I want to suggest a variation on the launch catapult. The rocket assembly is accelerated vertically through momentum transfer, by a reaction driver on the ground. In concept the driver would be a machine gun, or a jet of water. The rocket assembly has a bumper plate (as in Project Orion). Unlike most catapult concepts, there is no track. One resulting problem is the projectiles (or jet) could miss the bumper, and hit the payload (very bad).

Conceptually, this is a low performance ground-mounted rocket engine, and the mass consumption might be an order of magnitude+ greater than the first stage it aspires to replace. High aerodynamic loads are avoided for the rocket assembly, though they certainly affect the jet or projectiles. A distance of 10 km at 2.5g corresponds to 2500 kph (Mach 2). Aiming at that distance is a question; I have no reason to hope a jet of water would stay together, but can't rule it out. Ice composite projectiles might be possible.

Under Project HARP a projectile reached 180 km altitude, much greater than needed here. The necessary rate of fire would be challenging. Projectile launch velocity would need to be modulated, using a magnetic accelerator, to make closing velocity something reasonable.

Perhaps this could get the rocket assembly above the atmosphere, with enough velocity to be useful.

  • $\begingroup$ This is a question and answer oriented site, not so much debate/discussion. Is there a question here, related to your proposal? $\endgroup$
    – Saiboogu
    Feb 9, 2019 at 15:36
  • $\begingroup$ I had almost exactly the same idea a few years ago, and have been thinking about it since. After a few iterations I've come to something I see as more near term feasible: medium.com/@brysgo/high-altitude-mass-volley-9ee9e94f6007 $\endgroup$
    – brysgo
    Jul 25, 2019 at 6:33

1 Answer 1


I don't see this working very well.

  1. you can forget about using liquids as the reaction mass. The highest fountain in the world reaches only 300 m, and the jet loses integrity far below that.

  2. using solids as the reaction mass brings its own problem: erosion of the bumper plate. Every bullet fired at the plate deforms the plate.

  3. To get to orbit, you need a speed of about 9 km/s. A railgun reaches about 1-2 km/s, far short of what you need, and every projectile you fire will be slowed down by both gravity and atmospheric drag.

  • $\begingroup$ What about dry ice? or something else that sublimates $\endgroup$
    – brysgo
    Jul 25, 2019 at 6:42
  • $\begingroup$ Also, what about light-gas guns instead of rail guns: en.wikipedia.org/wiki/Light-gas_gun $\endgroup$
    – brysgo
    Jul 25, 2019 at 6:47
  • $\begingroup$ There are a number of 'catapult launch' questions on the site that address the more general problems with any catapult system. My item #3 is a major problem that changing the catapult mechanism can't solve. $\endgroup$
    – Hobbes
    Jul 25, 2019 at 7:12
  • $\begingroup$ I'm pretty sure the system I describe here solves #3 pretty well: medium.com/@brysgo/high-altitude-mass-volley-9ee9e94f6007 $\endgroup$
    – brysgo
    Jul 25, 2019 at 16:20

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