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Disclaimer: I've never worked in the space industry. I don't really know much about space travel apart from my musings in the internet and what I've picked up as an engineer. I haven't spent a lot of time refining this idea, and any input may well give me cause to alter it to make it more feasible or
entertaining.
So, you know SpaceShipOne that made history in 2004 by being the first privately funded space craft to make it to space? They decreased the size needed for a traditional fuel/oxidant mix rocket (lots more weight / volume) by having a carrier craft with air-oxygenated turbines gains the first 14km and then the rocket powered plane went the other 86km. The craft didn't achieve stable orbit because as well as the power required to get 100km up, it needs the energy to fly fast enough round the earth to counteract the earth's centripetal gravitational force.
Bring in the Space Kite: The SpaceShip remains attached to the supersonic carrier craft by a strong tether, allowing it to effectively utilise the carrier craft's engines as well as it's own rocket. I imagine that it will still need a rocket as with the atmosphere being less existent wings won't help an awful lot - but it may help up to about 120km. I'm trying to work out how it can avoid just skipping along behind the carrier whilst still allowing the carrier to contribute. Any ideas? By getting help from the tether to reach a higher altitude, the rocket energy can be used to achieve orbit velocity instead, keeping the whole edifice a lot smaller.
When the spacecraft reaches the maximum height such that it can no longer usefully benefit from the aircraft, it detaches and uses its own rocket to reach stable orbit velocity and height, much like an infant's umbilical is discarded after birth. By getting help to get higher and faster it needs less fuel and therefore size to achieve stable orbit!
ALTERNATIVELY:
Attach the space kite and tether to a blimp until it reaches 14km, at which point the aircraft, having made its own way up to that height, picks up the end of the tether. The huge structure required to spool the tether has it's own mini-blimp that is aerodynamically shaped and is towed some 50m or so behind the air craft. This tether is quite strong, has lifting surfaces attached periodically, and is hollow so that the cooled outlet from an ancillary jet engine on the mini-blimp can be passed up to the space craft and used as propulsion. The pressure in the tether caused by the air flow will ensure that it is kept rigid and does not sag. Once the air craft / mini-blimp / tether / space craft combination has reached optimum height(s), a trebuchet attached to the top of the tether flings the space craft that little bit higher, after which it's own tiny little rocket will accelerate it to the required orbital velocity. The rejected trebuchet dispatches a parachute to control the descent and re-spooling of the tether which is now neither attached to the air craft or the space craft. The air craft flies around for a bit trying to find where the tether spool mini-blimp landed, whilst the blimp is landed for re-use. Simple!
Stacked Flexifoils
http://www.kitepower.com/stacking.html
A sort of self-lifting tether [wagster, Aug 04 2009]
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The idea seems to be in the third paragraph, but after the first sentence you've lost the kite. And both spacecraft seem to be powered but attached by a tether. I don't get why this is supposed to do anything. |
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Let me see if I get it. The carrier craft is like a kid running around on a meadow, trying to fly a kite. The space ship is like a kite. A real kite goes up because that's the easiest way to go under the forward force of the running kid and the air hitting its shape; the space ship would go up and around the earth, up because of its rockets, and around because it has to follow the carrier craft to which it is tethered. |
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Transmitting the forward momentum of the carrier craft upwards through a tether is a good deal because that way you don't have to fly up the fuel (just the tether), and because the rocket is easier to pull forward through the thinner atmosphere. |
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I guess the question is, does anyone have 86km of extremely strong, extremely light twine lying around? |
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Ldisch - I think the idea is that the aircraft acts like a tug-
plane and the space-ship proper behaves like a glider being
towed (or, if you prefer, like a kite). It makes sense in
that the atmospheric craft (which is cheap and easy)
provides the propulsion, or much of it, for the spacecraft,
via the tether.
[TLAOB]
It sounds like quite a good idea, but I'm worried about a
150km tether being dragged at high speeds through the
upper atmosphere (even though the air will be very thin). |
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Presumably, the "tug" will help get the spacecraft up to
altitude, but not up to the 8km/s velocity it needs to
maintain orbit - are you envisaging the spacecraft's own
engines providing that velocity? And how fast will the tug
itself go? |
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// the space ship would go up and around the earth, up
because of its rockets, and around because it has to follow
the carrier craft to which it is tethered.// |
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[jutta] No, I don't think that's what's meant. The analogy
with a pulled-along kite or a towed glider is closer; the
uptitude comes from (or mainly from) the "kite" effect;
the forward velocity is only really a means of providing lift,
and isn't a significant contributor to the orbital velocity
which will eventually be needed. |
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(Even at 100km, there's enough atmosphere to give kiteoid
lift at high enough speeds; that's why re-entry is so hard.) |
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//Disclaimer: I've never worked in the space industry.// |
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Do we have an aerospace engineer around? I'm skeptical about the amount of atmosphere up there giving us proper lift, even though MB's faith is very convincing. |
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If the speed is enough, there's enough lift to bounce a re-
entering spacecraft right back to where it came from. The
key question is how fast you can drag the tether... |
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So the supersonic aircraft pulls on the tether, and the tether is very long and the spacecraft is very far out and flies out even further by centrifugal force? Is that the idea? (I kinda prefer ideas where you don't have to guess what it is.) |
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It'd work but you'd need to do it in stages, ie: if your tether can safely be 1km long then you need x amount of intermediate kites each of which is capable of lifting a 1km length of tether... |
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tether, kite, tether, kite.....tether, kite, tether, spacecraft... |
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// the spacecraft is very far out and flies out even further
by centrifugal force? // |
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No no no. Forget orbits and circularity - this is just about
helping get the spacecraft up to orbital altitude. For all in
tents and porpoises, you can think of the earth as flat on
this scale (orbit is only 100miles up). |
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//What's the point? Space travel is supposed to be more
efficient because of the reduced drag// |
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Yes, but on the other hand propulsion in space (rockets) is
horrendously inefficient because you need to get all your
fuel and engines up there in the first place. If you could
generate your propulsion in a ground- or air-based system
and convey it to a spaceborne craft, you would be vastly
better off. This is exactly what this idea proposes. |
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As the kite increases altitude, it actually goes faster than the tow craft - a bit like water ski jumpers. |
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\\I guess the question is, does anyone have 86km of extremely strong, extremely light twine lying around?\\ Wait.... no, sorry. |
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//orbit is only 100miles up// |
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If it were actually in orbit, the orbit would have to much higher than that. If the towing aircraft were traveling at 1 mile/sec along the equator, it would take about 7 hours to make one revolution. For a satellite to rotate around the earth in that time, it would have to be almost in geosynchronous orbit, and the tether would have to be weightless and spiral around the earth several times. A million miles long or so. |
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At lower altitudes, the "kite" would actually be towing the aircraft. |
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Yes, I was aware of the problems posed by that awkward customer Physics, that's why I left it fairly vague. Thanks for those buns flying my way, though! |
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To achieve stable orbit at low altitude, so MB informs me, you need 8km/s, which is far faster than the carrier craft can do. The idea is not to do away with rocket propulsion, just to help it along as much as possible. Atmospheric oxidated jet engines start to wheeze at about 14km, but helping the space craft get to 100km with a little speed still and having expended minimal fuel will reduce the size of the rocket required substantially, particularly considering the rapidly reducing returns issues with earth-to-orbit. That is to say that if you make it bigger to get higher / carry more load, most of your increased capacity is taken up with carrying your increased size. Conversely, if you can find a way of reducing the size required to get into orbit, you reduce the amount of fuel you need to carry up there and therefore don't need to be as big anyway... this is why Rutan and co went for a two-stage launch in their attempt. |
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As for the tether, I can help a little there, having been a composites engineer. I envisage a kevlar / carbon fibre / teflon composite tape as the tether, not much more than 100km long. I'd better do the maths sometime to make sure it can work, but not now. |
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I'm pleased that someone thinks the kite effect may still work at 100km - the question is at what speed? Similarly, if the kite effect doesn't quite cut the mustard, will the rocket be able to maintain altitude whilst the mother ship maintains rotational velocity, or will the two effectively be working agin one another, adding nothing to just letting the rocket go on its own? |
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The tension on the cable will break it. I don't care if aliens come down and give us a cable made of carbon nanotubes, it won't be strong enough. |
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And for like 1,000 other reasons, [-] |
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//The tension on the cable will break it.// What is the
tension in the cable? |
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//And for like 1,000 other reasons// Can you give us, say 2%
of those reasons? |
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Well, anytime you have a cable there is tension. That is why "high tension" powerlines sag even though they are technically pulled taught. A cable that long would snap under its own weight. Or the weight of the cable would pull both vehicles out of the sky. |
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Now if the kite is getting its upwars motion from rockets while the aircraft pulls it along, that is just incredibly inefficient. If no rockets are involved the kite part would simply fall into line behind the tug - a kite with no wind. |
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//atmocraft// :D you mean.... "airplane" ? |
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anyways, if you can get a spacecraft up there without using on-board fuel, then you have more fuel to do other stuff with. |
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The atmocraft needn't stay in flight the whole time, just somewhat longer than would be the case without the tether. |
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Perhaps the vacucraft could be towed like a glider on the end of the tether, and at the appropriate time allowed to swing upwards (like a glider launched by a ground based towing vehicle) and released, at which point it would have greater altitude and speed than the atmocraft is capable of. Its rocket motors would then take over. |
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There may be no need for heroic lengths of cable. |
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[DIY] I thought "high tension" was just an anachronistic term for "high voltage", when referring to powerlines. |
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// I thought "high tension" was just an anachronistic term for "high voltage", when referring to powerlines. |
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That's what I was taught at University, too - very high voltage ("high tension") reduces current and therefore resistive losses. In any case, were the wires under high tensile stress they would still sag - this is because they have a relatively low resistance to bending moment. Approximating this resistance to zero, you can infer from resolving forces into parallel and perpendicular parts to the wire that the wire would need to be under infinite stress to support its weight in a horizontal straight line.
I don't care if the line sags a little (you'll notice that kite strings do and the principle still works), and as the space craft will be above the aircraft the sag will be minimal. As for the weight of the line, its length can be set at the maximum that is useful and no more. If this is 20km rather than 100km, that still helps the space craft get higher without using its own fuel, reducing the size of rocket needed to acheive stable orbit. Available material properties would constrain the scope of this invention just as available fuel specific energy constrains the scope of rocket engines. However, in the immortal words of cliche, "every little helps". |
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//anytime you have a cable there is tension.// Yes, and
anytime you have a burger you have ketchup. That's why I
asked "how much tension" - as in, a number. |
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The reason I ask is that it is easy to have a gut reaction
like that, but not really very useful. What is needed are
numbers. You may be right, or you may be wrong, but just
saying 'there'll be tension' isn't an answer. |
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//Now if the kite is getting its upwars motion from rockets
while the aircraft pulls it along, that is just incredibly
inefficient. If no rockets are involved the kite part would
simply fall into line behind the tug - a kite with no wind.//
First sentence is not applicable. Second sentence - why?
Assuming there's some atmosphere and a sufficient lifting
surface, the analogy with a kid running with a kite on a
string is fine. |
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OK. NObody is thinking about this quantitatively, even
roughly. So let's try a semi-quantitative thought
experiment. |
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First, let's assume that the "tug" is cruising at 15km (this is
what Concorde cruised at). Let's assume also that we want
to tow our spaceship up to 50km, which is a very useful
part of the way up to orbital altitude, and would save a
disproportionately large amount of weight. |
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Now, next, a big and very rough assumption. I am
assuming that wing-lift and atmospheric drag for a glider-
like structure are proportional to one another. In other
words, a bigger lifting surface will have proportionately
bigger drag. Does that seem fair, give or take a factor of
two? |
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OK. So, the "glider" is going to be gliding at 50km, where
air pressure is a small fraction of that at seal-evil.
Therefore, it will need bigger wings (for a given payload).
Based on the above assumption, though, the drag on the
"glider" will be the same as the drag on a conventional low-
altitide glider with the same payload. |
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What does this mean? It means that **apart from the
weight and drag of the tether**, which we'll come back to
in a moment, towing a "space glider" at 50km altitude from
a plane at 15km altitude is not significantly different from
a regular "tow" where the glider is at the same altitude as
the tug-plane. |
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What does that mean? Well, it's not difficult to envisage a
jet-airplane/glider arrangement that would work, even
with a conventional jet-airplane, is it? Would anyone
doubt that (if we had to) we could design a supersonic
glider that could be towed (at the same altitude) behind
Concorde, with a payload of a few tons? Fine. |
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So, the only real question is the weight and drag of the
tether. Remember, the plane is at 15km and the glider is
at 50km. I have no idea of the optimal angle for the
tether (ie, how far behind the plane the glider is), but I'm
guessing that a 100km tether would do it. So, we have a
100km tether being dragged through the air at (say)
2000kph ; the air density is going to range from that at
15km (tug height) to that at 50km (glider height). |
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First, the drag on the tether. I have no idea how to
calculate this, but let's start by guessing that the tether
will be 1cm in diameter (we can come back to this if it's
wrong). So, the tether has a total frontal area of 1cm x
100km, or 1000m^2. Most of it is above 15km altitude
and, given the way air density drops off with altitude, I'm
going to take a guess and say that this is equivalent (in
terms of air resistance) to 50m^2 of area at 15km altitude. |
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So, is it possible to drag 50m^2 at 15km altitude at 1500kph
? Yes, that's roughly half frontal area of Concorde itself
(wings and all). |
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Problem is, of course, that a 1cm-diameter tether will not
take a tension which is equivalent to half Concorde's
thrust (which, as we've just seen, will be needed). This
thrust is about 100kN, whereas Kevlar will take about
4kN/cm^2. |
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So we need to make the tether thicker, but this increases
drag. If we make the tether 5cm in diameter instead of 1,
then its x-sectional area in increases 25-fold (so it'll take
100kN), but it's frontal area increases 5-fold (so, drag is
increased again). If it's 10cm in diameter, though, we're
OK - it won't snap under its own drag, at the speeds we
need (ie, we've increased x-section 100-fold, and increased
drag only 10-fold, so we're in the right ball-park) |
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So, where are we? Oh yes. We need a 100km long, 10cm
diameter kevlar tether. The plane (if it's something like
Concorde with, say, double the engine capacity to drag
itself, the tether and the glide) will therefore need to lift
about 1000 tonnes of tether. (This is pessimistic - it
assumes that we can't design the tether itself to generate
some lift, which we probably could, but it's messy.) |
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Ooopsie. This is going to cause issues. The beefiest plane
around (an Antonov thingy) can lift about 250 tonnes - a
factor of four too small. |
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Hang on, though - a factor of FOUR? That's nothing! Given
the gross assumptions I made above, it could mean
anything between well-within- current-engineering and
impossible-by- a-factor-of-100. |
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But it is not a silly idea, and certainly not completely out
of the question. |
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And if anyone doubts the advantage of getting giving a
rocket a "free" ride to 50km, for the price of operating a
very hefty but otherwise fairly normal airplane, then
they're wrong. |
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All that heavy lifting. You must have sinewy thews on your lobes ;-) |
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//And if anyone doubts the advantage of getting giving a rocket a "free" ride to 50km, for the price of operating a very hefty but otherwise fairly normal airplane, then they're wrong.// |
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Getting to that height alone supplies about 1-2 percent of the total energy it would take to get into low earth orbit. (1.0 MJ/kg to get to 100km, and >30MJ/kg to establish a low earth orbit.) And the contribution of the kinetic energy at 2000 km/hr is even less. So yeah, I doubt the advantage of it if your goal is to put something into orbit. |
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Yes. Have you calculated the impact on launch mass? No. |
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The launch mass must be greater in this case if you count the tether and the aircraft. |
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Yes, certainly. But aircraft are cheaper to fly than
spacecraft. A big passenger aircraft costs, what $40K in
maintenance and fuel for an 8hr flight? How much does a
shuttle launch run? I'm guessing 1000 times as much. |
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Basically, spacecraft are horrendously expensive, and a
kilogramme at one altitude costs you ten or a hundred
kilos at launch. Aircraft, on the other hand, are dirt cheap
to build and fly by comparison. So, getting any
contribution from an airplane is a big saving. |
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As the poster pointed out, SS1 was designed on exactly
this principal, even though the "airplane" stage only got
the rocket up to "airplane" altitudes. Clearly, it made
economic sense. Now, if the tether can double or triple
the altitude available from the airplane stage, it would be
a tremendous advantage. |
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// if the tether can double or triple the altitude available from the airplane stage, it would be a tremendous advantage.// |
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Actually not, because you're in the 1% advantage territory, which is a lot less than "tremendous." |
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No, what I meant was that it was worthwhile using a glorified
airplane to get SS1 up to airplane altitudes. Therefore, it
must be worthwhile using a glorified airplane to get a
spacecraft up to two or three times that altitude. Or is
there a flaw in that? |
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I never thought that SS1 made any sense at all. It's a stunt more than anything. Sounding rockets do far better, getting up to 1,500 km. |
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I'm with [ldischler] - a suborbital hop getting to 'space' altitude is not really any use. You need to get up to orbital velocity as well to do anything useful. |
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[neutrino] yes, granted - no point in suborbital hops. The
point was, though, that this system gives you a boost on
altitude (though not velocity). Depending on the
numbers, that alone may make it worthwhile as a stage in
an orbital flight. |
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[21Quest] yes, as I pointed out, the payload is four times
too much for an Antonov to get off the ground with.
However, a factor of four is peanuts given the other errors
in the calculations - it could be a factor of 1 or a factor of
10. I'm just saying that it's not a completely infeasible
idea - whether it would work in practice would need more
sophisticated calculations than I can do. |
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MB - thank you very much for your fag-packet mathematics (on which many of the world's greatest inventions were devised), I could have done it but had not the time, and in any case could not have done it as beautifully. I particularly like the lift vs drag insight. |
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So, the conclusion is - possibly feasible, but not massively likely. A perfect candidate, it seems to me, for the HB. |
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How big would this kite need to be? I'm guessing my 6 foot Flexifoil isn't going to cut it. What is the feasibility of building a kite with, say, a 500m2 area? Big things like 747's seem to provide better lift-to-weight ratio than little things like Flexifoil kites so I'm imagining that much like the string it needs to be as big and thin and strong as possible. |
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MB, I think you need to consider that it's the glider that carries the tether... |
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Ling - that is a deeply insightful point, truly. |
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With that in mind, I guess we would need to reckon on a
tether that also has lifting surfaces (maybe something like a
glorified rope ladder, where each rung is a winglet). |
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Has anyone got another fag packet? No no, a full one will do
fine.... |
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//I'm guessing my 6 foot Flexifoil isn't going to cut it//
I've got a ten and a couple of sixes - we could stack them. |
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Actually, that's not such a bad idea. Stacked Flexifoils are effectively a tether with lift. Here's a piccy of 16 in a stack (link). Now imagine that the links between them are 500m instead of 5m. There's your tether, Maxwell.* |
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*NB - Assumes constant windspeed and direction at all altitudes |
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Given that these things are being dragged along at several
hundred mph, I think we can assume that relative windspeed
will be to all in tents and porpoises uniform. |
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One could also make the tether hollow (ie: a pipe) and fuel the space end from there as well... for that matter the atmocraft (su) could also be a processing plant: distilling oxygen and harvesting hydrogen. |
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And small gardens on the topside of the lifting surfaces could grow tomatoes to feed the astronauts. |
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// guess we would need to reckon on a tether that also has lifting surfaces (maybe something like a glorified rope ladder, where each rung is a winglet// |
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And holding the bottom end of it, a small boy running. |
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Does the towcraft *have* to take off with the weight of the tether and the glider? Why not hoist them to altitude by another, slower method ... a blimp, or ground tethered kite. When the towcraft takes off, it would only have to lift its own weight and fuel off the ground. Then at a certain point it would pick up the slack in the tether and *whoosh* away we go. |
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I can't help feeling that a trebuchet is the only element
missing here. |
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//Then at a certain point it would pick up the slack in the tether and *whoosh* away we go.//
Yes, like that Hercules that had the big forked grabby thing for plucking ditched airmen from the water, like they did in that Bond movie. |
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// I can't help feeling that a trebuchet is the only element missing here. |
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Duly added (check updated description). We may need to move up to the back of cigar boxes due to the added complexity. |
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Whom has possessed [21Quest]? Give him back! |
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He's usually quite brilliant. I guess I never realized that Rocket Science isn't his forté. |
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(Can I give an bun to [MaxwellBuchanan]? Impressive...) |
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Can we make the tether ovoid or teardrop shaped in cross-section, rather than cylindrical, to reduce frontal surface area and thus drag? Mould in little winglets along its length to both maintain forward orientation and provide lift. Somewhat like so: |
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If we make the length of the tether shorter, we reduce the weight and the tensile strength requirement. Surely there must be a sweet spot for h (height above towcraft) where h > 0, and the advantage gained balances the weight, cost and feasibility. |
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//winglets// would work better than my idea of a kite every nnn-metres... be a pain to roll up, though. |
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I would think that kites every n meters would be even more of a bugbear for coiling/uncoiling. The laughably crude ASCII diagram above suggests an approximately rectangular overall cross-section, which should be coilable. And rather than making the winglets larger to compensate for reduced air pressure, just have more of them per meter. |
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No argument there, I like the winglet idea better anyways. Here's something to consider though: |
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The cable at higher altitudes would need bigger winglets to hold it's own weight. But a bunch of big winglets would provide too much drag at lower altitude (and you might have the bizarre problem of too much lift). So you can't unreel the cable high end first at a low altitude. |
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So... the entire mess goes inside a huge glider wing that the spacecraft sits on and is unreeled from there. |
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but as you approached the big wing (from the bottom) it would probably look more like wagster's <link>, gradually increasing in size... like vertical window-blinds (with lots of room between slats) |
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Those difficulties are overcome by my previous suggestion of using a blimp to raise spacecraft, kite and one end of the tether to altitude before the towcraft takes off. |
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//blimp to raise spacecraft, kite and one end of the tether to altitude// but maximum "at altitude" for a balloon that isn't weighed down by a spacecraft, multiple kites/winglets and kilometres of cable is only 50km. |
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Am I the only one that sees a naked lady? |
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That was no lady, that was ....
(I'm sorry, I tried not to post that.) |
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[FT] That's OK. Once the blimp is at altitude, and the towcraft takes off and picks up the slack, the tether and kite will generate its own lift. The whole assembly can then fly higher than the blimp itself allows. The towcraft then doesn't have to be engineered to carry all the weight, merely to provide enough thrust. |
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heheh, I just pictured some kid standing off to the side glowering while dad adds all sorts of contraptions to his kitestring. |
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