Which gets back to my statement that, while they may not be planning to
build one, if someone offered them orbital propellant storage at the
right price, they'd probably buy it.
On 2019-08-27 08:51, Henry Vanderbilt wrote:
RE the commonly-assumed one tanker flight per Starship Lunar
mission... When in doubt, run the numbers. Delta V = LN(MR) x Vexh
isn't just a good idea, it's the law. (That's natural logarithm of
the ratio of the ship's pre-all-burns-mass/post-all-burns-mass, time
the rocket motor's exhaust velocity, equals the overall ship's
velocity change. All you need is a calculator with an Ln key and you
too can play rocket scientist on the internet!)
That said, ahem, I believe SpaceX's proposed LEO-Lunar missions for
refueled Starship will require multiple tanker flights.
LEO-Lunar surface requires delta V in the neighborhood of 5.5 km/s,
the return roughly 3 km/s less with aerobraked reentry. Ship data:
Wikipedia figures (all of which are labeled "needs updating") say
Starship dry mass is 85 tons, full propellant load 1100 tons, LEO
payload 100 tons. And a good LOX-CH4 engine should manage a vacuum
Isp around 360 seconds, AKA exhaust velocity around 3.5 km/s.
So let's assume we're delivering the max 100 ton payload to Luna and
coming back empty. So we break the trip down into two parts.
We'll calculate coming back first, because the propellant we'll need
to carry for that has to be part of the outbound leg number.
With 3.5 km/s exhaust velocity, the rocket equation says to reach 2.5
km/s delta V to return from Luna to Earth reentry, you need mass ratio
2. So for an 85 ton Starship, that's 85 tons of propellant. Not bad
so far, less than one 100-ton Starship payload earth-to-LEO.
But for the outbound LEO-to-Lunar-surface leg, our total ship mass at
landing on Luna has to be 85 tons of Starship plus 100 tons of payload
plus 85 tons of return propellant, 270 tons. And for the outbound
delta V of 5.5 km/s, we need a mass ratio of 4.8.
(Propellant+delivered-mass/delivered-mass) So, 3.8 times 270 tons
equals 1026 tons of additional propellant in LEO. With the 85 tons
return propellant, 1111 total tons of propellant in LEO at the start
of our Lunar trip. Given we're using ballpark numbers, that's
remarkably close to Starship's listed max propellant load of 1100
tons.
So, that's 11 Starship tanker flights at 100 tons propellant each to
support one full-load Starship flight to Luna. You might cut that to
9 or 10 flights by using a stripped-down version of Starship as
tanker, but at 85 tons dry for that capability there's not a lot to
strip.)
My take, mentioned earlier, is that SpaceX is skipping a LEO
propellant depot for these missions not because it wouldn't make sense
for them - having a loaded crewed Starship hanging around in LEO
through ~10 tanker arrivals is both a cost and a risk - but because a
tanker version of Starship is less current load on their already
highly-loaded engineering (and likely also fiscal) bandwidth than a
separate depot.
I think this is a good business decision, mind - time enough to spend
resources on a LEO propellant depot when they get to the point where
the costs of ~10 tanker rendezvous per Lunar flight start becoming
obvious.
Henry
On 8/27/2019 7:48 AM, James Fackert wrote:
Maybe the orbiting fuel depot as a separate entity is not optimal.
The fuel depot implies that there is a requirement for a much larger
supply of fuel than a single tanker can bring up.
SpaceX's plan seems to be to launch a big tanker to an appropriate
orbit, then launch the ship that needs refueling to meet it, tank up
and move along to whatever destination it is headed.
Tanker returns home, to load up to supply the next mission.
When you have a big ship with lots of supplies and living space and
stuff for a real long term mission rather than a touch and go, you
also have a ship capable of being a tanker that can bring enough
fuel in one flight to refuel that ship for an extended lunar or
interplanetary mission and return.
Now all you need is a nice big reusable booster to loft those guys.
Sound like a plan?
