[AR] Re: SpaceX Single Stage to Orbit
- From: Craig Fink <webegood@xxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 29 May 2019 18:31:40 -0700
On Tue, May 28, 2019 at 2:05 PM Nels Anderson <nels.anderson@xxxxxxx> wrote:
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.
You touch on the other force that counters the "effective gravity"
Lift = AirdynamicLift + sin(Thrust) = - "effective gravity"
If pure rockets took advantage of this, their Thrust would decrease and
their ISP would increase as they approaches Orbit. I would say that rockets
essentially ignore what "effective gravity" is doing, because they can. I
know of only one case where the "effective gravity" was used properly,
there may be others. A Shuttle engine out Trans-Atlantic Abort, if the
remaining engines were left at full thrust, the Space Shuttle would not
make it to the landing site. But shut down another engine and throttled
down, the "time" of the burn could be lengthen to fight gravity loses and
make it.
Lift = 0.0 + sin(Thrust) = -"effective gravity"
But, for the most part rockets ignore this effect because acceleration is
going through the roof and they need to throttle down to reduce gee loads.
Flight to Orbit, it's advantageous to design the vehicle to use both
aerodynamic thrust to offset "effective gravity" and aerodynamic lift will
go to zero before orbit as the vehicle is still thrusting, so it's going to
have shed aerodynamic lift as some point. Roll to 90 degrees, aerodynamic
lift (z-axis) is zero. But, we're not through with lift yet if this vehicle
is going to make it to the Moon and Mars. It will have to point the lift
vector down to stay in the atmosphere and continue thrusting. Even
decreasing it's altitude for greater lift as it approaches a trans-lunar
injection.
This is how to achieve a 60 Minute Orbit of the Earth, the winner of the
award will be determined at some future date.
--
Craig Fink
WeBeGood@xxxxxxxxx
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