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Can aircraft carrier-like catapults contribute to solutions?

Posted by: Ekkehard Augustin - Mon Nov 15, 2004 8:27 am
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Can aircraft carrier-like catapults contribute to solutions? 
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Post Can aircraft carrier-like catapults contribute to solutions?   Posted on: Mon Nov 15, 2004 8:27 am
To launch an aircraft from an aircraft carrier catapults are used to save runway length because the whole carrier is to short to provide normal runways.

Might such a catapult save propellent if it is used to launch vehicles like Wight Knight/SpaceShipOne from a runway of normal and usual length? If so - round about how much propellent could be saved? The saved amount would be available for increasing the altitude or increasing the velocity - it would be available for reaching the orbit.

What about this? How probable is it that Scaled might think of including this in their concept?



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Post    Posted on: Mon Nov 15, 2004 9:19 am
From the perspective of White Knight/SS1 the important thing was for White Knight to gain altitude. Not speed. Given that propellent loading for White Knight was a trivial problem compared to the other obstacles they overcame and contributes very little to the ceiling altitude of an SS1-loaded White Knight ... I would say that a catapault launch system would not be of any real benefit at all.

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Post    Posted on: Mon Nov 15, 2004 10:15 am
I agree, for a system like White Knight/SS1 it would not work. The problem lies in the the increadable speed you'd need at ground level if you'd like to add anything real benefit. That causes huge stresses on the ship, thus calling for a heavier construction, which would negate any advantage. However, note that the idea you propose is not new. Think about the magnetic rail guns that have been proposed. This is basicly the same idea.


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Post    Posted on: Mon Nov 15, 2004 10:29 am
I have been thinking of the "magnetic rail guns" when I remembered the Transrapid an hour ago.

But some amount of advantage seems to be there - otherwise catapults wouldn't be used at aircraft carriers.

Is there a difference between the engines of military airjets and the engine of Wight Knight? The jets are very heavy but get more speed by the catapult than the wight Knight has.

I don't want to discus the catapult as a new idea but as a little add-on or improvement. Scaled's/Mojave's deal with Branson requires redesign, redevelopment and improvements - ...



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Post    Posted on: Mon Nov 15, 2004 11:09 am
Ekkehard Augustin wrote:
Is there a difference between the engines of military airjets and the engine of Wight Knight? The jets are very heavy but get more speed by the catapult than the wight Knight has.

Well the reason why jets use catapaults to leave aircraft carriers is to reach take-off speed, this is a mechanism for overcoming the problem of not having a long runway to build up sufficient take-off speed. The cruising/max speeds of these jets is unchanged by the launch system. Ordinarily, a WhiteKnight/SS1 type combo does not have to overcome the problem of a short take off runway.

The only reason I can think that a WhiteKnight/SS1 type of combo would need this sort of system is if they needed to launch from a ship at sea. E.g. a kind of two-stage to orbit thing leaving from the equator (and the only reason they would use a catapault is because the ship isn't long enough to build up take-off speed). But I imagine that the stresses involved at such a launch are, as André pointed out, significant.

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Post    Posted on: Mon Nov 15, 2004 11:22 am
Yes, catapults are used to reach take-off speed within a shorter distance - but am I wrong, that this saves propellent otherwise required to reach take-off speed on a long runway? The catapult requires energy which isn't provided to it by the propellent of the jets. Do I misunderstand something?

The stresses will be significant - but what about the possibility that the future Branson vehicle could resist that stresses?



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Post    Posted on: Mon Nov 15, 2004 2:30 pm
Using a rail gun to accelerate a spacecraft is not a good idea, the huge EM pulse created by them would likely fry most the crafts electronics.

A few years back I was asked to do some EMC testing on one and its size would be prohihibitive to use it for this purpose. It had a bank of over 100 capacitors to store the necessary energy, each one was the size of a 45 Gallon oil drum. The projectile was only a couple of kgs in weight but the "rails" had to be rehoned after every launch because the current warped the 7.5 cm metal.

Granted that the same sort of velocities would turn any crew member to instant mush, so a much less agressive acceleration would be required I think a steam or compressed air catapult would be much better or even a pair of small solid rockets like the USAF used to launch their planes with.

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Post    Posted on: Mon Nov 15, 2004 2:51 pm
I remembered the Transrapid and I have been thinking of the "magnetic railway guns" - but I don't think of them as useful technologies for launches.

I'm considering a special fact only:

The touch of an aircraft's whells to the ground as well as the touche of a train's wheels to the rail is a source of energy losses because by the touch the kinetic energy is transformed into thermal energy partly.

The Transrapid trains don't have wheels and they don't touch the magnetical rail while in motion - the trains are much faster than conventional trains. So the technology provides the advantage of less energy losses during motion.

The Transrapid is accelerated by additional magnets at the rail - this part of the technology shouldn't be used for launches. The acceleration should be left to the engines used now.

A horizontal launch by the used engines from a runway made of magnetical rails to remove the close touch to the runway. The spacecraft would have the magnets of the Transrapid trains, the magnetic field of the rail(s) never would change during one launch and the acceleration would be done by SpaceDevs engines produced for SSO for example.

I don't know for sure wether the magnets of the spacecraft require power - but it seems very very likely to me. Perhaps this problem could be solved easyly because the magnets are no longer required after take off.

To this a catapult could be added.



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Post    Posted on: Mon Nov 15, 2004 3:55 pm
A couple of small solid rockets would give the same acceleration (or better), give extra height as well as acceleration, probably be cheaper, be much easier to produce and not require a rail structure fixed at one location. Also no need to worry about friction with rails or the ground.

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Post    Posted on: Tue Nov 16, 2004 12:11 pm
What about searching for way to get advantages by combining both?

It often seems to me that may ideas proposed here are considered as competing stand-alone solutions excluding other solutions.

But it may be possible to combine at least some of them in a manner providing a greater advantage than each of the combined solutions alone.

When I wrote my initial post of this thread I had in mind that savings of WK-propellent could be used to reduce volume and weight of WK's tank and to increase volume and weight of SSO's tank and SSO's amount of propellent. Then SSO's engine could have a longer burn, give more acceleration which would increase end velocity and altitude.

Adding your recommendation could increase all that by an additional amount.

I have academic considerations in mind up to this point.

What could be added?



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Post    Posted on: Tue Nov 16, 2004 12:52 pm
I think that it would be a question of weight. If the overall spacecraft is not to heavy, say under 100 tons then the addition of a catapult system may be advantageous to increase the intial velocity/height.

If the craft becomes heavier then I suspect that the catapult system would become to complex and expensive to build. A smaller catapult is likely to be mobile aloowing it to be deployed at different locations.

How about instead of a catapult on a rail system hanging the craft from a tether and spinning it with a counterweight, once sufficient velocity was achieved you could release the craft, flinging it straight up in the air. Something modelled on those fairground attractions that turn a godola upside down might do. Once the craft was airborne the rockets could ignite giving a further push.

I dont know what sort of stresses this would put on the crew and craft, but a velocity of 3 or 4Gs should be OK (probably not much more than the original fairground ride).

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Post    Posted on: Tue Nov 16, 2004 1:04 pm
That idea I proposed earlier this year for use in orbit - I would like it too. But it how difficult is it? And what about dangers?



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Post    Posted on: Tue Nov 16, 2004 2:23 pm
The timing would have to be just right to act as a sling shot with the rockets firing at the optimum point but it should be doable.

I think that a ground based system mounted on a tower of 100m or maybe even overhanging a cliff should give a big enough boost to make it worth it, the craft and its counterweight tethers could be played out from the centre resulting in ever increasing arcs until a diameter of nearly 200m was achieved before the craft was released. In this way the stored energy in the craft could be increased and the effect on the crew would be gradual. There is of course the question of what to do with counterweight, you would probably have to release it at the same time to stop the machinery throwing itself in the opposite direction.

Such an apparatus might give a boost of a couple of 1000 meters which coupled with a pair of solid rockets could be increased significantly. This is an interesting idea but the more I think about it the more I think that the gain that you get does not warrant the inherant complexity and danger associated with such a device. I think I would stick with just the pair of solid rockets.

Of course there is a much easier solution to gaining extra altitude, airlift your rocket to a suitable mountain and take off from there. I wonder why launch sites are not at higher altitudes, even a relatively small mountain would probably make a difference to the amount of fuel carried.

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Post    Posted on: Tue Nov 16, 2004 3:07 pm
One big problem with a device like this would be that you'd need to attach the craft at it's centre of gravity, which is probably somewhere around the middle. While you're bussy slinging, you're producing huge G-forces on the craft. These forces are predominatly directed in a direction perpendicular to the one you'd like to travel in and the one in wich the forces will be directed once the normal rocket ignites. That causes problems in the construction. While I have not calculated it, I guess the additinal stresses will result in a weight increase, which, in turn, negates the gain of the slingshot.


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Post    Posted on: Tue Nov 16, 2004 4:19 pm
You could possibly put the craft in some sort of craddle so that it was supported underneath and the centrafugal force would help to keep it in place but like I said the more I think about it the more a pair of small solid rockets look a much better bet. Probably explains why someone hasn't done it yet. :)

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