Jet engine 1st stage

wrote in message
ups.com...
I am sure this topic has been covered here at some time but I missed
it.

Would it be feasible to use air breathing jet engines as the 1st stage
(or as strap-ons) of a launch system? Of course, that is exactly what
White Knight is but what about on a larger scale unmanned? This would
mean you do not have to lift the oxidizer for the 1st stage but max
altitude is limited. Re-use would be a problem if you drop em in the
ocean.

If you go the first 5 miles straight up with jet engines, you only have 195
miles left to low orbit. I heard it's an insignificant amount and a total
waist of time.

wrote:

Though modern carbon fibre technologies also make possible low cost,
custom aircraft, as proposed by T-Space:

Proposed projects are *always* low cost and high performance. When
metal is bent (or in this case, fibers glued), that may or may not
remain true.

D.
--
Touch-twice life. Eat. Drink. Laugh.

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL

Derek Lyons wrote:
wrote:

Though modern carbon fibre technologies also make possible low cost,
custom aircraft, as proposed by T-Space:

Proposed projects are *always* low cost and high performance. When
metal is bent (or in this case, fibers glued), that may or may not
remain true.

Not always. The A380 was expensive when proposed, and slightly more
expensive when completed (+~10%).

Carbon fibres, like plastics, however do lend themselves to low cost
low volume production. And a basic aircraft is actually a fairly simple
item. Rutan's design would basically be a scaled up glider with an
engine, and very large wheels.

"Derek Lyons" wrote in message
...
wrote:

Though modern carbon fibre technologies also make possible low cost,
custom aircraft, as proposed by T-Space:

Proposed projects are *always* low cost and high performance. When
metal is bent (or in this case, fibers glued), that may or may not
remain true.

How true. Just look at the space shuttle. ;-)

Seriously though, when it started flying, it couldn't launch the payload
mass advertised. It never met its projected high flight rate and was
therefore never able to meet it's low cost per flight goal.

Hope for the best, but plan for the worst.

Jeff
--
Remove icky phrase from email address to get a valid address.

Henry Spencer wrote:
Yes, but with the exceptions of air launch from an
existing aircraft, and systems which use the jets
for some other purpose as well, using rockets is better.

Mike Miller wrote:

That "for some other purpose as well" raises a
question that's been kicking around in my head
for a while.

If a launcher has jet engines for cruise back/landing,
what benefit can those engines deliver during launch?
Would it be enough to help minimize the weight penalty
of the engine and landing fuel?

There has been consensus in sci.space.tech for many
years that jet engines are too heavy as the replacement
for the first stages of rocket launchers. The most recent
attempt was Darpa's Rascal:
http://www.aviationnow.com/avnow/new...s/09223top.xml

The first stage of the Rascal was a steamjet, which
is a turbojet or a turbofan cooled with copious amounts
of water and liquid oxygen. It is described in U.S.
patent 6,202,404. Rascal failed because the idea does
not make sense. What makes sense, however, is either
using conventional airplane to quickly deliver small
rocket launcher to any place and launch it to any
orbit (Pegasus), or lifting a vacuum optimized rocket
launcher above the dense part of the atmosphere. The
latter idea is much more interesting because it can
substantially reduce the launcher cost.

As we all know, rocket launchers are shaped like
pencils to minimize their aerodynamic drag in dense
part of the atmosphere. The optimum shape of a rocket
flying in the vacuum is the opposite of the pencil shape
-- it is a disk having extremely large exhaust nozzle
exit area. The large exhaust nozzle exit area improves
thrust, exhaust gas velocity, or both. The bottom end of
the Soyuz launcher is conical to improve its performance
in the vacuum. Soyuz is a compromise. A much better
idea is to use a small first stage or a steamjet to lift
the disk shaped launcher above the dense part of the
atmosphere. Steamjet is better because it is reusable.
To minimize aerodynamic drag in the dense part of the
atmosphere, the disk shaped launcher flies through
the atmosphere sideways, like the rocket cluster:
http://www.islandone.org/LEOBiblio/SPBI1010.JPG

In article ,
wrote:
(Henry Spencer) wrote:
...
Design group after design group has come up with an elegant jet/rocket
design, and as an afterthought compared it to an all-rocket approach...
and been startled to discover that the all-rocket system looked to be
simpler, more capable, and cheaper both to develop and to operate.

Maybe this is drifting into sci.space.tech.psychology, but why does this
keep coming up? Is it because we are used to living at the bottom of
our ocean of air, and we think of air as a fairly dense and useful
substance?

Where as, at high altitude, there isn't enough air to be useful, it only
slows you down.

Is it a lingering "jet fighter" mentality in the design groups? Is it
that they all want to design highly-optimized, high-performance systems?
As opposed to systems which may not be optimal in some senses, but are
more practical and cost-effective.

Which begs the question, why don't we use them for aircraft? I'm not
trying to argue that we should, it's just that every time I see this kind
of discussion, that comparison is left hanging.

What is it about space launches that makes rockets better, and what is it
that is different about atmospheric flight that makes jets better? Is it
reusability? Fuel economy (and why doesn't that matter for space
launches)? The altitude/pressure thing (surely that wouldn't be such an
issue for first stages)? Weight (why isn't that a problem for aircraft)?
Raw peak thrust? Maybe it's the fact that space launches are so brief we
need less fuel?

At the speeds and altitudes that aircraft run, the air is useful. It is
dense enough to use as lift for wings. It has enough oxygen to use as
an oxidizer. It has enough density to use as reaction mass.

The thing to keep in mind is that rockets have to go really fast to have
their payloads reach orbit. You want to get _out_ of the atmosphere as
quickly as possible, so that it doesn't slow you down.

James Graves

"Nog" wrote in message
...
If you go the first 5 miles straight up with jet engines, you only have
195
miles left to low orbit. I heard it's an insignificant amount and a total
waist of time.

You didn't spell waste correctly. That and orbital velocity takes far more
energy to reach than orbital altitude.

Jeff
--
Remove icky phrase from email address to get a valid address.

wrote:
Henry Spencer wrote:

Yes, but with the exceptions of air launch from an existing aircraft, and
systems which use the jets for some other purpose as well, using rockets
is better.


That "for some other purpose as well" raises a question that's been
kicking around in my head for a while.

If a launcher has jet engines for cruise back/landing, what benefit can
those engines deliver during launch? Would it be enough to help
minimize the weight penalty of the engine and landing fuel?

I did some trajectory simulations that suggested that the cruise
back/landing jet seemed to be almost, kinda, somewhat free, *if* you use
them during launch. As in you didn't lose *performance* by replacing
rocket engines with jet engines (allowing for oxidiser mass and tankage
etc. etc.)

So you'd think that jet engines would be better, but it looked like it
wasn't worth taking off on pure jet engines, because the dry mass and
hence cost of the vehicle goes up (basically because jet engines'
thrust/weight ratio is relatively pathetic compared to rockets).

But having just enough jet engine to fly you back probably works better,
particularly if you use it during ascent.

One problem is that the flyback fuel comes directly out of the second
stage mass (because you have to carry it all the way to stage
separation), so a flyback booster or first stage is significantly bigger
than an expendable in other ways as well.

Precooled jet engines change the equation though- that means you can use
a flatter trajectory and gain more of the speed within the atmosphere
without burning out the jets. And they have a very high thrust/weight
ratio. So concepts like SABRE are probably a win (and look like they're
going to work.) But any bog-standard jet doesn't cut it.

Mike Miller

Andrew Nowicki wrote:
There has been consensus in sci.space.tech for many
years that jet engines are too heavy as the replacement
for the first stages of rocket launchers.

Andrew, that reply has nothing to do with my question.

Mike Miller

wrote in message
...
(Henry Spencer) wrote:
Which begs the question, why don't we use them [rocket engines] for
aircraft?

There might be some "aircraft missions" for which rocket power would make
sense. Some of the design principles of the Saenger antipodal bomber
(militarily impractical; briefly documented at
https://h30172.www3.hp.com/DT00605) might be reused in the design of a
hypersonic transport. Of course, that would be an "aircraft" only to the
extent that it used the atmosphere as a trampoline :-)


Jim McCauley