Bill,
Why then the huge first stage? Because they're not designing their
system for that one market, but for a range of markets.
As for the glide-back strap-ons (I /like/ that for a pop band name...)
sure it's doable. But, it's a whole separate new supersonic vehicle
development project. AKA an expensive distraction and a gross violation
of the KISS principle.
Henry
On 8/28/2019 10:57 AM, William Claybaugh wrote:
Henry:
Good point about suborbital markets: as I recall from pre-X33 studies, Edwards to Woomera was an about a Mach 22 problem; thus a single stage trans Pacific capability would be close to SSTO and might be enough for a lunar surface return mission from LEO. But why then the huge first stage?
Observe that there is no reason a strap-on can’t be fully reusable: glide-back RTLS is possible up to Mach 3 and jet back up to Mach 6.
Bill
On Wed, Aug 28, 2019 at 11:49 AM Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>> wrote:
Bill,
The correct person to ask these things is named Elon, and good
luck with that.
If you want my speculation on the matter however, I'm happy to
share. I might guess the big-picture answer to your question is
that, future markets being guessable but not knowable, SpaceX may
be aiming to maximize overall system versatility, rather than
optimizing for one particular mission. With the observation that
"a few strap-ons" is not nearly as obvious an option for someone
focused on high operability as it may be from a trad-space
perspective.
Speaking of versatility, and lunar missions aside, a few strap-ons
short of orbit is also not that bad a place to be for single-stage
long-distance Earth point-to-point, a market that SpaceX has been
openly courting.
If it all means that the LEO payload is, say, 100 tons rather than
120, them's the trades. (Figuring 20 tons extra tankage should be
good for 400-500 tons CH4-LOX at ~20-25:1 tank mass fraction.)
(I would expect that if in the long run LEO cargo is a big enough
market, a short-tank version of Starship might well then emerge.)
Henry
On 8/28/2019 9:48 AM, William Claybaugh wrote:
Henry:
I’m just asking....
If the upper stage is able to refuel in LEO and then do a full
lunar surface and return mission then it must be a near SSTO
vehicle on it’s own; which raises a question as to why it would
need that huge first stage...a few strap-on’s would be sufficient
to put it in LEO.
If the two stage vehicle is velocity optimized for LEO then the
second stage would require at least two refuelings to complete a
lunar surface mission (presumably one in LEO and one in LLO,
although other options exist).
Bill
On Wed, Aug 28, 2019 at 10:17 AM Henry Vanderbilt
<hvanderbilt@xxxxxxxxxxxxxx <mailto:hvanderbilt@xxxxxxxxxxxxxx>>
wrote:
Bill,
Keep in mind that the Wikipedia Starship dry mass and
propellant mass data I worked from was all labelled "needs
updating". In fact, the remarkably close agreement between
my 1111 tons of propellant needed for a LEO-lunar mission and
the Wiki listing of 1100 tons capacity leads me to suspect
the possibility that someone may have backfigured those Wiki
numbers from SpaceX's statements that Starship would be
capable of such missions.
I might guess from the utter public lack of hard data that
final stage masses and overall delta V split of that
two-stage system have not in fact yet been set by SpaceX. I
would not be surprised if they're waiting till they have more
flight data from the various prototypes (and possibly more
data as to who the most likely customers are) before they
finally pin all that down.
Which makes perfect sense to me. The more data you have when
you attempt to optimize a system, the better your chances are
of actually arriving at something close enough to optimum to
be economically viable.
That said, it occurs to me they may well make the decision to
include enough additional tankage on their upper stage to
allow it to fly lunar surface missions even if that extra
capacity isn't used on routine LEO launches, accepting the
additional dry mass penalty for the extra utility.
In fact, some such extra capacity is implicit in the idea of
being able to deliver to LEO either 100 tons dry payload or
100 tons propellant...
Henry
On 8/28/2019 7:16 AM, William Claybaugh wrote:
Jim, et al:
Help me out here: SpaceX’s vehicle is a two stage system.
If they have optimally splint delta-v then the upper stage
is good for about Mach 15, flying on it’s own. That is not
enough energy to get to the lunar surface and return.
It looks to me like the upper stage is probably capable of a
circum-lunar mission if fully refueled but is going to
require a second refueling to get to the surface and
return. Is that not correct?
Bill
On Wed, Aug 28, 2019 at 7:38 AM James Fackert
<jimfackert@xxxxxxxxx <mailto:jimfackert@xxxxxxxxx>> wrote:
Henry is the king of analysis on a table napkin!
So about 10 tanker flights to refuel a Starship to head
for the moon. Still no need for a permanent fuel depot.
First tanker parks in refueling position optimal for the
mission, 9 (?) more tanker flights meet it, dump their
payloads and go back for more.
Mission Starship meets it, tanks up and heads out, and
the prime tanker heads home for the next mission.
As has been pointed out, there is no optimal position
for a depot and no need to develop a specialized depot
until the traffic on the lunar rail line warrants it.
jim fackert
On 8/27/2019 8:51 AM, 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?
