Naked Science Forum
General Science => Question of the Week => Topic started by: katieHaylor on 20/11/2017 13:16:17
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Anthony asks:
Would a rocket be assisted or hindered by being launched from a tube, like a bullet from a rifle barrel?
What do you think?
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The rifle bullet is aided by the barrel because the pressure from the explosion is available through the whole length of the barrel and the effect of this overrides the friction and air resistance effects, so muzzle velocity is greater for a longer barrel - within limits obviously.
A rocket has a continuous burn which would provide greater pressure from behind, but you need to consider the negative effects. To keep a seal and use the rocket pressure you need a tight fit, meaning friction and damage to the thin skinned rocket. Also the rocket has a much greater diameter than a bullet so needs to be able to push the large column of air out of its way, remember air resistance is proportional to velocity squared plus the rocket will compress the air column using energy.
Without calculating, my guess is that friction damage and air resistance crushing the rocket would put a limit on final velocity making it difficult to achieve escape velocity.
However, would be interesting to consider short, loose fitting tubes as in bazooka or mortar.
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It was done in the fifties using two naval gun barrels welded together by the same man who worked on the Iraq super gun project, more when I find the references.
Take a look here
http://www.slate.com/blogs/atlas_obscura/2013/07/03/project_harp_a_giant_space_gun_in_barbados_is_now_abandoned.html
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Rockets can be critically damaged by the frozen condensate on the outside of the rocket. Any benefit to acceleration would be negated by the inevitable explosion that follows.
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This question has now been answered on the show. You can listen here - https://www.thenakedscientists.com/podcasts/question-week/could-space-rocket-be-launched-gun (https://www.thenakedscientists.com/podcasts/question-week/could-space-rocket-be-launched-gun)
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As mentioned on the podcast, a rocket launched from a gun has the problem that the gun is on the ground, so the rocket still has to power through the atmosphere.
Some rockets are launched from a high-altitude aeroplane, which provides some significant benefits over a ground-launched rocket, for example:
- At launch, the rocket is already above most of the Earth's atmosphere, so it doesn't waste so much fuel fighting atmospheric drag
- It already has a significant velocity boost
- The carrier plane can be air-breathing, so it is much lighter than if it had to carry cryogenic oxygen (like most rockets)
- The carrier plane can use wings, so it doesn't have to fight gravity directly - the wings provide considerable lift
Unfortunately, this only works for smallish rockets aiming for tourist flights to the edge of space, experimental telescopes that need to get above the atmosphere, to launch small satellites into low-earth orbit, or to attack satellites in low-Earth orbit.
See: https://en.wikipedia.org/wiki/Air_launch_to_orbit