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  Pixelation, 7 Apr 2016 @ 5:33pm

  
  

  "Perhaps you've seen comparisons of the sizes"
  
  It always has to devolve into a conversation about size, doesn't it. Next you'll be discussing the quality of the payload.
  
  At least I'm above that.

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  Lawrence D’Oliveiro, 7 Apr 2016 @ 11:41pm

  
  

  You Want Comparisons Of (Fictional) Spaceships?

  Feast your eyes on the grandmother of them all...

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  • 

    Anonymous Coward, 8 Apr 2016 @ 5:19am

    
    

    Re: You Want Comparisons Of (Fictional) Spaceships?

    And yet none of them compare to Dahak.

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  Robert, 8 Apr 2016 @ 2:49am

  
  

  Cheap Launch System

  Getting people into space is not that hard. Build a long tunnel (say 1 to 10 kilometres long, even longer ie final assembly site to a suitable mountain top), ensure it is air tight and can handle being in vacuum or high pressure. Build in electro magnetic suspension.
  Place the launch vehicle in the tube, in vacuum and electro magnetically suspended.
  Ensure there is a relatively tight fit between the launch vehicle and tunnels walls.
  When you are ready to go fill the space behind the launch vehicle with compressed air, preferably hydrogen and oxygen which you ignite for maximum pressure gain and away you go (multiple ignition points would be required along the length of the tunnel to better balance and maintain pressure).
  Interesting design element, mass of launch vehicle is now a plus, as it provides inertia upon exiting launch tube, with mass just limited by pressure required to generate the designed acceleration.
  Zero mass wasted on getting the launch vehicle to orbit and so maximum payload and life support achieved and that launch vehicle can use extra mass needed for inertia to punch it through the remaining atmosphere to orbit to, so in fact it can be armoured.
  Biggest steam rings imaginable, along with the exit of the launch vehicle as it coasts into space. By far the cheapest per tonne (forget pounds, so last millennium) launch system possible.

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  • 

    Anonymous Coward, 8 Apr 2016 @ 6:07am

    
    

    Re: Cheap Launch System

    There are a few problems with your suggestion. The first is that it can't put a vehicle into orbit (on it's own). Or rather, it can but the orbit would be one that intersects with the Earth at some point, which isn't a useful orbit. The velocity needed to enter orbit has two components, the velocity needed to reach a certain altitude and the velocity needed to remain at that altitude. Let's call them launch velocity and orbital velocity (and yes, I'm talking vectors, not scalars). This system can essentially only impart the launch velocity. You need to have another system to impart the orbital velocity. The lower the orbit you're targeting the higher lower the launch velocity required, but, ironically, the higher the orbital velocity required, so the less use this system would be. To reach low Earth orbit, this system could impart around a sixth of the needed velocity. Now how long does that tunnel have to be? Well, just to get to LEO, you need a speed (yes, scalar now) of around 1.4 km/sec. Suppose you will accept an acceleration of 3g or roughly 30 m/s/s. Using the good old acceleration equations, it will take roughly 47 seconds in the tube to acquire that speed, over a distance of approximately 33 kilometers. Slightly longer than you envisioned. Of course you could increase the acceleration, but the higher you go, the fewer people who will live through the experience... Plus that's only to LEO and you only have a sixth of the velocity you need... Next there's what happens when you exit the tube. You encounter atmosphere. At this speed, that will be something akin to hitting a brick wall when travelling in a car, in terms of acceleration. Here is where your inertia will come in handy, though strictly speaking it isn't inertia that counts in overcoming air resistance, it's the ratio of mass to cross-sectional area. For the same shape, mass scales with the cube of length, while cross section scales with the square, so larger size is a benefit. Still, unlike a rocket, your speed will be highest where the air is thickest, which means the air resistance will be ferocious. Not only will this mean you need a significantly higher speed than would be need on an airless planet with that same gravity, but there will be enormous heat problems (remember those ceramic tiles on the shuttle? Needed because of heating from air resistance. At 6 times the velocity, admittedly, but in air maybe one hundredth the density. To make matters worse, this system can't launch straight up because 33 km up and down is impossible (either because we can't build something that tall, or because the Earth is too hot at that depth, while a turn at the end to deflect the launch vehicle would impart a high enough lateral acceleration to kill any crew members. Eventually the curvature of the Earth would take the surface far enough away from the launch vehicle that would be travelling straight-ish, but it does mean that the vehicle would travel a far greater distance in the densest part of the atmosphere than a rocket does. Rockets don't require heat shields in the launch because they are never travelling fast enough in the air at any particular density to heat up enough to require shielding. (As they accelerate, they rise and the atmospheric density drops).
    
    And no, the launch vehicle will not "coast into orbit". This is not a launch system, just part of one at best. You would still needed an enormous rocket to impart the orbital velocity.

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  Roger Strong (profile), 8 Apr 2016 @ 7:13am

  
  

  > Not only will this mean you need a significantly higher speed than would be need on an airless planet with that same gravity, but there will be enormous heat problems
  
  An example of this: The 900-kilogram steel plate cap for the test shaft of the Pascal-B nuclear test.
  

  Before the test, experimental designer Dr. Brownlee had estimated that the nuclear explosion, combined with the specific design of the shaft, would accelerate the plate to approximately six times escape velocity. The plate was never found, but Dr. Brownlee believes that the plate never left the atmosphere, as it may even have been vaporized by compression heating of the atmosphere due to its high speed.

  
  Granted, we now know much more about meteorites, including that the heat of re-entry will still have them arriving at the ground with a cold interior. Ablation can remove heat, which is how the fibreglass Apollo heat shield worked, and a Russian wooden heat shield.
  
  If any bit of it cleared the atmosphere, it would mean that America put an object into solar orbit a couple months before Sputnik reached earth orbit.

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  Pixelation, 8 Apr 2016 @ 7:56am

  
  

  "The use of a subterranean shaft and nuclear device to propel an object to escape velocity has since been termed a "thunder well"."
  
  I have something similar in my bathroom. If my wife walks in inadvertently, she will nearly reach escape velocity getting out.

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  • 

    Roger Strong (profile), 8 Apr 2016 @ 11:36am

    
    

    Re:

    That reminds me of an anecdote I just read in IGNITION! - An Informal History of Liquid Rocket Propellants - by John D. Clark.
    Many of those propellants are more than a tad dangerous, including Chlorine trifluoride, or "CTF"...
    

    All this sounds fairly academic and innocuous, but when it is translated into the problem of handling the stuff, the results are horrendous. It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water —with which it reacts explosively.
    It can be kept in some of the ordinary structural metals — steel, copper, aluminum, etc. —because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminum keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.
    And even if you don't have a fire, the results can be devastating enough when chlorine trifluoride gets loose, as the General Chemical Co. discovered when they had a big spill. Their salesmen were awfully coy about discussing the matter, and it wasn't until I threatened to buy my RFNA from Du Pont that one of them would come across with the details.
    It happened at their Shreveport, Louisiana, installation, while they were preparing to ship out, for the first time, a one-ton steel cylinder of CTF. The cylinder had been cooled with dry ice to make it easier to load the material into it, and the cold had apparently embrittled the steel. For as they were maneuvering the cylinder onto a dolly, it split and dumped one ton of chlorine trifluoride onto the floor. It chewed its way through twelve inches of concrete and dug a three foot hole in the gravel underneath, filled the place with fumes which corroded everything in sight, and, in general, made one hell of a mess. Civil Defense turned out, and started to evacuate the neighborhood, and to put it mildly, there was quite a brouhaha before things quieted down.
    Miraculously, nobody was killed, but there was one casualty — the man who had been steadying the cylinder when it split. He was found some five hundred feet away, where he had reached Mach 2 and was still picking up speed when he was stopped by a heart attack.

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