Towards a combined-cycle SSTO.

On Jan 26, 5:18*am, Robert Clark wrote:
Looking at the numbers I'm now convinced you can make a single stage
to orbit vehicle with a combined ramjet/rocket engine, and without
having to use scramjets.

AFAIK the only advantage to a jet mode is not having to carry
oxidizer for that part of the ascent within atmosphere dense enough
*at a specific velocity* to obtain sufficient oxygen to take the place
of the oxidizer. "Sufficient" tails off with height of course, so you
have to tailor velocity profile to density (modulo oxygen
concentration per altitude).

Rather than favoring a particular design a priori, it would seem
that the first step is determining the maximum altitude at which any
kind of air-breather will work *better than a rocket* at that
altitude. That's your potential final engine configuration before
going to pure rocket.

Anybody done that?

Then, determine the altitude and velocity domains in which a
specific type of jet is most efficient. Then, determine the
feasibility of combining the winners so they can transition from one
mode to the next *without awkward loss of thrust* during transition.

IOW, work backwards from a high-altitude efficiency benchmark to
determine what sort of engine you use to launch with. I'll mention
recalling reading that at sea level, piston engines are more efficient
than any kind of jet. I'd love to see a SSTO with props...

Heard of the Pulse Detonation Engine?

http://en.wikipedia.org/wiki/Pulse_detonation_engine

Still under research but very promising.

The idea is to combine the turbo-ramjet/rocket into a single engine.
This is what Skylon wants to do with their Sabre engine. But the Sabre
will use hypersonic airbreathing propulsion up to Mach 5.5 before the
rockets take over. This will require complicated air-cooling methods
using heat exchangers with flowing liquid hydrogen for the Skylon.

ISTM that as long as the air-cooling tech is passive and doesn't
introduce enough drag to offset the no-oxidizer-aboard advantage, and
the hydrogen cooling tech isn't as heavy as the oxidizer would have
been, then fine. Otherwise, no point.

But the above suggested analytical path may indicate it isn't
worthwhile to include either turbojet or ramjet mode.


Mark L. Fergerson

On 29/01/2012 5:14 PM, wrote:
On Jan 26, 5:18 am, Robert wrote:
Looking at the numbers I'm now convinced you can make a single stage
to orbit vehicle with a combined ramjet/rocket engine, and without
having to use scramjets.

AFAIK the only advantage to a jet mode is not having to carry
oxidizer for that part of the ascent within atmosphere dense enough
*at a specific velocity* to obtain sufficient oxygen to take the place
of the oxidizer. "Sufficient" tails off with height of course, so you
have to tailor velocity profile to density (modulo oxygen
concentration per altitude).

Rather than favoring a particular design a priori, it would seem
that the first step is determining the maximum altitude at which any
kind of air-breather will work *better than a rocket* at that
altitude. That's your potential final engine configuration before
going to pure rocket.

There doesn't appear to be a theoretical limit, so at any altitude,
there is a velocity at which an airbreathing engine will out perform a
rocket.

In practice, that velocity becomes unmanageably (and indeed unreachably)
high as the atmosphere thins into the interplanetary medium, but its
existence means that your approach isn't going to work - there is no
altitude at which no kind of air breather willl work better than a rocket.

It's an engineering problem, not a theoretical one.

Sylvia.

On Sun, 29 Jan 2012 23:27:46 +1100, Sylvia Else
wrote:


There doesn't appear to be a theoretical limit, so at any altitude,
there is a velocity at which an airbreathing engine will out perform a
rocket.

Actually, there is. Consider the simple ratio of the energy available
from the combustion process to the energy of the captured airstream.
On a per mass basis this is the heat of combustion divided by the
total enthalpy of captured airstream.

http://preview.tinyurl.com/87gz29g

See 3.1 Airframe Integrated Scramjet Design Challenges 1st paragraph.

On 30/01/2012 5:39 AM, me wrote:
On Sun, 29 Jan 2012 23:27:46 +1100, Sylvia Else
wrote:


There doesn't appear to be a theoretical limit, so at any altitude,
there is a velocity at which an airbreathing engine will out perform a
rocket.

Actually, there is. Consider the simple ratio of the energy available
from the combustion process to the energy of the captured airstream.
On a per mass basis this is the heat of combustion divided by the
total enthalpy of captured airstream.

http://preview.tinyurl.com/87gz29g

See 3.1 Airframe Integrated Scramjet Design Challenges 1st paragraph.

All that means, to my mind, is that the performance of an ideal air
breathing engine aproaches that of a rocket engine asymptotically from
above as velocity increases.

Sylvia.

On Mon, 30 Jan 2012 17:18:12 +1100, Sylvia Else
wrote:


All that means, to my mind, is that the performance of an ideal air
breathing engine aproaches that of a rocket engine asymptotically from
above as velocity increases.

Not sure by what rules your ideal system plays, but there is nothing
which says an air breather need make positive thrust.

On 1/02/2012 9:23 AM, me wrote:
On Mon, 30 Jan 2012 17:18:12 +1100, Sylvia Else
wrote:


All that means, to my mind, is that the performance of an ideal air
breathing engine aproaches that of a rocket engine asymptotically from
above as velocity increases.

Not sure by what rules your ideal system plays, but there is nothing
which says an air breather need make positive thrust.

In an ideal system there are no entropy gains. The captured airstream
undergoes adiabatic compression with an associated temperature increase.
This process is completely reversible, so that the the air can be
expanded with a lowering of temperature and return to its original
state. There would be no net force on the engine.

If heat is added between the compression and expansion, then the
expansion starts from a higher pressure, which can clearly be used to
generate thrust.

Achieving, or even approaching, an ideal system is not easy, of course,
but that is, as I said, and engineering problem.

Sylvia.