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A conversation with Elon Musk



 
 
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  #1  
Old October 9th 19, 05:52 AM posted to sci.space.policy
[email protected]
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Posts: 654
Default A conversation with Elon Musk

On Thursday, October 3, 2019 at 6:49:33 AM UTC-7, David Spain wrote:

Nozzle design helps to. It would appear to have sea level *and* vacuum
optimized nozzles is the win over aerospikes at least for a TSTO
vehicle. Though Elon remains open to any "gifts", in this regard.

Dave



What happened to nozzle extensions?

"For first stage rocket engines, the engine works with nozzle extension in
disposed position during the first minutes of flight and expands it at some
predefined level of air pressure. This scheme assumes the outer skirt of the
bell is extended while the engine is functioning and its installation to working
position happens in the upper layers of the atmosphere. It excludes problems
with flow separation at sea level and increases efficiency of the engine in
vacuum."

See:

https://en.wikipedia.org/wiki/Nozzle_extension
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  #2  
Old October 9th 19, 12:50 PM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 1,994
Default A conversation with Elon Musk

In article ,
says...

On Thursday, October 3, 2019 at 6:49:33 AM UTC-7, David Spain wrote:

Nozzle design helps to. It would appear to have sea level *and* vacuum
optimized nozzles is the win over aerospikes at least for a TSTO
vehicle. Though Elon remains open to any "gifts", in this regard.

Dave



What happened to nozzle extensions?

"For first stage rocket engines, the engine works with nozzle extension in
disposed position during the first minutes of flight and expands it at some
predefined level of air pressure. This scheme assumes the outer skirt of the
bell is extended while the engine is functioning and its installation to working
position happens in the upper layers of the atmosphere. It excludes problems
with flow separation at sea level and increases efficiency of the engine in
vacuum."

See:

https://en.wikipedia.org/wiki/Nozzle_extension

Added complexity. It's non-trivial to extend while firing the engine.
For a vehicle like Starship/Super Booster which stages "early" compared
to its expendable counterparts, I'd imagine it's harder than designing
an extension for a fully vacuum optimized engine. For example, the RL-
10 engines with extensions like this extend *before* the engine starts
firing. This takes a bit of time, so wouldn't be a good trade for an
upper stage that stages early due to gravity losses during the nozzle
extension period.

All IMHO of course.

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.
  #3  
Old October 9th 19, 04:09 PM posted to sci.space.policy
David Spain
external usenet poster
 
Posts: 2,593
Default A conversation with Elon Musk

On 2019-10-09 7:50 AM, Jeff Findley wrote:
In article ,
says...

On Thursday, October 3, 2019 at 6:49:33 AM UTC-7, David Spain wrote:

Nozzle design helps to. It would appear to have sea level *and* vacuum
optimized nozzles is the win over aerospikes at least for a TSTO
vehicle. Though Elon remains open to any "gifts", in this regard.

Dave



What happened to nozzle extensions?


Added complexity. It's non-trivial to extend while firing the engine.
For a vehicle like Starship/Super Booster which stages "early" compared
to its expendable counterparts, I'd imagine it's harder than designing
an extension for a fully vacuum optimized engine. For example, the RL-
10 engines with extensions like this extend *before* the engine starts
firing. This takes a bit of time, so wouldn't be a good trade for an
upper stage that stages early due to gravity losses during the nozzle
extension period.

All IMHO of course.

Jeff


I might add that it appears from what I last read that Elon plans to
attach the vacuum Raptors on Starship to the "airframe",

https://twitter.com/elonmusk/status/1131433322276483072

with the sea level Raptors being able to gimbal but the vacuum ones do
not. So real complexity there to try to add extensions to the sea level
Raptors which need to also move. Also consider cost. It might actually
cost *more* to put extensions on the SL Raptors than just add Raptors
dedicated to vacuum operation given the fact that the engine mfg. is
already vertically integrated into your company and therefore by
definition your are obtaining the engines *at cost*. As I understand it
these engines are somewhat cheaper to build for SpaceX anyway. So it
makes sense to me to just use more...

Dave
  #4  
Old October 10th 19, 01:54 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 385
Default A conversation with Elon Musk

On Oct/9/2019 at 11:09, David Spain wrote :
On 2019-10-09 7:50 AM, Jeff Findley wrote:
In article ,
says...

On Thursday, October 3, 2019 at 6:49:33 AM UTC-7, David Spain wrote:

Nozzle design helps to. It would appear to have sea level *and* vacuum
optimized nozzles is the win over aerospikes at least for a TSTO
vehicle. Though Elon remains open to any "gifts", in this regard.

Dave


What happened to nozzle extensions?


Added complexity.* It's non-trivial to extend while firing the engine.
For a vehicle like Starship/Super Booster which stages "early" compared
to its expendable counterparts, I'd imagine it's harder than designing
an extension for a fully vacuum optimized engine.* For example, the RL-
10 engines with extensions like this extend *before* the engine starts
firing.* This takes a bit of time, so wouldn't be a good trade for an
upper stage that stages early due to gravity losses during the nozzle
extension period.

All IMHO of course.

Jeff


I might add that it appears from what I last read that Elon plans to
attach the vacuum Raptors on Starship to the "airframe",

https://twitter.com/elonmusk/status/1131433322276483072

with the sea level Raptors being able to gimbal but the vacuum ones do
not. So real complexity there to try to add extensions to the sea level
Raptors which need to also move. Also consider cost. It might actually
cost *more* to put extensions on the SL Raptors than just add Raptors
dedicated to vacuum operation given the fact that the engine mfg. is
already vertically integrated into your company and therefore by
definition your are obtaining the engines *at cost*. As I understand it
these engines are somewhat cheaper to build for SpaceX anyway. So it
makes sense to me to just use more...


I think that SpaceX didn't do much optimisation of its rockets. They
could be improved quite a lot. As you say it isn't obvious that
extending the nozzle in flight is the way to go, but there is a good
chance that it would improve performance. There are many other things
that could be done to improve performance. And I think that such work
will be done in the not too far future.

Now don't read me wrong. I'm not complaining that SpaceX didn't do it
the right way. The main problem in rocketry was that rockets were used
once. The important thing to do was to make them reusable. If cars were
thrown away once the fuel in the gas tank is all used, the important
thing to do wouldn't be to make a hybrid electric/gas car that can run
longer on that single tankful of gas. You don't make the car more
expensive so it can run longer on the limited fuel. You make the car
reusable. But once that is done, yes making the car more fuel efficient
is important. Until recently, the cost of fuel in a rocket launch was
irrelevant, something like 0.1%. SpaceX is now making it relevant
because all other costs have gone way down.


Alain Fournier
  #5  
Old October 10th 19, 11:42 AM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 1,994
Default A conversation with Elon Musk

In article , says...
I might add that it appears from what I last read that Elon plans to
attach the vacuum Raptors on Starship to the "airframe",

https://twitter.com/elonmusk/status/1131433322276483072

with the sea level Raptors being able to gimbal but the vacuum ones do
not. So real complexity there to try to add extensions to the sea level
Raptors which need to also move. Also consider cost. It might actually
cost *more* to put extensions on the SL Raptors than just add Raptors
dedicated to vacuum operation given the fact that the engine mfg. is
already vertically integrated into your company and therefore by
definition your are obtaining the engines *at cost*. As I understand it
these engines are somewhat cheaper to build for SpaceX anyway. So it
makes sense to me to just use more...


I think that SpaceX didn't do much optimisation of its rockets.


This is a loaded statement right off the bat. Firstly, what are you
optimizing for? When people start talking extending nozzles and the
like, they're likely talking about optimizing performance. O.k., so
we're back to minimizing propellant mass and/or minimizing dry mass like
an expendable? Why?

Someone did the calculations to figure out the cost of propellants for a
single Starship/Super Booster flight. The final cost was on the order
of $1 million. Think about that. Why would you try to minimize
propellant mass? Let's say you shave off 10% of that mass. You've
saved on the order of $100,000 per flight. But at what cost? And by
cost, I mean money. If it's by adding an extensible nozzle, what costs
does that add in terms of cost of the added complexity in terms of the
initial build and in terms of ongoing maintenance and testing (because
it's got to work right because you don't want to risk a failed extension
damaging a nearby engine).

Secondly, as I've said on this group before, we're looking at Mk1 and
Mk2 right now. Musk has already admitted they *aren't* optimized. Why?
Because they're the early test vehicles used to gather data which will
be used to refine the design! Actual flight data is always a *good
thing* to have.

NASA was able to use flight data from Columbia to refine the designs of
the later orbiters by making them lighter. Same thing will happen with
Starship/Super Booster but I'd imagine, based on Falcon experience, that
we'll see far more iterations of the design than the space shuttle
orbiters that only saw minor tweaks to the design as a result of the
data gathered by Columbia.

They
could be improved quite a lot. As you say it isn't obvious that
extending the nozzle in flight is the way to go, but there is a good
chance that it would improve performance.


And there is a certainty that it will increase costs because you're
adding moving components that must work and not fail, increasing
complexity, and increasing costs. Will those increased costs cover the
cost of propellant saved? Doubtful, IMHO, since we've already figured
out propellant costs and they're still small compared to all other costs
involved in launching and maintaining Starship/Super Booster.

There are many other things
that could be done to improve performance. And I think that such work
will be done in the not too far future.


This I agree with. But, I seriously doubt extensible nozzles will be
one of them.

Now don't read me wrong. I'm not complaining that SpaceX didn't do it
the right way. The main problem in rocketry was that rockets were used
once. The important thing to do was to make them reusable. If cars were
thrown away once the fuel in the gas tank is all used, the important
thing to do wouldn't be to make a hybrid electric/gas car that can run
longer on that single tankful of gas. You don't make the car more
expensive so it can run longer on the limited fuel. You make the car
reusable. But once that is done, yes making the car more fuel efficient
is important. Until recently, the cost of fuel in a rocket launch was
irrelevant, something like 0.1%. SpaceX is now making it relevant
because all other costs have gone way down.


I'd argue we're still a long way from propellant costs being a dominant
cost for orbital launch.

Still, SpaceX chose a propellant combination that's relatively cheap and
has several advantages over the alternatives. Liquid hydrogen is more
expensive to buy, handle, and store (because of its very low bulk
density) than liquid methane. Liquid methane results in a smaller
launch vehicle, compared to liquid hydrogen, because it's more dense
than liquid hydrogen. Merlin's kerosene is cheap, but it is harder in
terms of inspection and maintenance due to its ability to coke in
cooling passages and the like in a liquid fueled rocket engine. So,
liquid methane is likely a "sweet spot" for rocket engine fuel,
especially for lower stages.

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.
  #6  
Old October 11th 19, 01:30 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 385
Default A conversation with Elon Musk

On Oct/10/2019 at 06:42, Jeff Findley wrote :
In article , says...
I might add that it appears from what I last read that Elon plans to
attach the vacuum Raptors on Starship to the "airframe",

https://twitter.com/elonmusk/status/1131433322276483072

with the sea level Raptors being able to gimbal but the vacuum ones do
not. So real complexity there to try to add extensions to the sea level
Raptors which need to also move. Also consider cost. It might actually
cost *more* to put extensions on the SL Raptors than just add Raptors
dedicated to vacuum operation given the fact that the engine mfg. is
already vertically integrated into your company and therefore by
definition your are obtaining the engines *at cost*. As I understand it
these engines are somewhat cheaper to build for SpaceX anyway. So it
makes sense to me to just use more...


I think that SpaceX didn't do much optimisation of its rockets.


This is a loaded statement right off the bat. Firstly, what are you
optimizing for? When people start talking extending nozzles and the
like, they're likely talking about optimizing performance. O.k., so
we're back to minimizing propellant mass and/or minimizing dry mass like
an expendable? Why?

Someone did the calculations to figure out the cost of propellants for a
single Starship/Super Booster flight. The final cost was on the order
of $1 million. Think about that. Why would you try to minimize
propellant mass? Let's say you shave off 10% of that mass. You've
saved on the order of $100,000 per flight. But at what cost? And by
cost, I mean money. If it's by adding an extensible nozzle, what costs
does that add in terms of cost of the added complexity in terms of the
initial build and in terms of ongoing maintenance and testing (because
it's got to work right because you don't want to risk a failed extension
damaging a nearby engine).

Secondly, as I've said on this group before, we're looking at Mk1 and
Mk2 right now. Musk has already admitted they *aren't* optimized. Why?
Because they're the early test vehicles used to gather data which will
be used to refine the design!


That's exactly what I was saying. They didn't do much optimisation. So
Musk agrees with me.

Actual flight data is always a *good
thing* to have.

NASA was able to use flight data from Columbia to refine the designs of
the later orbiters by making them lighter. Same thing will happen with
Starship/Super Booster but I'd imagine, based on Falcon experience, that
we'll see far more iterations of the design than the space shuttle
orbiters that only saw minor tweaks to the design as a result of the
data gathered by Columbia.

They
could be improved quite a lot. As you say it isn't obvious that
extending the nozzle in flight is the way to go, but there is a good
chance that it would improve performance.


And there is a certainty that it will increase costs because you're
adding moving components that must work and not fail, increasing
complexity, and increasing costs. Will those increased costs cover the
cost of propellant saved? Doubtful, IMHO, since we've already figured
out propellant costs and they're still small compared to all other costs
involved in launching and maintaining Starship/Super Booster.

There are many other things
that could be done to improve performance. And I think that such work
will be done in the not too far future.


This I agree with. But, I seriously doubt extensible nozzles will be
one of them.

Now don't read me wrong. I'm not complaining that SpaceX didn't do it
the right way. The main problem in rocketry was that rockets were used
once. The important thing to do was to make them reusable. If cars were
thrown away once the fuel in the gas tank is all used, the important
thing to do wouldn't be to make a hybrid electric/gas car that can run
longer on that single tankful of gas. You don't make the car more
expensive so it can run longer on the limited fuel. You make the car
reusable. But once that is done, yes making the car more fuel efficient
is important. Until recently, the cost of fuel in a rocket launch was
irrelevant, something like 0.1%. SpaceX is now making it relevant
because all other costs have gone way down.


I'd argue we're still a long way from propellant costs being a dominant
cost for orbital launch.


Yes we are a long way from propellant costs being dominant. But until
now, propellant costs were totally insignificant. That's no longer the
case. The costs of developing and building the first Starship-Super
Booster is not known but seems to be in the low billions. I would guess
that the great majority of that is in the developing part not the
building part. This is just a guestimate, but I would say that building
a Starship-Super Booster, after development is paid for, will be in the
hundreds of millions. If I recall correctly, SpaceX wants to fly them
about 100 times each. Assuming they are successful in flying them 100
times (a big assumption) the cost of the rocket hardware should be a few
millions per flight. That's the same order of magnitude as the cost of
the fuel. It used to be that the costs of the rocket totally dominated
the launch costs. Now the costs of the fuel which used to be totally
insignificant is of the same order of magnitude as the cost of the
rocket (per launch). There are other costs for the launch. SpaceX is
supposedly trying to minimise those other costs too, and from what I can
tell they are successful at it.

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.

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.

Still, SpaceX chose a propellant combination that's relatively cheap and
has several advantages over the alternatives. Liquid hydrogen is more
expensive to buy, handle, and store (because of its very low bulk
density) than liquid methane. Liquid methane results in a smaller
launch vehicle, compared to liquid hydrogen, because it's more dense
than liquid hydrogen. Merlin's kerosene is cheap, but it is harder in
terms of inspection and maintenance due to its ability to coke in
cooling passages and the like in a liquid fueled rocket engine. So,
liquid methane is likely a "sweet spot" for rocket engine fuel,
especially for lower stages.


Yes, I like their choice of fuel. I like their idea of making reusable
rockets much more, but their choice of fuel makes a lot of sense.


Alain Fournier
  #7  
Old October 11th 19, 12:25 PM posted to sci.space.policy
Jeff Findley[_6_]
external usenet poster
 
Posts: 1,994
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 ber 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.
  #8  
Old October 12th 19, 01:13 AM posted to sci.space.policy
Alain Fournier[_3_]
external usenet poster
 
Posts: 385
Default A conversation with Elon Musk

On Oct/11/2019 at 07:25, Jeff Findley wrote :
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 ber 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.


Yes we're mostly in agreement here. I think they will look at propellant
costs a little earlier than you. I think that when they will have flown
a single Starship 10 times AND, after checking it out, think that it
will be good to fly 100 times, they will start looking at propellant
costs. That's just start looking at propellant costs, not make it the
most important priority. Until then they will be looking at improvements
that will probably be needed to get to that 100 flights, I don't think
the first iteration will do it.

They will probably also be optimising other things like work needed to
be done between landing and relaunch before propellant. But ultimately,
the objective is to make the thing more like aircraft operations, and
air plane manufacturer work hard to save fuel. Once again, we're mostly
in agreement here.


Alain Fournier
  #9  
Old October 14th 19, 05:06 PM posted to sci.space.policy
David Spain
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Posts: 2,593
Default A conversation with Elon Musk

On 2019-10-14 2:52 AM, JF Mezei wrote:
On 2019-10-10 06:42, Jeff Findley wrote:

This I agree with. But, I seriously doubt extensible nozzles will be
one of them.


Wouldn't 3 engines with variable nozzle weight less and cost less than 6
engines (3 optimized for vacuum and 3 for sea level) ?


Weigh less, yeah there is a distinct possibility. Cost less? No. For
three reasons. One the NRE to design them, two the cost to retrofit the
Raptor to accommodate them and three they wouldn't be necessary for
Super so now you have a specialized nozzle just for Starship and
produced in far less quantity that that of sea level Raptor. Thus losing
out on the saving to be had in mass manufacture of the same nozzles.

And there is another reason. A variable geometry nozzle optimized for
Earth won't be of much help on Mars. So now you are talking about yet
another design. Here's a thought tho. Maybe you take another set of Mars
optimized nozzles along as cargo and swap out the Earth nozzles once on
Mars. Of course, over optimization is the bane of engineering. If it
isn't necessary, why do it? Elon has raised this point again and again.
I'm not going to second guess him.

Dave


  #10  
Old October 14th 19, 05:30 PM posted to sci.space.policy
David Spain
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Posts: 2,593
Default A conversation with Elon Musk

On 2019-10-14 3:45 AM, JF Mezei wrote:
On 2019-10-11 07:25, Jeff Findley wrote:

But, as Musk also said, raw materials for carbon composite structures
cost a *lot* more than commonly available 301 stainless steel.


Caveat: the comparison was the cost per pound of material not the cost
to build a part of X strengtn with one or the other.

Since you need fewer pounds of carbon fibre to achieve the same strength
as steel, the extra cost of carbon fibre is _not as bad_ as Musk made it
look. Steel is likely cheaper, but you end up with it being heavier.


That is really a remarkable comment. Not as bad? How do you know? You
building Starships?

Here's some interesting figures quoted from Tim Dodd:

https://everydayastronaut.com/stainless-steel-starship/


"With carbon composite, you need to cut the fabric, impregnate it with
high-strength resin, which can be difficult and then make 60 to 120
layers! There’s also approximately a 35% scrap rate of material too,
which makes it so carbon composites are terribly expensive. As a matter
of fact, the advanced carbon composites cost about $180 per KG by the
time you factor in the scrap material. So how’s that compare to
stainless steel? $3. $3 per KG….

Uhhh yeah… 60 times to cheaper to manufacture. SIXTY TIMES CHEAPER. I
don’t care what business you’re in, when something is 60 times cheaper,
readily available today, and outperforms the other material, you’d
better hop on it! Which makes me wonder how the heck does Rocket Lab get
away with it, they make it look so easy! Well, these two vehicles aren’t
in any way shape or form comparable… so let’s not even do it haha"

....and there was this comment at the end of the article by Bruce Dunn:

Bruce Dunn
January 30, 2019 at 2:09 pm
Reply

"There is a further reason for using stainless steel rather than carbon
composite – fracture toughness. Carbon composites are typically very
brittle, and small flaw can initiate cracks which rapidly spread and
destroy the structure. This means that extreme care needs to be taken
not to mar the surface of carbon composite structures. On January 17
1997 there was a failure in the carbon composite case of one of the
solid rocket boosters of a Delta II launch vehicle. This caused the
spectacular loss of the vehicle shortly after takeoff. A subsequent
lengthy investigation determined that the case of the booster had likely
been slightly damaged during ground handling, providing a flaw which
initiated a crack which rapidly spread."

Here is the video of that anomaly:

https://www.youtube.com/watch?v=z_aHEit-SqA


Note the debris field. An issue raised later during the development of
Aries I / Orion (The Stick). It wasn't clear the nylon chutes of the
Orion would have survived contact with any burning solid fuel remnant
debris field from an exploded SRB should it be so unlucky as to descend
through it after launch abort.

If I were a betting person, I'd prefer to take my chances on
characterization of thermal reactions of stainless and iterate on that...

Dave
 




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