Jet powered rocket?

What would happen if you strapped a few super powerful jet engines onto a rocket body and filled it up with jet fuel? Wouldn't it work better then propellers? (Refering to topic : "Win X-Prize with a propeller?")

Or for that matter, would it be cheaper to use jet fuel instead of LOX and LH?
Maybe you would still have to use LOX or a ramjet, but wouldn't it be at least a little cheaper?

The problem with the jet engines is that they're very difficult to get to work properly closing speeds of M3 and altitude of 100.000 feet; that will not provide enough kinetic energy to reach 100 km. A quick calculation with the energy conservation equations:

Assuming we have achieved mach 3 at an altitude of 30.000 meters (a VERY notable accomplishment for any home-built!) . Mach 3 at 30.000 meters equals 915 m/s

The equations for kinetic and potential energy are

Ek= 1/2* mv^2

Ep=mgy

respectively,
where

m = mass
v = velocity
y = altitude

Combining equations we get

mgy = 1/2 * mv^2

from which we get the altitude

y = mv^2 / 2*mg :where the masses cancel each other out;
y = v^2 / 2*g
y = 915^2 / 2*9,81
y = 837225 / 19,62
y = 42672,02 m

Which added to initial altitude, 30.000, equals to 72.672,01 meters, which is not even close and still the equations doesn't even take the air resistance into consideration.

J-58 engines (SR-71 engines) bypass the engine core to overcome the mach number problem, using the afterburner as a ramjet, which just manages to mach 5. At such speed, the vehicle could, with no air resistance taken into account, reach about 150 km altitude... however, a mach 5 jet is not the optimum solution for suborbital flight, I'd think! One thing is that jet engines capable of propelling any mass into such velocities is very large and heavy, with rather adverse effects to the air resistance and payload.

But a ramjet could serve as a very efficient first stage--I believe that the SR-71 can get up to about 30 km--then let the rockets take over from there. The advantage is not having to carry the oxidizer for the first stage, noting that a large fraction of the energy is spent getting through the lower atmosphere. The disadvantage is the need for a winged first stage. The solution is to build a "flying wing" configuration--a huge fuel tank shaped like a wing. Easier said than done, of course. Let the mechanical engineers take over from here.

The initial post tries to compare the application of propellers by a rocket to the application of a jet engine by a rocket.

The first alternative already was realized by the Rotary Rocket while the other never was tried really as far as I remember. But what abot a jet-driven White Knight?

And what about air launch by a propeller driven unit that can eave the point of releasing the rockt as quickly as required? What about this in comparison to the jet-powered White Knight?



Dpl.-Volkswirt (bdvb) Augustin (Political Economist)

Check out the SABRE engines. Jet, Ramjet, then Rocket.