[AR] Re: SpaceX Single Stage to Orbit
- From: Nels Anderson <nels.anderson@xxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Tue, 28 May 2019 17:04:54 -0400
On 5/28/19 12:06 PM, Craig Fink wrote:
Gravity Losses are a function of Time, so flying to orbit with an
air-breathing engine is going to be acceleration limited along the
way. Meaning more time spent doing it. But as the vehicle gets closer
and closer to orbit, the amount of lift necessary to offset the
"Effective Gravity" goes to zero at orbital insertion.
Effective Gravity = GravityEarth - CentripetalForce
So, the faster the vehicle flies, the required lift decreases to zero
at insertion.
It's a function of the product of time and the sine of the flight-path
angle (maybe Time with a capital T is defined as that product? :)). But
the reduced effective gravity benefits pure rockets, too, through
decreasing gravity and steering losses as they approach (circular)
orbit. If there's a case that air breathers have much lower gravity
losses, it's going to have to be made early in the trajectory, where
pure rockets are brute-forcing their way up. It seems perfectly
reasonable that air breathers would have lower gravity losses. What's
not clear to me is that they 1) would be *much* lower (in the context of
a total delta-V budget of 9000+ m/s), and 2) would not eaten up by
larger drag losses.
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