About reusable orbital stages, SSTO's or upper stages, I was surprised in
most discussions of them it assumed they are able to reenter to achieve an
only 100 m/s terminal velocity. This is true whether vertical or propulsive
landing is being discussed. See for example here:
http://yarchive.net/space/launchers/horizontal_vs_vertical_landing.html
It's also discussed here:
https://gravityloss.wordpress.com/2007/10/03/powered-landing-analytical-solution/
With gravity drag the delta=v that needs to be cancelled out would be
somewhat higher but still less than 200 m/s.
As the example of the space shuttle shows, this is not so surprising for
winged, horizontal landing. But the thing is this is assumed even for
vertical propulsive landing. This is surprising because this means for a
cylindrical rocket stage, without wings, reentering broadside or a conical
stage entering base forward, they can cancel out almost all of the ca.
7,800 m/s orbital velocity to get down to only 100 m/s terminal velocity
from aerodynamic drag alone.
Actually, I haven't seen the argument for this. Anyone have a reference for
the idea you can get down to 100 m/s terminal velocity even without wings?
But if that's so why all the big debate over vertical vs. horizontal
landing? For either method nearly all the 7.8 km/s orbital velocity will
already be cancelled out before either landing method comes into play, and
even then they only have to account for a *measly* 100 m/s.
That is surprising though that *without even needing wings* you can cancel
out nearly all of orbital velocity to get down to terminal velocity. Given
that, on Earth reentry for an orbital reusable isn't even particularly
hard. So why all the hullabaloo about how difficult it is to get full
reusability because you also need to return the orbital stage from orbital
velocity?
*In fact orbital stage reusability might even be easier than first stage
reusability:*
For the first stage, you need a reentry burn to cancel out forward
velocity, then a boostback burn to return to the launch site then you need
the final landing burn. This results in quite a bit of lost payload because
of the propellant that needs to be kept on reserve.
But for the orbital stage you just let Earth rotate beneath you until the
launch site is back under you before you initiate reentry, so you don't
need the boostback burn. Then aerodynamic drag cancels out nearly all of
orbital velocity even without wings so you don't need the reentry burn.
Then you only need the landing burn and this is for only ca. 100 m/s.
Yes, I know for the orbital case you need more thermal protection than for
the first stage but this is not particularly heavy anyway. Even for the
space shuttle, this was only ca. 8% of the landed weight, and SpaceX's
PICA-X weighs half that to only 4%. And if GW Johnson's thermal ceramic
<https://www.youtube.com/watch?v=3MXYY3jnNr0> really is as lightweight as
he estimates the thermal protection would be a minor portion of the vehicle
weight.
Then when you consider the either wing weight (with carbon composites) or
propellant weight needed for the final 100 m/s of the landing would be 5%
or less, the total lost payload for the orbital stage would be less than
10%. Compare this to the 30% to 40% lost payload with reusability for the
two stage Falcon 9 or BFR.
Taking these facts into account *a reusable SSTO may actually be more
economical than a reusable two stage vehicle.*
Bob Clark
On Sun, Oct 22, 2017 at 11:34 AM, Robert Clark <rgregoryclark@xxxxxxxxx>
wrote:
The SpaceX BFR tanker can serve as a reusable SSTO by switching to a
winged, horizontal landing mode:
SpaceX BFR tanker as an SSTO.
https://exoscientist.blogspot.com/2017/10/spacex-bfr-tanker-as-ssto.html
Bob Clark
On Sat, Oct 7, 2017 at 9:50 AM, Ken Biba <kenbiba@xxxxxx> wrote:
Thoughts? Particularly about economics and reusability.
*Elon Musk and SpaceX Want to Fly From NYC to Shanghai in 39 Minutes*
The Atlantic
Elon Musk’s proposed Earth transport system better stock barf bags. Read
the full story <https://apple.news/AcHtLtLC7Q2-RsSe_1gMcQQ>
Shared from Apple News <https://www.apple.com/news/>
Ken Biba
Novarum, Inc.
415-577-5496 <(415)%20577-5496>
