In article <fn39c3$nmq$1@[EMAIL PROTECTED]
>,
Tim McDaniel <tmcd@[EMAIL PROTECTED]
> wrote:
>In article
<f858a0ec-27ab-4edd-8e25-3809c9f5efad@[EMAIL PROTECTED]
>,
>David E. Siegel <siegel@[EMAIL PROTECTED]
> wrote:
>>Correct me if I'm wrong, but isn't it also possible to do that on the
>>way up, by having your space vehicle take-off as a more or less
>>conventional airplane, and switch to rocket power only when above the
>>effective atmosphere?
>
>How do you save orbital velocity with any technology? A stable
>low-Earth orbit has a spee of 8 km/s, period, regardless of how you
>get it. You need 8 km/s by rocket, by jets, by giant slingshots, or
>whatever else. [....]
>
>If you mean "it can take less energy if you use airbreathing jets part
>of the way instead of rockets all the way": aside from that not being
>the point,
>
>That notion cropped up enough times in sci.space.tech in the years
>that I read it that it might be an FAQ. I think Henry Spenser
>debunked it several dozen times before he got tired of the effort.
>
>- The only possible saving is weight of oxidizer.
That's not true; the other big saving is reaction mass. Instead of
carrying all of the reaction mass onboard you can react against
the air using a wing, propeller, or turbofan. This is an *enormous*
advantage, since if reaction mass is free you can use low "exhaust"
velocities wih high flow rates (e.g. wings, rather than downward-
pointing rockets). Compare the relative fuel usage of a fixed-
wing aircraft maintaining a constant altitude, a hovering Harrier,
and a hovering rocket.
It still doesn't make spaceplanes worthwhile, I guess.
I think the reason the spaceplane concept is so attractive is that
people think of attaining orbit as being a problem of gaining
altitude. It isn't, mostly. As you rightly point out, it's a problem
of gaining horizontal velocity, and the required velocity is high
enough that you can't do that down here in the soupy-thick air.
If the air's thick enough to be useful for your engine or wing,
it's too thick to move through at orbital velocities (scramjets,
AURORA, etc., possibly notwithstanding).
That doesn't mean that rocket/airplane hybrids can't be at all
useful. Consider SpaceShip One and Two; they use an air-breathing
fixed-wing carrier to get to about 15 km, then rockets for the
other 85 km. But the jets and wings are left behind.
--
Wim Lewis <wiml@[EMAIL PROTECTED]
>, Seattle, WA, USA. PGP keyID 27F772C1
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