didn't think you were being rude -- and thanks for the kind words about my Dad's website. He's a fairly nifty character.

Anyhow, I guess the shuttle *did* kinda spill my drink, in that it kinda ruined most people's perception of space travel -- making it seem like something which was both hideously complex and psychotically boring. Quite an accomplishment, I suppose, but not one that I appreciate.

Anyhow, on the "mass ratio" thing: If you're going orbital, you need a lot of fuel. In order to carry that fuel, you need more fuel and larger tanks to hold it. And in order to carry *that* fuel... et cetera. It's a nightmarish recursive problem. For a pure LOX-H2 SSTO system, every pound of fuel that you need towards the end of your burn will end up requiring about 100 pounds of additional fuel throughout the total system. For propellant combinations with a lower energy-density, the problem is even worse. Staging can alleviate it to a degree, but the bottom line is that your vehicle is going to mostly fuel.

The term "mass ratio" refers to the ratio of fuel to structure. A 95% mass ratio is pretty typical (for something like an Atlas booster), whichac means that the vehicle is only a few percentage points away from having the structural integrity of a soda can (around 98.5%). Actually, the soda can analogy is apt, because the mass ratio needs to include not only the tankage, but the engines, avionics, airframe, landing gear, heat shield, payload, et cetera. When you're designing an orbital vehicle, you have every motivation to make these things as light as possible, so that you don't end up with tanks with the structural integrity of tissue paper. In most cases, you'll end up with roughly a roughly soda-can structure. Now, imagine what would happen if you tried to fly a soda can at mach 7 or 8 (which is where most vehicles hit their maximum dynamic pressure). If you guessed that it would get squashed like, yes, a soda can, then you're exactly right. Getting such a fragile structure to *not* squash like a soda can is the primary problem of engineering an orbital vehicle.

Now, if you're not coming back from orbit, then you don't need land gear or a heat shield, which means that you can use their weight for stronger tanks, more payload, or whatever. It's definitely an easier problem, but that's not because *coming back* is difficult -- it's because coming back requires more system *weight*, and that screws up your mass ratio.

Just for comparison, an X-prize time vehicle can be quite successful with a mass ratio of 60% -- which, in comparison to a mass ratio of 95%, isn't much of an engineering challenge at all.

A final note on re-entry: there are three things which *can* make it quite difficult, all of which the Shuttle did. One is to have a non-aerodynamically-stable shape. The shuttle required continual adjustment from its RCS thrusters in order to maintain the correct attitude; on a passive re-entry it would have flipped around like an airborne mattress. Compare this to stable shapes ala Apollo or Soyuz, and it's obviously more difficult than it needs to be. The second thing is that the maximum heating is a function, if I remember correctly, of the cube of the radius of the re-entry surface. This means that blunt shapes have a relatively low maximum temperature (allowing you to use those oak planks as your heat shield), while sharp edges will get very hot indeed (requiring you to use exotic materials which will cost you millions and shatter when you spit foam at them). The third thing is the ratio of surface area to weight; in this case, a high mass ratio is actually your friend (up to the point where it gets you squashed). This is because the more surface area per weight, the *sooner* your spacecraft will decelerate. If you use a lifting-body shape, then you can prolong your deceleration at high altitudes, reducing the overall dynamic pressure, giving yourself a longer time to dissipate the total heat flux, and reducing the maximum heating. A dense re-entry vehicle, in contrast, will plunge through the atmosphere like a brick, decelerating quickly and getting REALLY hot it the process. Which is exactly what the shuttle did.

Hope this answers some of your questions. I don't usually make blind assertions like in my last post, but I was pinched for time then.


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Post    Posted on: Mon Jun 28, 2004 4:34 pm
Wow. Thanks very much for a detailed and well-written response. I am very glad that you were able to spend the time on it that you did. It certainly highlighted a few things for me, particularly the differences between engineering challenges associated with simply going higher and those associated with the significantly different goal of achieving orbit.

Once again, thanks very much!

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Post    Posted on: Mon Jun 28, 2004 4:56 pm
Thanks as well, I am learning alot.

Dr keith wrote

"It just seems overly complicated to use a (disposable?) launch vehicle to run up a bunch of (disposable?) heat shields in anticipation of using them on orbital RLVs. Not to mention an additionally dangerous procedure for the RLV crew (you have to dock with your cargo boat, get out the shield, install it correctly, with noone around to say "oh look, you forgot to screw that bit down" and make sure you don't flame yourself on re-entry)."

I don't know if the heat shields should be disposable or reusable.

The Space ship that brings them up should not by disposable, it should either serve as a (part of a?) space station and hold the heatshields itself, or it should be a more expensive heavy lifting vehicle that carries its own heat shield-Well, with the heat shields in orbit it might not need to bring its own up the next time.

Either way it would allow other ships to bring up more cargo on their next fights.

Btw maybe it should bring up fuel for slowing down the craft before reentry instead of heatshields?

Btw I have nothing against the Swiss. Or Australians :)


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Post    Posted on: Wed Jun 30, 2004 12:51 pm
My first idea was just as dumb as the topic headline made it appear....


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Post    Posted on: Wed Jun 30, 2004 12:52 pm
Not that I did not learn anything in this thread, I did.

This site has been useful for "downloading" knowledge.


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Post    Posted on: Wed Jun 30, 2004 12:56 pm
Yeah, someone once called it Passive Research. How much more is learnt from the telly and the internet. School was where we learn how to learn, and of course "girls".

:roll:

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Post    Posted on: Wed Jun 30, 2004 1:39 pm
Texan wrote:
This site has been useful for "downloading" knowledge.

I think you mean speculation ... not knowledge (well a bit, but the knowledge to speculation ratio here is microscopic) ... and some of the speculation is pretty wild. But hey! It's all good. :lol:

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Post    Posted on: Wed Jun 30, 2004 3:10 pm
That was fascinating. Dr Keith skilfully used his Aussie traits to annoy Skybum and then used the diplomatic skills of the Swiss to goad knowledge out of his head! :lol: A highly successful manoevere which led to a very informative post. Well done all. And I think I have just gone sub-orbital too, woo hoo! :D :D :P

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Post    Posted on: Wed Jun 30, 2004 4:50 pm
And thus the interest in Beamed Energy Propulsion...


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Post    Posted on: Thu Jul 01, 2004 1:09 am
My "crazy idea" has to be one of the stupidest ideas ever posted on this site.

:oops:


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Post    Posted on: Thu Jul 01, 2004 1:40 am
Texan wrote:
My "crazy idea" has to be one of the stupidest ideas ever posted on this site.

:oops:


nothing stupid about it.. you thought about space.., development, the future, ... most people even don't... and maybe you should consider your idea as "in development" ;) most ideas in "development" are not possible :D that's why they test and change some parts :)

You posted it, other people who know more about it, tested it.. and now you can modify it ;)

Maybe when you're older and learned a lot more about space, you can "litterly" invent something that all our lives will change..

I'm not really a space guru myself, so I guess I only know maybe a little more as you do... but with programming code.. I remember very well what 'funny' things I asked people.. and now I'm a game developer with an own company...so just never give up ;)

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Post    Posted on: Thu Jul 01, 2004 2:14 am
Thanks


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Post    Posted on: Thu Jul 01, 2004 10:43 am
May be this could be a good way to deliver fuel needed for deceleration as preparation to reentry or for changing between orbits.

To get to orbit this way first the distances between the ends of the trajectories of suborbital flights should made much wider and the ascents should be made by a smaller angle. Before this could be done a significant increase of altitude is required.

Provided the wider distances, smaller angles and increased altitudes the period of time the crafts are falling back by low velocity wil be longer and the chances to get to orbit this way might be better.

But the crafts have to be given best navigation abilities.

Dipl.-Volkswirt (bdvb) Augustin (Political Economist)


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Post    Posted on: Tue Aug 03, 2004 3:31 pm
TerraMrs wrote:
the real reason orbit is hard is reentry heating. the real question will be whether or not it's cheaper to do a powered deceleration before reentering, or to use one of the insanely expensive heat-resistant materials currently used to survive reentry. personally i think the former is ultimately the answer, but that's just a guess.

"Powered Deceleration"? Tell me more, I thought we had no choice but to enter the atmosphere at 18,000mph?

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