Accelerator Turbojet for SSTO

"John Schilling" wrote in message
...
"johnhare" writes:

Having studied the matter a bit, I would like to see if other people have
reached the same conclusion that I have concerning accelerator turbojet
engines.

An SSTO VTVL with Kero/LOX engines requires a mass ratio of 16+- to reach
LEO. If a mach 0 to mach 3 jet were added, the mass ratio could drop to
as
low as (optimistically) 8. Of a 100 ton rocket SSTO, 1 ton is engine
mass,
and 2 tons for tank mass totalling about 3 tons for propulsion system
mass.
The remaining 3.25 tons is guidance, payload, etc.

Are you including structure and aeroshell under "tank mass" or "etc" here?
There will be some overlap either way, but your gross numbers are OK.

Structure and undershell under etc, tank mass as 2% of fuel load.

Using dry mass as the figure of merit, the equivilent jet assisted SSTO
would have a GLOW of 50 tons with the available propulsion system mass
of
3 tons divided about equally between the rockets and jets. The 3.25 tons
other mass must remain unchanged for equality. With 1.5 tons available to
the turbojet system,

Only one ton, actually - you had two tons out of three for tankage in
the 100 ton rocket case, so you still need one ton of your three-ton
propulsion budget for tankage here.

The rocket mass also halves for the other half ton.

a 33.3 T/W ratio would be required of those engines just to hover.
Realistic acceleration for a VTVL would require a T/W of 67 of the air
breathing system just to make it performance competative with the rocket.
If one went HTHL, then the wings and gear would have to be charged off to
the jet system with a maximum mass for those two items of 1 ton for a 50
ton vehicle.

Careful here. Winged, airbreathing vehicles have to be done as integrated
designs. Which may be an argument against winged, airbreathing vehicles
in itself, on development complexity grounds. But the same wings that
enable jet propulsion at plausible weight, are the major components of
your recovery and landing system. And they reduce gravity losses a bit
during ascent, but increase drag losses, and they alter your re-entry
in a way that is both good and bad for the TPS designer.

For the SSTO, any excess mass that is required because of the
propulsion system should be charged that way, though your point
that some of it comes from the orriginal landing system is valid.

You can't just charge the wings against the jet propulsion mass budget.
The first-order solution is to charge the mass of the wings and wheels
minus the mass of whatever recovery and landing system you otherwise
would have used, against the jet propulsion system, and judging by
the HL/VL holy wars there will be disagreements over the *sign* of
that budget item.

Agreement.

But this doesn't change the conclusion. Even with free wings, the
turbojet needs a mass ratio of 40:1 or so, and there ain't no such
beast. My first rule of airbreathing propulsion is that you never,
ever try to send a turbocompresser assembly into space.

Possible disagreement based on some concepts of mine. 40/1 does
not exist, and won't by tweaking existing designs.

I don't buy ramjets as they become usefull only as aerodynamic heating
becomes a concern. The mass to handle that heating from mach 2-6 must
also be charged off to the jet system, and wings also for the flight
profile required for this type engine system.

Here, I start to disagree at the conclusion level. Again, a winged
airbreather is an integrated system without a seperate propulsion
mass budget. You only pay the marginal cost of wings and TPS beyond
what you'd otherwise be using on the return trip, which is tough to
determine.

When I BOTE such systems, though, the mass budget starts to close
easier with mach-6 ramjets whose T:W is anywhere above 25:1, and
that is a plausible figure for ramjets. If the state of the art
for ramjet development were anywhere near that for rockets or
turbojets, I'd recommend winged rocket/ramjet SSTOs unreservedly.
Trying to resurrect the lost art of ramjets, though, is a problem.

To me, ramjets are a study in intake design. If you can get a high
pressure recovery with low spillage and parasite drag across the
whole range of mach numbers, then we might agree. Otherwise
there is too much acceleration time in the heat soak range. IMO
of course.


My point, and question, is that an airbreathing accelerator system must
have
a T/W within a factor of 2 of the rocket accelerator system just to
compete
on SSTO vehicles. Leaving out economics of jet engine costs vs rocket
engine
costs, can anyone that has done the numbers find fault with this
conclusion?
A usefull jet assist needs a thrust to weight of 60 or more including
intakes and afterburners to match performance with rockets on an SSTO,
while
retainining a 4 digit fuel Isp.

Again BOTE, the T:W needs to be at least 120/M, where M is the range
of mach numbers over which you can run the engine with a four-digit
fuel Isp. This assumes a proper integrated design, where the wings, etc,
carry their weight on both ascent and descent.

No turbo-anything is going to fall above the 120/M line. Likewise
scram-anything, and of course any sort of combined-cycle whachamajig
that is hiding a turbo- or a scram- behind the name.

I think we might have some fun with this 120/M turbo. Scram nearly equals
scam to me. Physics does not prohibit the 120/M, just engineering and
material science, which for our purposes nearly amounts to the same thing.

Ramjets, ejector rockets, and combined-cycle systems based on same, are
potentially quite useful if anyone wants to go figure out how to build
them into a spaceship properly.

Discussing this is sort of the purpose of this thread. I will describe
a new turbo engine design in a new thread in a day or so. I wanted
to use this thread to preempt the airbreather to orbit to save LOX
guys, and to establish that I wasn't proposing such.

If you're dealing with a TSTO that splits the ascent delta-V evenly,
substitute 60/M for 120/M, and good turbojets are not wholly absurd.
They still won't carry their weight, so to speak, but they won't
kill the mass budget and the operational benefits for recovery and
ferry may tilt the balance in their favor.

At this time, the good turbojets don't exist for this purpose. I don't
believe that a pure accelerator design has been done anywhere as
opposed to highly efficient cruisers.

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Johm Hare