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Maglev launch

Posted by: Garnetstar - Thu Sep 11, 2003 1:24 am
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Maglev launch 
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Post    Posted on: Mon Nov 03, 2003 2:19 am
Let me start by saying that i'm not an expert, but i know alot about the physics of launching rockets. So i thought that i could help clear something up about maglev-launch.

using maglev to boost a ship to ignition speed for hypersonic engines, would require a takeof speed of mach 3 to march 5, and doing that at sealevel would put to much strain on the ship.

If we look at the pure benefit of giving a ship an initial velocity, it is very inefficiant to do so close to sealevel. So to gain from an exstra boost, you have to launch as high as posible. which mean build the tracks up mount everest.:) Or, as some of the X-prise teams have found out, diliver your ship at an altitude of 10km or higer. I did some simulations that shows that launching from high altitude will yield a little differens in the change of altitude, but launching with an initial speed yeilds a big differens when launhing at high altitude.

so in conclusion maglev might not have that much to offer, unless you can have the track end at more than 8km altitude, and have a sensible exit direction from the track.

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Post    Posted on: Mon Nov 03, 2003 8:16 am
Really? That suprises me.

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Post    Posted on: Mon Nov 03, 2003 5:51 pm
Incont does have a point, just tossing the rocket up at mach one at sealevel won't do jolly squat for the required energy, however, there is a certain benefit of mach one, it's the first-stage ramjet operations, which produce much higher Isp than a rocket engine, and could reach speeds up to mach 4 or 5, thus allowing for a much lighter first stage.

In the links at the X-Prize website there's a paper that quite clearly demonstrates why a mach 1 initial speed won't do much difference in launch weight; As kinetic energy is the square of the velocity, the required velocity of 28 machs require an energy of 28^2 = 784 times greater to that getting to mach 1, which would mean a save of 0.13% in launch energy... not very spectacular.


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Post    Posted on: Thu Nov 13, 2003 2:37 am
The other issues, however, are that the air is densest (and resistance highest) at sea level, and the vehicle at takeoff must carry all of its fuel. Hence, the vehicle moving at Mach 28 will have far less mass, and face far less air resistance, than when it's moving at Mach 1. I haven't yet done a calculation (fuel weight loss is straightforward but tedious, while decreasing air density coupled with increasing speed is tricky), but I expect the energy saving to be significant.

The advantage of using a ramjet is that you don't have to carry oxygen, only the very light LH2. The disadvantage is that you need to give a good initial kick. I don't like the hybrid (SR-71) approach because it means more complicated engines that weigh more. The traditional approach is to push it off with solid fuel rockets. (Somehow, I can't help but think of the redneck who attached a JATO unit to his big ol' Chevy.)

I've suggested Maglev as an alternative, but while that approach has potential, it has its challenges.

1.) Can you build a system that can handle a quarter megamp current? (Picture what the fuses will look like!)

2.) You need a track about 2 km long to get to Mach 1 with the occupants in one piece. Hence,
a. a gradual rise, so that the vehicle will have to obtain all its altitude after it leaves the track,
OR,
b. a vertical or near-vertical track, either extending high into the sky, or deep into the ground, or some combination thereof (picture the construction techniques);
OR,
c. a curved launch track that can point the vehicle upwards without having to be a mile high. As has been discussed, the transverse g forces will rule out a winged vehicle in this context.

THE GOOD NEWS: If such a track could be built and made to function correctly, it could be used repeatedly and perhaps recoup the initial costs.

ALSO: The vehicle upon leaving has a speed of Mach 1, is (hopefully) pointed upwards, and is running with very efficient air-breathing engines. Those engines should take it to about Mach 4-5 and about 30 km altitude, at which the rockets take over. It just might work!


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Post    Posted on: Thu Nov 13, 2003 10:01 am
Can't Ramjets, with the proper modification, also act as rocket engines when the air gets too thin? Say, pump in a little LOX to mix with your chosen fuel, and hey presto, they keep on running. This would reduce the need to bring along a second propulsion system.

Just a thought. Perhaps I read too many sci-fi books.

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Post    Posted on: Fri Nov 14, 2003 3:54 am
The problem with that scheme is that the large air intakes of the ramjet would have to be stopped in order to provide the pressure--there won't be enough air rushing in. The amount of metal and ceramic you'd need would far outweigh a rocket engine.


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Post    Posted on: Fri Nov 14, 2003 1:51 pm
I wonder what kind of solution would a rocket-ramjet hybrid bring? The gas generator in the jet engine would be a rocket thruster instead of combustor chamber; the excess heat from the rocket could bring a substantial amount of extra thrust through expansion, just like in normal jet engine. When the ambient atmosphere has thinned sufficiently, the rocket engine is shifted to full thrust, and the ramjet shroud could be jettisoned to reduce weight (For later use, obviously).


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Post    Posted on: Fri Nov 14, 2003 3:32 pm
Vendigo, rather than having a ramjet shroud, wouldn't it make more sense to use a variable geometry inlet, similar to the SR71's engines? Then, when it was time do convert from ramjet to rocket, the plenum could slide/screw forward, covering the inlet.


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Post    Posted on: Fri Nov 14, 2003 4:14 pm
Yay, that was the type of thing I had in mind. Perhaps the whole intake wouldn't need covering, because of the reduced atmosphere, so maybe an internal closing just prior to the combustion section of the engine would suffice?

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Post Maglev Launcher   Posted on: Sat Nov 15, 2003 4:17 pm
Why not bulid it up the side of a mountain or simply use a large escartment, either would do especially if it was on the equator.


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Post    Posted on: Mon Nov 17, 2003 10:16 pm
There's no need to close the intake if the combustor is replaced by a rocket chamber. This way, the engine wouldn't use the outside air as oxidizer, which would reduce the ISP but also simplify the engineering quite a bit; and it would still use the surplus heat to expand the air, giving extra thrust. When the atmosphere has thinned past minimum operating air density, the rocket inside the engine would continue to provide thrust, independent of the metal around it.


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Post    Posted on: Sun Dec 24, 2006 8:02 am
This technology seems to be getting a chance now. The german journal Wirtschaftswoche of 18th of December 2006 is reporting that the californian company Launchpoint Technologies plans to launch satelites of up to 100 kg weight this way.

A ring of superconducting electromagnets will be applied that has a diameter of 5 km and accelerate the satellites up to 27,500 km/h.

Accordeing to the article this could be used to service the ISS or to send water and foof to the Moon.

The project is furthered by the Pentagon who will check the feasability two years long.

If it turns out to work on Earth then it should be ideal for the Moon. The payloads that could be launched there this way could be much heavier than here.

It also should be a good technology for Mars since that planet is smaller than Earth and has less gravity. If the Amperes and the voltage required were available it could be considered in the electricity-on-Mars-thread and thus included also in the later calculation of the costs of colonizing Mars.



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Post    Posted on: Sun Dec 24, 2006 9:48 am
So, in essence you have a large partical-accelarator-like ring (like they have seomwhere in Switzerland underground) where a heavy object will be driven around with 27,500 km/h and then simply guided into the air? And just 100kg? 100 kg is a lot of weight, but for something really usefull it is not much. Consider only that you have to have some kind of maneuverability up there including fuel, so there goes your 100 kg.

Wouldnt it take way much more energy to speed it up to 27,500 km/h then to put it on a small booster rocket? Only advantage is probably that you can do it whenever you want. But i bet the electricity costs will make sure that the launchcosts won't get anywhere close to what we need.

Besides, don't they take the tremendous friction into account when you launch a sattelite from the ground up with a speed of 27,500 km/h? That's pretty much mach 25. They probably gonna need the 100 kg mass for heatprotection :P


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Post    Posted on: Sun Dec 24, 2006 10:29 am
Hello, Stefan,

I don't know the kwh etc. required yet and would be interested in them and in the other data you are mentioning also.

But the article says that they calculated costs of $ 50,000 for the launch. So the electricity etc. required can't be too much - might that have to do with the superconductivity of the magnets?



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Post    Posted on: Sun Dec 24, 2006 10:29 am
I would have thought that the air friction would have slowed the projectile enough that it would stop it getting in to orbit or only allow a couple of orbits before it re-entered.

For such a small payload it makes much more sense to use a small rocket, the cost involved in building this accelertor ring will be huge compared to that of something slightly larger than a sounding rocket.

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