Saturday, September 23, 2006

Verne's Cannon Part I

I've been thinking that I've been a bit too much of a wet blanket lately on various subjects. I've tried to define things that I've felt were unfeasable, or unworkable, or didn't make any sense. Esp with my recent posts on energy (since I feel both that many highly touted alternatives simply will not scale, as well as that it's important to provide this energy somehow).

But anyway, enough with what can't be done. Now I'm going to get downright whimsical.

I've read threads on the space elevator recently. If we ever manage to get a material that has a high enough tensile strength versus density, we may yet be able to build the thing. However, nanotubes are still problematic, seeing as how they slide against one another. The macroscopic material may not end up with the strength of the individual fibers. (There I go again)

So, I was thinking about other non-rocket methods of launching into LEO. One of the ones that has intrigued me for a while is a launch catapult.

The launch catapult idea has been looked at by NASA as part of it’s 90’s X-plane efforts. In their formulation, the catapult will accelerate a rocket up to a few hundred miles per hour so that it can start a scramjet. However, it’s not intended to provide any meaningful contribution towards the total deltav required to get into orbit. The envisioned space-plane will still have to provide almost all of it’s own dv, which means it will still have to carry substantial amounts of fuel (and be aerodynamic besides, seeing as how it has to operate in the atmosphere for extended periods of time to get any advantage from it’s scramjets).

However, I was interested in a mostly non-rocket launch method. What if we could get almost all of the dv needed to make orbit from the catapult? Maybe a circularization burn once the spacecraft clears the atmosphere, but have the spacecraft fire off the catapult at 8000m/sec or so, blast through the atmosphere, and off into orbit?

This would basically be a re-creation of Jules Verne’s cannon. The system I envision is more like a 1000km long mag-lev acceleration track. (1000 km comes from the requirement to achieve this exit velocity while limiting the acceleration of the spacecraft to <5 g-forces. Shorter tracks can be made for unmanned rockets). I concatenated a lot of my older simulation code together into a simulation of a spacecraft ascending through the atmosphere, using Earth’s standard atmosphere model, under such conditions. In the simulation, I had the track firing the spacecraft off at a 0.5 degree angle with the ground. Assuming a light drag coefficient (around 0.05), my 200 ton simulated vehicle made it’s way to 25km altitude in less than 40 seconds (basically in a straight line). This is where my standard atmosphere model quits, so I assumed zero atmosphere afterwards, though this isn’t really the case. (a point of refinement for later models) Even though the drag forces are huge, they apply only for a very limited time. Dynamic pressure can become problematic.

At this velocity, dynamic pressure is around 400 atmospheres (what a submarine would be experiencing at a 4km depth.) You might think this would sink the whole project. 400 atmospheres is a bit much to withstand for an aerospace vehicle. Furthermore, you’ll also have supersonic stagnation, and supersonic temperature for the few seconds you’re within the lower atmosphere. A normal plane or rocket would be crushed like a can.

However, the mass limits for the launched vehicle aren’t determined by any normal aerospace consideration, but rather by the capabilities of the launching system. If you’ve already invested enough to build some 1000km long launch track, you’re probably going to want to beef it up as much as possible. Furthermore, a more massive vehicle is an advantage, since it helps the vehicle punch through the atmosphere. So why not build it like a submarine? Better yet, encapsulate the rocket in a steel pressure-vessel hull designed to blow off after the vehicle has exited the atmosphere. (freeing the payload to circularize into a stable orbit, and jettisoning the shield mass). The shell can ablate in blazing glory upon ascent and bomb out in the ocean. While you might think this is a waste of good steel, steel is cheap and plentiful. Remember, material costs are a minor fraction of the cost of a vehicle, and machining steel is absurdly cheap and well understood compared to most high-performance aerospace processes. We could easily mass produce all the ascent shells we would ever need as part of a major space effort.

This whole approach is surprisingly workable, assuming you’re willing to build such a piece of infrastructure. Obviously this won’t be cheap or easy. It would have to be built across several states, (or, while I’m in this whimsical frame of mind, out in the ocean, between several platforms). It would only be justified if we really wanted to put mass into space on a daily basis, as part of a colonization effort or something. Other negative side effects include a car overturning mach 25 sonic boom at the exit. Still, this could be made to work (to my knowledge) with current material technology, and a boatload of powerful track magnets. If the space elevator doesn’t work out, this could end up being another railroad into space.


Anonymous CosmicConservative said...


I think one of the posts you were referring to was my post on space elevators, where I discussed a story I wrote almost 30 years ago which featured "skyhooks" instead of a space elevator. Well, I didn't mention it in that post, but the story also utilzed linear accelerators to launch the vehicle to the height needed to get picked up by the skyhooks.

In my story the linear accelerators (basically electro-magnetic catapults) were built into strategiclaly located mountains along or close to the equator. This allowed them to both utilze natural geography to gain the optimum trajectory and allowed them to launch into the much thinner air at high altitude, thus reducing drag.

Because the payloads were only being launched a few miles up and being picked up by the skyhooks at the peak of their ballistic trajectory, they did not need full escape velocity for the trip, which meant they needed no specialized materials or heat shields.

Your idea is one that has been put forth many times in the past. I think it is a completely workable one, and all of your points seem valid, with two minor quibbles.

1. The system does not scale easily, building the 4th such system is no easier than building the first.

2. The power needed to accomplish this is substantial. You may be able to run it from the power grid (I am too lazy to do the math) but even if you could, the political reality is probably that you'd have to build your own power plant, and that means either having your own hydroelectric plant, or a nuclar one. Both are problematical to get permission.

Still, I do think your proposal is no more "whimsical" than if someone for the first time proposed launching payloads into space on barely controlled chemical bombs.

Saturday, September 23, 2006 9:47:00 PM  

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