On Mar 21, 7:47 pm, Justin
wrote:
On 3/12/12 8:34 AM, wrote:

On Mar 7, 2:26 am, wrote
:
Let's bring SABRE into the mix. Would it be possible to desich a ro
cke
t
to use the atmospheric O2 when available and then switch to an on boar
d
supply?

Yes, it is possible. It won't be competitive with turbofans for
atmospheric cruising, though. Rockets and turbojets waste a lot of
horsepower that turbofans and turboprops capture.

Sorry for the delay, been traveling for job interviews!

I think I understand. A rocket wouldn't be cost effective for
atmospheric travel - even those SABRE's that I just wikied!
Why carry your O2 when you can pull it out of the air?



It might help to have some more explanation. I read your initial post
asking for a usable range, normal cruise speed, size, etc. But what do
you want to do with this rocket plane? (Don't hold back - I've posed
some odd thought experiments on usenet, too.)

It's embarrassing... But here goes!
I was at an interview in February and the guy I interviewed with has a
model plane on hie desk. It looked like a mini space shuttle. It tu
rns
out it was a "Farscape" toy! I downloaded a few episodes and I have to
say the show sucked, but that rocket plane mini shuttle thing looked
pretty neat.
From what you, Dave and Jeff said, something that small wouldn't be
able to carry enough fuel and O2. I was just wondering if we were even
remotely capable of a craft that small capable of air and space flight.
There was a scene where the pilot took the craft into an atmosphere and
did a dogfight.

A jet engine gets oxygen from the atmosphere, but much more
importantly, it gets reaction mass. Using rocket fuel at a high
exhaust velocity is much less efficient than using jet exhaust at
lower velocity. In fact, modern engines are fanjets - basically
shrouded propellers - and are twice as efficient as earlier turbojets
which had higher exhaust velocity.

John Halpenny