Single Stage to Orbit Vehicle

[Kaito]

Well, we COULD just start of slower....

 

[Jarvitä]

Another problem: You can't launch live humans on a railgun. The acceleration involved would reduce any live payload to strawberry jam.

You could if you reduced the acceleration and increased (substantially) the muzzle length.

 

[spcefrk]

I'm with urwumpe on this one. I think optimally, you have a TSTO vehicle. In fact, I've got one on the drawing board ATM . Stage 1 is a flyback booster with a lifting body and linear aerospikes. Stage 2 is an orbiter with a payload for Station Resupply/Crew Return using conventional bell rockets.

The catch on Aerospikes is that they're much less efficient than bell nozzles at high altitudes, but a bit more efficient at all other altitudes. So you use the aerospikes in lower altitudes to build speed at minimal drag, then release the second stage and let it circularize the orbit.

I'm still tweaking things (the biggest question is at what altitude do you release?) and with a lot of demands on my time and low experience with Orbiter, an addon is far away, but it's fun to toss around ideas.

Bottom line: SSTO is possible on current tech. The X-33 would have probably accomplished it if the composite tank had been designed by people who knew what they were doing (LMSW not NASA). But the performance margins are slim to none. TSTO offers more comfortable margins and can be easier to design (I just need to sit down and investigate a couple of designs).

 

[Andy44]

X-33 was a suborbital test bed. You're thinking of the VentureStar.

SSTO is easy; you just have to build a big enough rocket. It wouldn't be practicle, but it would work.

 

[Linguofreak]

You could if you reduced the acceleration and increased (substantially) the muzzle length.

Granted, though another problem comes from the fact that such a railgun installation would pretty much have to be at ground level, and would be launching its payload at a good fraction of orbital velocity into the lower atmosphere.

 

[Kaito]

Right, the Rail gun provides external energy to get us out of a good section of the atmosphere so the ship has enough fuel to reach orbital velocity.
The point of the rail gun is to provide the initial boost, not get it up to orbit in one shot. The ship would use its fuel to reach and sustain orbit.

 

[Biscuit]

Why not use maglev?

http://en.wikipedia.org/wiki/Maglev_train

Evidently it has already run at speeds of 361 mph (581 km/h), with the potential to hit 600 mph (900 km/h).

Nowhere near orbital speeds, however, the tech already exists, and a 'first stage' using a maglev cart running up a long sloping incline to a high altitude, oh, say a mountain on the equator with a long sloping west face, would be a good place to launch from.

Of course, this is a pretty old idea, used in quite a few scifi novels in the past.

Nobody seems to think about it much anymore, though.

Finding the mountain would be the hard part, though ... not a lot of height available compared to a jet ... Everest is not even 9km.

 

[cjp]

Why not use maglev?

My idea. Well, not only my idea of course; as you say it's quite old.

Your discussion about reaching jet altitudes made me wonder how a maglev would compare to launching from a jet. I guess you'd need quite a large jet for a manned launch, and using a custom designed jet for a couple of launches is quite expensive. Would a 747 or an A380 be large enough? How would both systems compare in terms of complexity, cost, and possible payload mass?

Would a ramjet second stage help? I read that ramjets can be efficient if they are optimized for a specific cruising speed, but can they be used to accelerate over a wide range of velocities, maybe using a variable geometry?(*)

I guess the jet (or maglev) will be re-usable. Making the second stage re-usable is probably the difficult part. You'd probably want to make the orbiter as small and light as possible, to minimize the weight of re-entry and landing equipment (heat shield etc.).

Anyway, back on topic: the idea to "have a giant stick of solid fuel and have your engine burn it from the bottom up" sounds quite inefficient, unless you have some way to direct those exhaust gases downwards. Maybe some kind of nozzle construction on the lower end of the stick, that moves itself upwards as the fuel burns up?(*) Anyway, AFAIK most solid fuels

1. are not really capable of maintaining the shape of the rocket properly when multiple G-forces act on it
2. Don't have enough ISP to make them interesting for SSTO

(*) :fool: forgot to run to the patent office first!

 

[YL3GDY]

As far as I know, current projects of railgun are a sort of Jules Verne's space cannon with all its limitations. Looks like the only solution is to make a rail VERY long, e.g. about 2000 km long. But would it be even a bit profitable?

For low-weight satellites a fully-reusable single stage booster is being developed in Russia called Tu-2000. Basically it's a big hypersonic plane equipped with liquid fuel engines to perform a jump from upper atmosphere (where is sufficient air density for scrams) to orbit. Payload mass taken to LEO is claimed to be 6-10 tonnes. It will use liquid hydrogen+liquid oxygen fuel. Looks like they managed somehow stop fuel heating, but the whole project development would cost a giant amount of money and time. But it would be theoretically most energy-efficient way.

Quite interesting idea of "1.5 stage" device: a vehicle is being taken to ~60 km height using a baloon (similar to ones which are used in meteorology). Then the booster will perform tradicional ascent. Pros: high-density atmosphere layers are passed almost without energy waste. Cons: only light-weighted satellites can be brought this way.

For heavy satellites, IMO, two- and three-stage boosters (Proton, Delta, Ariane etc.) wouldn't have any profitable alternative.