View Single Post
  #7  
Old October 11th 19, 12:25 PM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 2,307
Default A conversation with Elon Musk

In article , says...
It used to be that trying to save rocket fuel was a sure way to increase
overall costs. But now it seems that SpaceX will be so successful in
decreasing all other costs, that saving fuel becomes a good target for
reducing overall costs. Of course that is assuming that Starship-Super
Booster lives up to expectations.


Kind of, but not really when the big decisions were made. SpaceX's
choice of 301 stainless steel as the material for building
Starship/Super Booster goes completely against the typical motivation of
aerospace engineers who have minimized dry mass in order to maximize
payload. That's why they started testing very large composite tanks
initially and had already paid a lot of money (no doubt millions of
dollars) for a huge cylindrical jig for making composite sections for
BFR/BFB.

It's not clear yet how much mass they'll be able to shave off the
"final" design of Starship/Super Booster. Musk has said that the Mk 1
and Mk 2 Starships (that will fly to 20k feet and test aerodynamics and
landing) mass 200 tons! For comparison, the dry mass of the space
shuttle orbiters was on the order of 86 tons. So, Starship Mk 1 and Mk
2 will mass over twice what a shuttle orbiter massed! He said they'll
refine the design of later Starship prototypes based on flight data, and
the like. He mentioned 120 tons, but then he also said he'd be *very*
happy if they could get the mass to under 100 tons (obviously an
aspirational Musk goal that may or may not ever be met). So even "mass
optimized" Starships are going to be quite a bit heavier than the space
shuttle orbiters which were primarily made of aluminum with some
titanium.

But, as Musk also said, raw materials for carbon composite structures
cost a *lot* more than commonly available 301 stainless steel. It's
also a lot more time consuming to build a large carbon fiber structure
(just look at how long it takes Virgin Galactic to build a Spaceship
Two). I'd wager that's a far bigger cost savings over say 100 flights
than the tiny savings in propellant you'd get by doing an all carbon
fiber structure. And 301 stainless steel is easier to maintain (you can
cut out bad pieces and weld in new pieces relatively easily). Carbon
fiber is a lot harder to "fix" when it breaks to the point that you
might as well scrap the whole thing and start over.

So, IMHO, we're still in an era where saving money on the vehicle build
and maintenance dominates propellant costs.

The devil is in the details here. Musk said that SpaceX ditching carbon
fiber for 301 stainless steel for Starship/Super Booster is probably the
best decision he's ever made.

When you have a reusable rocket, it can make sense to increase the
cost of the rocket to save fuel because you pay the rocket once,
the fuel many times. Obviously, even with a reusable rocket, you
don't go back to performance über alles. But increasing the price
of Super Booster by a few millions to save a few percent fuel
might make sense.


True, but early Mk Starships won't fly very many times, IMHO. So making
those sorts of optimizations to them just doesn't make as much sense as
flying them earlier knowing that they're likely a few tons "overweight".
Gaining flight experience necessary to gather more data will allow for
better optimization later.

This is the same sort of thing we saw with Falcon. The design iterated
over time based on actual flight data and post flight inspections.

I guess the key difference here is that you want to do your optimization
at the right time in the development cycle. NASA and other traditional
aerospace contractors will spend years doing computer simulations and
optimizations to shave mass off a design (often getting lost in the
weeds which is called "analysis paralysis"). Traditionally, these
organizations also have "critical design reviews" and fix the design
well before even flying the first copy!

SpaceX simply doesn't do that. They fly early and often with a "minimum
viable product" and then tweak the design based on what they see from
data in flight and from post flight inspections. That prevents
"surprises" in case your simulations don't line up perfectly with
reality, as is often the case. It also eliminates the guesswork in
simulations when you really don't know your boundary conditions very
well (e.g. actual aerodynamic and thermal loads in flight).

Actually, I'm sure we're mostly in agreement here. I think that
eventually SpaceX will worry more about propellant costs than they do
today. But I'm thinking that time won't be for a decade or more. Once
they actually fly a single Starship 100x, maybe they'll worry a bit more
about propellant costs.

But then again, Elon Musk is still the "chief engineer" and has said he
wants to eventually make the methane and liquid oxygen on earth using
solar energy (instead of fossil fuels), which might end up costing
SpaceX more money for propellant than if they'd just bought them in
quantity. Musk is always pushing the state of the art even if it means
dumping a bunch of money into the development of new technologies that
"traditional" companies simply don't want to touch.

Jeff
--
All opinions posted by me on Usenet News are mine, and mine alone.
These posts do not reflect the opinions of my family, friends,
employer, or any organization that I am a member of.