In article ,
Charles Talleyrand wrote:
The best bets, in my opinion, are (a) carbon-fiber or *possibly* nanotube-
composite structures, (b) innovative engine designs with rather better
performance than conventional approaches, and ...

In what way will these engines be better than the current ones?
I understand that the current engines opperate at a very large fraction of
the theoretical performance. So I assume you're talking about either lower
weight or lower cost. Is that correct?

"Performance" has a number of dimensions.

Current engines are not too far from the limits on Isp, although
incremental improvements remain possible and can make a substantial
difference to vehicle performance (because the relationship between
the two is very nonlinear).

Current engines are (in my opinion) *nowhere* *near* fundamental limits on
thrust/weight, even without magic materials like nanotube composites.
Improving that means lighter engines for the same thrust, or more thrust
in the same package. This matters both directly -- engine mass is a
significant part of the orbited dry mass -- and indirectly -- many RLV
concepts have center-of-gravity problems for reentry because of all that
engine mass in the tail.

The ability to operate efficiently over a wide range of altitudes (i.e.,
ambient pressures) would be very useful for a first-stage or SSTO engine.

Even such a small, mundane thing as being able to operate with very low
pump-inlet pressures -- that is, a reduced requirement for tank
pressurization -- could significantly ease vehicle design.

Manufacturing cost, maintenance workload, and working lifetime are all
important.

Reliability and robustness are important for costly, long-lived vehicles.
This insane business of safety factors of 1.25 or less has got to stop.

Is this also correct: you do not believe that concepts like ORTAG are
the way to go? Why? I have to admit the concept appeals to me.

There are limits to how far you can reduce costs with expendable rockets,
even mass-produced ones with cheap components. More subtly, there are
limits to how reliable they can be, since it is impossible to test-fly one
before entrusting a valuable payload to it. (Today's expendables have
failure rates that any other branch of transportation engineering would
class as criminal negligence, and the situation does not seem to be
improving significantly.)

As George has pointed out, they remain of some interest in the short term,
but they're not what people want in the long term.
--
MOST launched 30 June; first light, 29 July; 5arcsec | Henry Spencer
pointing, 10 Sept; first science, early Oct; all well. |