A vertical rocket launch seems to me that wastes most of its initial fuel/thrust as the exhaust is deflected sideways to atmosphere.

 

If such launch could be done from inside a 'barrel' as a bullet in a rifle, its own exhaust gases push would be way, way more efficient. Would it?

 

Do rockets launched from silos need much less fuel to overcome inertia ? Are their combustion gases vented or compressed within the silo until the rocket clears the surface opening ?

Conservation of momentum for the rocket applies for the gas right as it's leaving the exhaust. What happens afterward has no effect on this. If you're on a frictionless surface and you throw a rock, it doesn't matter if the rock hits a tree a meter away from you or travels on an uninterrupted path.

 

A rocket launch from a silo might have an advantage of pressure buildup behind the rocket as an additional force, but one must consider that there is a maximum acceleration many payloads can endure.

Externet is right. A significant part of the exhaust gases are going sideways. A nozzle must be designed for a specific surrounding pressure (atmospheric rockets have a different nozzle design than rockets for outer space).

 

If you use a barrel, this problem should be overcome. Obviously, you might get friction instead though.

 

This website (wikipedia) shows formulas and pictures of different nozzles.

This website (aero.org) seems even better.

I took "deflected" to mean what happens to the exhaust due to e.g. the lunch pad. If that's not what was intended then disregard that part of my response.

It would be more efficient, but the "launch pad" would be much more expensive, especially if you wanted accelerations that would not flatten people and just about anything else.

 

If you want efficiency, I'd go with an air breathing engine, or a nuclear powered rocket.

Thanks for the replies.

Yes, a fragile payload would not survive. Unless the thrusters are driven gently enough to be the gases the ones that would do the initial smoother propulsion with much less fuel payload.

The friction could be nearly eliminated, with a loose fit in a vertical bore inside a mountain, or a duct on its slope.

In the hobby of model rocket altitude record attempts (IE, max altitude given an Estes "A" engine or similar) there are what's known as "piston launchers." They do indeed provide a bit of a boost via more efficient use of fuel for those first few instants of flight. HOWEVER, as others have stated, they are much more complicated. In the "real world" (IE, world beyond the artificial limits of a competition) it is much cheaper/easier to simply have a slightly larger fuel tank.

 

A piston launcher...