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Space Planes
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Marsman
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PostPosted: Mon Aug 13, 2007 12:28 pm 
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I have been reading up on the various space plane developments and came across this statement on the debate between rockets vs planes- "
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The problem with planes is that wings contain very little fuel, but they have a big surface area, so they're heavy. Their advantage is that they can generate lift in the atmosphere so that a launch vehicle uses much less propellant in the first part of its trajectory. But that isn't enough of an advantage to overcome their weight problem - so HTOL (Horizontal Take-Off and Landing) SSTO (Single Stage To Orbit) is impossible with existing or near-term materials and propulsion technology.


I then started looking through all the different ideas and designs and thought of a possible solution for the "wings are heavy" problem. Couldn't wings (and body) covered in a carbon nanotube skin and containing carbon nanotube framework, mixed with aerogel overcome the weight issue? If you check out the properties of both substances you will see what I mean.

Plus if need be couldn't they be retractible? Like some of the fighter jets we see? SSTO HTOL planes would cost big $$$ in development, but for what they can deliver to the space industry isn't the cost worth it? There are some great ideas out there and some reputable looking people working on it like these guys so what's the reason governments aren't willing to invest more in this?
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louis
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PostPosted: Mon Aug 13, 2007 4:51 pm 
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Marsman -

Retraction won't solve the weight problem I guess - but you might be right about using lighter materials.

I haven't looked into this but given your comments, presumably someone must have thought in terms of using a high altitude plane to get a rocket up to say 20KM and then have the rocket take of having been dropped off the plane. Presumably then you get the advantage of the air lift without the subsequent disadvantage of the weight.

One other thought - is there any role to played by high energy beams? As I understand it microwave beaming of energy from space is perfectly feasible. Could it have a role in powering a craft? I'm thinking here of maybe actually running the equivalent of a maglev system in space? I don't know enough of the subject to say what could get the craft moving. Would you be creating some sort of magnetic force in front of the craft - kind of like a donkey with a carrot? If anything like this was feasible presumably the great advantage would be that you weren't actually having to lift the rocket fuel or the big aircraft engine and wings - you woudl just be lifting the payload craft.

That said, for the Mars project I don't think we should wait on any of these developments. Rockets are proven technology.
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PostPosted: Mon Aug 13, 2007 8:06 pm 
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louis,

I don't think we should depend on any unproven technology either. I don't think anyone here is thinking that.


marsman/louis,

materials engineering is always progressing toward lighter and stronger composite materials. The problem with carbon nanotubes is simply that they are still too difficult to fabricate in bulk. If you look at the last figure on the last page of this article, you'll see what I mean. ($1500/gram) As the technology becomes available to make those materials, developments in spaceplane technology are sure to follow.

Retractable wings are an interesting topic. There is a balancing act to be had, though. For that to work the wings would retract as the vehicle nosed up into a near verticle climb to space, correct?

But to build a high altitude aircraft to get it part of the way up there, you need wings with a very high Aspect ratio which is exactly what causes the weight problems you have described. Thats exactly why the high altitude (and high aspect ratio winged) White Knight didn't follow SpaceShip1 up to space.

That being said, the spaceplane is a very promising technology, but I see it as the predecessor to the space elevator. We can develop it, but it will be a tough road.
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PostPosted: Mon Aug 13, 2007 9:14 pm 
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Marsman wrote:
I then started looking through all the different ideas and designs and thought of a possible solution for the "wings are heavy" problem. Couldn't wings (and body) covered in a carbon nanotube skin and containing carbon nanotube framework, mixed with aerogel overcome the weight issue? If you check out the properties of both substances you will see what I mean.


CNT composites will help a lot of things (there is a reason the company is called Scaled Composites), but the point that the original poster was making is that, just by geometry, the ratio of the volume enclosed by a wing to the surface area of the wing will always be less than the same ratio for a cylinder.

Wikipedia has a reasonable SSTO (and problems therewith) article ...

http: en wikipedia org wiki Single-stage-to-orbit

See also, Mr. Bell ...

The Cold Equations Of Spaceflight
http: www spacedaily com news oped-05zy html

Marsman wrote:
Plus if need be couldn't they be retractible?


This ability would likely add mass, rather than reduce it.

Marsman wrote:
SSTO HTOL planes would cost big $$$ in development, but for what they can deliver to the space industry isn't the cost worth it?


As you point out, there are lots of people pitching designs, so the answer to this question is "very nearly." I think investors are trying to work out if development cost estimates are realistic. "Did we say $20 billion? Oh, yes, well, it turns out we need $200 billion."

Marsman wrote:
what's the reason governments aren't willing to invest more in this?


Most governments just wait on the tech leader to burn no one knows how many billions in R&D, and then build version 2.0. It's actually a pretty good strategy if national security (or perhaps national pride) doesn't depend on being the first to build version 1.0.

louis wrote:
I haven't looked into this but given your comments, presumably someone must have thought in terms of using a high altitude plane to get a rocket up to say 20KM and then have the rocket take of having been dropped off the plane. Presumably then you get the advantage of the air lift without the subsequent disadvantage of the weight.


This is basically what SpaceShipOne did, and is the basis for a lot of TSTO designs.

louis wrote:
One other thought - is there any role to played by high energy beams? As I understand it microwave beaming of energy from space is perfectly feasible. Could it have a role in powering a craft?


Microwaves aren't energy dense enough, but laser launch has been examined as a low cost launch method. Unfortunately, the g-forces during laser launch are too high for people.

louis wrote:
Rockets are proven technology.


This is an issue for CNT composites. Aluminum and titanium alloys are now well-characterized for aerospace purposes, but it cost a lot, including some people's lives, to get to that point. It'll be a while (decades) before engineers trust CNT composites enough to use them for mainstream purposes.
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PostPosted: Mon Aug 13, 2007 9:41 pm 
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No doubt this will be shot down in flames -

But then you say "not energy dense enough" - surely that can only be for one beam. What if you have got 100 beams coming together - aren't you getting something similar to the solar furnace effect?
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PostPosted: Mon Aug 13, 2007 11:13 pm 
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Maybe Jeff Bell should be in charge of this issue? He makes some good points.

Here is a picture of a fully reusable TSTO which could solve the problems I'm reading about-

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Spacebus a design for a 2-stage passenger-carrying HTOL by the company Bristol Spaceplanes in Britain. The first stage uses jet engines for take-off, followed by rocket engines to climb to high altitude for separation, after which the upper stage uses rocket engines to reach orbit. This configuration has a number of advantages described at length in papers by David Ashford (including the recent "Space Tourism - How Soon Will it Happen?" and the 1990 book "Your Spaceflight Manual: How you could be a tourist in space within 20 years"). Using existing jet and rocket engines is very attractive in reducing initial development costs. And though new air-breathing rocket engines might be more efficient, they're not needed initially (the first stage just has to be a bit bigger and use a bit more fuel). And going high with the first stage makes separation easy as it's in thin air, so even hypersonic loads are small.

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PostPosted: Tue Aug 14, 2007 1:03 am 
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louis wrote:
But then you say "not energy dense enough" - surely that can only be for one beam. What if you have got 100 beams coming together - aren't you getting something similar to the solar furnace effect?


Yes ... but then you need to make 100 beams, so your launch costs are 100 times greater.

Marsman wrote:
Bristol Spaceplanes Spacebus


Yes, the TSTO designs are much more sensible.

My current favorite is ...

Andrews Space Gryphon Aerospaceplane
http: www andrews-space com content-main php?subsection=MTA5

... because the first stage doesn't go hypersonic, which avoids all sorts of issues and development costs.
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PostPosted: Tue Aug 14, 2007 1:50 am 
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My current favorite is ...

Andrews Space Gryphon Aerospaceplane
http: www andrews-space com content-main php?subsection=MTA5

... because the first stage doesn't go hypersonic, which avoids all sorts of issues and development costs.


Very cool looking. I'm wondering also that sleekness of shape might be important to the design but it will be good to see this sort of thing become a reality, I wonder how likely it is though?
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PostPosted: Tue Aug 14, 2007 2:20 am 
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Marsman wrote:
I'm wondering also that sleekness of shape might be important to the design


The faster the first stage has to go, the sleeker it has to be to reduce drag. But that's also a clue to development costs - the sleeker it is, the less like existing airplanes it is. Hypersonics ( > Mach 6 ) is not a walk in the park.

Marsman wrote:
how likely it is though?


Well DARPA gave Andrews Space $1 million to pursue it's design ...
http: www hobbyspace com nucleus index php?itemid=1264
... so there is some interest there.

As to TSTO RLVs in general ... I think they are very likely, but as to timing? I'd say no earlier than 2030, and probably closer to 2040.
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PostPosted: Tue Aug 14, 2007 3:59 am 
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But then you say "not energy dense enough" - surely that can only be for one beam. What if you have got 100 beams coming together - aren't you getting something similar to the solar furnace effect?


This won't really work. With one beaming source, like a high powered laser, you can point the beam directly up on the spacecraft as it ascends. With multiple beams, you must beam from multiple sides. That means that a great deal of power is lost as directional kinetic energy vectors cancel. This also means a devil of a time keeping those beams aligned on the target when it's a hundred miles away and traveling mach thirty.

Also, for any given wavelength size it is impossible to decrease the size of the rectenna without huge power losses. Attempts to focus multiple phased arrays for a smaller focal point meet with what's called the thinned array curse, which states that the power density of the smaller point will ultimately be the same as if you used a full sized rectenna. The same power density in a smaller receiving point simply means less power.

So, even if you increased the intensity of the beam with a very high powered microwave (and I think this is possible), you still can't get around the fact that a beam which can pierce atmospheric disturbances and transmit as far as space will need a rectenna several kilometers wide. Whether you can transmit a great deal of power to this giant receiving end, with or without a great profusion of different microwave beam devices, is somewhat beside the point. A spaceship several kilometers wide is just impractical.
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PostPosted: Tue Aug 14, 2007 8:41 am 
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A spaceship several kilometers wide is just impractical.
Au contrare--it's the only way to go. Nothing else works.
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PostPosted: Tue Aug 14, 2007 10:07 am 
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Perhaps in space, but as a launch vehicle?
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PostPosted: Tue Aug 14, 2007 2:56 pm 
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Not sure if I follow what you are saying Mirari. You seem to be thinking in terms of a ground station.

I was actually think of microwave beams from orbital solar power stations - and possibly only coming into play after a space plane has delivered the craft to 20km. Couldn't multiple beams be focussed from one parabolic reflector?

I was thinking more in terms of some sort of magnet propulsion - a double magnet ring: one ring extends and then attracts the craft upward till it meets the ring; meanwhile the other magnet ring is extending in similar fashion ready to attract the craft further along - and so on. No idea if that is practical, but obviously maglev is a well established form of transport and one is just trying to find a similar principle that works with .

One point - I am not thinking in terms of a large craft, more of transporting lots of little craft vehicles. The magnetic levitation system might just be like a framework for containing tubes of different varieties. There would be a space station style grid in orbit where one would assemble the Mars or Lunar lander from the various tubes. We might be talking about tubes with a mass of say 500kgs. If the system worked well, there would be no problem with lifting these up at a rate of maybe say 5 an hour. So we could raise 30 tonnes in half a day.
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PostPosted: Tue Aug 14, 2007 6:58 pm 
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louis wrote:
I was thinking more in terms of some sort of magnet propulsion - a double magnet ring: one ring extends and then attracts the craft upward till it meets the ring; meanwhile the other magnet ring is extending in similar fashion ready to attract the craft further along - and so on. No idea if that is practical, but obviously maglev is a well established form of transport and one is just trying to find a similar principle that works with .


Maglev works by one magnet pushing against another that is securely attached to the Earth. In mid air, the magnet would be attracted to the craft, unless it were more massive than the craft, in which case the craft would be pushed backwards when the magnet was "extended." There would be no net forward motion in either case.
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PostPosted: Tue Aug 14, 2007 8:40 pm 
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Thanks Noos.

Doesn't it make a difference if there are two magnets - one say like a disc, the other like a life saver around the disc and they extend at different times, in rythmn?

I don't understand the point about the mass. I thought if the electricity input was powerful enough to drive a powerful motor that the magnetised disc or ring would be powerful enough to pull things with a larger mass.

Just to be clear:

My idea is:

Orbital microwave power station, using solar energy.

Station beams energy on to parabolic collector or whatever is appropriate to concentrate energy attached to the front end of the contraption.

Collected energy then drives motors which magnetise and de-magnetise as appropriate a disc and a ring in front of the craft secured on extendable rods. In turn the disc and ring are extended, and then magnetised, which causes the craft to move towards the magnet, at which point the next phase is taking place with an extension of the ring/disc as the case may be, which is then in turn magnetised, so attracting the craft forward.

The contraption is all one piece with the rods extending foward on to which the ring and disc sit. There is a central pole that passes through the disc and to which is attached the reflector and magnetising motors.
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