Space Forum / Space Flight / August 2003

There isn't really any point.
You end up with a rocket engine with very, very limited performance,
that's much heavier than standard.
There might be a very limited case for extending the altitude that it
will work at.

Despite the hype, rockets are less complex than jet engines, much
less complex.  A jet engine for a large aircraft (fighter jet or
jumbo jet) is itself the size of a large automobile, but a rocket
engine with the same thrust performance could easily sit on your
coffee table without straining it much.

Here's an example, the Pratt & Whitney model F100* jet engine,
designed for fighter jets and the engine which powers the F-15
and F-16C (among other aircraft) weighs 1900 kg (4100 lbs) and
outputs 145 kN (32,500 lb-f) maximum thrust.  For a thrust to
mass ratio of 78 N/kg.  Meanwhile, there's the X-405* LOX /
Kerosene rocket engine, of a decidedly older vintage (it
powered some of the stages of the Vanguard rocket), which weighs
191 kg and outputs 135 kN maximum thrust.  For a thrust to mass
ratio of 707 N/kg, nearly 10x that of the jet engine (and this is
for a rocket engine which is over 3 decades older than the jet).

Building rockets with jet engine thrust is no simple task, but
they can be bought at a reasonable price.  And, yes, they can be
made without too much difficulty either.  More importantly,
building a rocket engine designed not to maximize the thrust to
weight ratio as much as possible but simply to *exceed* the
exceedingly poor thrust to weight ratios (by comparison) of  jet
engines is a much easier task.  Which is why nobody even thinks
of using jet engines as you describe, it's simply not worth it.

An F-100 jet engine costs around $4.5 million dollars, an
NK-33 rocket engine costs around $2 million dollars but has
*eleven* times the thrust and weighs 1/3 less.

(*) http://www.pratt-whitney.com/3a/html/products_f100.html
(+) http://www.astronautix.com/engines/x405.htm