totm nov 2023 SpaceX Discussion Thread

[Spaceman.Spiff]

7 minutes ago, SpaceFace545 said:

What’s the minimum amount of raptors needed on superheavy to lift the whole starship stack?

I think atm they’re going to use 28?

[GuessingEveryDay]

8 minutes ago, SpaceFace545 said:

What’s the minimum amount of raptors needed on superheavy to lift the whole starship stack?

Just now, Spaceman.Spiff said:

I think atm they’re going to use 28?

He said minimum. Considering pictures of the thrust plate for Superheavy, looks like they only need 4 for an orbital attempt. TWR would be low, carrying Starship, though. 

[tater]

15 minutes ago, SpaceFace545 said:

What’s the minimum amount of raptors needed on superheavy to lift the whole starship stack?

28 is what the thrust structure seems to be plumbed for. I have seen lower numbers quoted, though. It also depends on the Raptors used. We have obviously see SL Rators fired/fly. There is a Vac version that can also fire in the atmosphere, and there have been statements of a fixed SL Raptor for SH that has no gimbal, no throttle, and higher thrust, reducing the number required.

4 minutes ago, GuessingEveryDay said:

He said minimum. Considering pictures of the thrust plate for Superheavy, looks like they only need 4 for an orbital attempt. TWR would be low, carrying Starship, though. 

No way it's 4. 4 could be for a lightly fueled hop. (just SH, nothing on top)

I'd think something like 20 SL Raptors is the bare min. Maybe fewer if the fixed ones generate more thrust.

Edited May 11, 2021 by tater

[Spaceman.Spiff]

I was thinking 28 is probably on the lower end because they’ve already reduced the number. 

[SpaceFace545]

Cost aside but why don’t they just develop a booster engine that is tailored for SH instead of repurposing the same engine everywhere?

[RealKerbal3x]

2 minutes ago, SpaceFace545 said:

Cost aside but why don’t they just develop a booster engine that is tailored for SH instead of repurposing the same engine everywhere?

Because, as you said, developing an entirely new engine is expensive and time consuming. They already have one of the highest-performance engines ever built, why not use it?

Plus, a common design for booster and ship means production is more streamlined and engines can be interchangeable if necessary. The large number of engines is also great for multiple-engine-out scenarios (as long as you don't have horrific quality control a la N1, and Raptor reliability seems to be steadily improving).

[Spaceception]

16 minutes ago, SpaceFace545 said:

Cost aside but why don’t they just develop a booster engine that is tailored for SH instead of repurposing the same engine everywhere?

Having a common fuel/engine type can ease development. In this case, a lot of what's been developed for Starship can directly apply to Superheavy as well, which means they can focus on the problems that Superheavy will specifically have to deal with (like plumbing and vibrations for 28 engines), instead of having to develop two separate systems in parallel on top of that (another tank design, engine, fuel plumbing, etc).

Methane as a fuel is a good general fuel for what they want to do anyway. Kerosene cokes the engine, which is a problem for Falcon 9 reusability, and Hydrogen isn't very dense, so you need an even bigger rocket for the same payload (And Superheavy is already massive as is). Methane is in the middle, and it becomes cryogenic at similar temperatures as oxygen, so you can use a common dome, simplifying design.

 

SN15 is getting ready to be lifted onto pad b. We'll see if they can do another flight, or just do another round of testing.

Edited May 11, 2021 by Spaceception

[tater]

Musk has said the target is for Raptors to cost ~$250k each.

You get there via mass production. I would not be surprised to find that Starlink sats cost something like that. Tom Mueller said that very early in SpaceX's history Musk asked him why an engine should cost more than his car (Think at the time Musk was driving a Macleren). He thought that the Merlin should not cost more than a (very expensive) car, and they worked towards that goal.

There are 3 variants. SL with gimbal (what we have seen fly), fixed vacuum Raptor (seen test fired, or at least the engine on a stand), fixed SL Raptor with no throttle. The last we have not seen yet.

[SpaceFace545]

8 minutes ago, tater said:

Musk has said the target is for Raptors to cost ~$250k each.

Please, in what timeline will rocket engines get down to the price of a cheap house.

[Hannu2]

27 minutes ago, SpaceFace545 said:

Cost aside but why don’t they just develop a booster engine that is tailored for SH instead of repurposing the same engine everywhere?

They save significant amount of money and time by using the same basic engine everywhere they can.

They tried to develop much bigger engine. However, full staged combustion cycle has certain optimum size, which is not as large as they hoped.  It is more advantageous to use large number of "small" engines.

[Spaceception]

7 minutes ago, SpaceFace545 said:

Please, in what timeline will rocket engines get down to the price of a cheap house.

Merlin is already around the $1 million mark, and SpaceX is dedicating more resources (or at least intends to, I don't know if they're there yet) for Raptor, so it's not impossible. But they need a high production rate, and the lower they get the cost, the more diminishing returns will creep up on them. So we'll see how close they get.

Edited May 11, 2021 by Spaceception

[RealKerbal3x]

6 minutes ago, SpaceFace545 said:

Please, in what timeline will rocket engines get down to the price of a cheap house.

When they're no longer bespoke, single-use machines that have to be built, transported and installed using highly specialised tooling and vehicles.

Raptor is already eschewing a lot of that - it's the first full-flow staged combustion cycle engine to leave the test stand and yet it's transported on a flatbed and installed with a scissor lift.

[JoeSchmuckatelli]

1 hour ago, RealKerbal3x said:

...combustion cycle engine to leave the test stand and yet it's transported on a flatbed and installed with a scissor lift.

And outdoors, not in a clean room or treated like a fairy princess - it's a high tech beast of a machine... And that attitude is the only way to get space 'normalized' / routine. 

Edited May 11, 2021 by JoeSchmuckatelli

[RealKerbal3x]

Insane 4K views from Cosmic Perspective:

 

[tater]

There's a reddit thread where an insider claims Raptor production is a few a week at this point.

[sevenperforce]

10 hours ago, mikegarrison said:

I think the key is actually the Starlink. SpaceX is making all these launches for mostly an internal customer. There is good synergy there -- they have a business where they need to make a *lot* of launches, and they have a rocket that needs a lot of launches to pay off the investment in making it reusable.

If they didn't have this endless number of Starlink launches, they wouldn't be making up on volume what it presumably cost to develop the capability.

If reusing a stage costs $1M and saves them the ~$18M it costs to build a new stage, and they can reuse stages ten times each, then it would take 66 launches at full commercial price to recover the $1B it cost to develop Falcon 9 first-stage reuse. That's a long time.

Instead they're reinvesting that capability by selling themselves Starlink launches.

10 hours ago, mikegarrison said:

Of course, whether that's all good for the business in the end depends on whether Starlink can make back the money for all the launches.

Bingo. More than half of their reflights are going to Starlink (since the F9B5 debut, there have been 26 operational Starlink launches and 22 commercial launches on reflown boosters). So that's more like 80-90 commercial launches to recoup the dev costs. And even if they make that, their investment in upper stages and launch costs for Starlink still has to be recovered. 

[tater]

1 hour ago, SpaceFace545 said:

Please, in what timeline will rocket engines get down to the price of a cheap house.

Why should a rocket engine necessarily cost a lot of money relative to the price of a house, or the price of a supercar?

Musk says they think they can get it to that level (~$250k), and I have no reason to disbelieve it. Doesn't really matter to people outside of SpaceX, external customers will only ever see launch cost.

Now if SS/SH was being paid for exclusively by the government (vs buying the services of a bespoke version), the cost might matter, as we'd be paying for dev, then each engine at some stunningly high value (Raptor, Be-4, and RS-25 are all of similar thrust, though RS-25 beats them in Isp). Be-4 is ~$6.67M per engine, and Raptor is apparently ~$1M.

[SpaceFace545]

1 hour ago, RealKerbal3x said:

it's transported on a flatbed and installed with a scissor lift.

That’s not a good thing. Rocket engines are a bit too delicate to transport on goosenecks.

[tseitsei89]

1 minute ago, SpaceFace545 said:

That’s not a good thing. Rocket engines are a bit too delicate to transport on goosenecks.

That is definitely a good thing. They can make a rocket engine that is not too delicate to be transported that way. They transport raptors like that and they still work...

[RealKerbal3x]

3 minutes ago, SpaceFace545 said:

That’s not a good thing. Rocket engines are a bit too delicate to transport on goosenecks.

Well, SpaceX does exactly that with Raptor and the engines seem to be working great.

As I said before, maybe it's time for everyone else to start building engines that are less delicate and bespoke and more ruggedised. It'll certainly save them the cost of all of that custom tooling.

Edited May 11, 2021 by RealKerbal3x

[tater]

3 minutes ago, sevenperforce said:

$1B it cost to develop Falcon 9 first-stage reuse.

I think I have seen that number before, is it from Musk or Shotwell?

Do we think that number is accurate? Seems like the actual cost of hardware for reuse was fairly low, and the landings were tested on flights that were completely paid for at retail.

[RCgothic]

21 minutes ago, sevenperforce said:

If reusing a stage costs $1M and saves them the ~$18M it costs to build a new stage, and they can reuse stages ten times each, then it would take 66 launches at full commercial price to recover the $1B it cost to develop Falcon 9 first-stage reuse. That's a long time.

Instead they're reinvesting that capability by selling themselves Starlink launches.

The difference between govt and a private company is that a govt must count the cost of every penny spent to taxpayers, but a private company never has to pay back its development capital as long as it can service the interest.

[zolotiyeruki]

4 hours ago, wumpus said:

That's mostly true, but the merlin engine was designed for Falcon [1], which they never really attempted to recover.  Future plans for both Falcon [1] and Falcon 9 were based around parachutes.  Oddly enough, the merlin engine doesn't appear to have had issues lighting while falling with air blasting into  the nozzle  while the raptor engine (which was designed with retropropulsion in mind) has crashed a few times thanks to failing to ignite while falling.

...

Don't forget, "plan A" not only failed for recovering boosters (parachutes), it also failed for fairing recovery (catching it in nets).  Plan B (fishing it out of the water) is working well enough that plan A was scrapped.

So while you might claim that Spacex was "lucky" that merlins have a property that was never a design goal, recovery appears to be more due to a dogged insistence of solving the problem than pure luck.  The original recovery plans wouldn't work (although presumably merlins *were* designed for long enough working life for reuse), and they simply kept at it.  You shouldn't be surprised if some parts are eventually found to have the properties needed to solve the problem in *some* way (if not the original plan, i.e. parachutes).

I would argue that catching fairings in a net *did* work. SpaceX just came to the conclusion that making the fairings water-tolerant was a better approach.

Even if the Merlin engine wasn't originally designed for relight, I'd be shocked if its design hasn't evolved to better facilitate relight.

And Raptor's failures on landing have generally been the fault of the fuel tanks, rather than Raptor itself.

[sevenperforce]

4 hours ago, tater said:

Wait, are they going to relaunch it after all??!!

4 hours ago, SpaceFace545 said:

I think SMART reuse is a great choice for ULA to pursue in the future. ULA really doesn't need to focus on recoverability as they focus on DOD and NASA missions and those people could really care less about launch costs and more about launch vehicle capabilities.

I had thought SMART was shelved but yesterday Tory said it wasn't, so...

Tory also wished my seven-year-old a happy birthday yesterday and did so with cool facts.

Elon may send Doge to the moon with his tweets but Tory's Twitter game is way better.

3 hours ago, tater said:

Right now the only really good thing ULA has is Centaur—which is a decent thing to have, Centaur is arguably the best rocket stage, ever.

That's partly because Centaur SEC has such low dry mass but mostly because the RL10A-4-2 is arguably the best upper-stage engine, ever. It weighs so little that even without the extra specific impulse of the RL10C-2-1 it usually wins. Unfortunately it really only works properly for very small payloads, and it can't be scaled up, and it can't be clustered well since it relies on radiative cooling.

3 hours ago, wumpus said:

Oddly enough, the merlin engine doesn't appear to have had issues lighting while falling with air blasting into  the nozzle  while the raptor engine (which was designed with retropropulsion in mind) has crashed a few times thanks to failing to ignite while falling.

Presumably the same geometry that chokes the flow at the nozzle throat serve to stagnate airflow at the throat and thus insulate the chamber. It's just as if it is restarting at a slightly higher ambient pressure than it otherwise would. 

The equation for the effective elevated air pressure inside the combustion chamber is given by 8ρv², and once the engine restarts then the equation for the effective air pressure it's pushing against becomes ρv²/2. Borrowing numbers from NROL-108, the entry burn starts with an effective in-chamber air pressure of 5.8 kPa, just 6% of atmospheric pressure at sea level. At the end of the entry burn the engine is pushing against an effective ambient pressure of 4.2 kPa, again a fraction of what it does at sea level.

The landing burn restart takes place at around 4.5 km, where the air pressure is already more than half of sea level. The total pressure at the nozzle throat here is the bigger challenge; it's actually about 630 kPa or more than six times the ambient pressure at sea level. But it's less than 7% of the Merlin 1D's actual chamber pressure so it's not a big deal.

Raptor hasn't had any issues with retropropulsion. It has had issues with starting up properly while falling sideways due to propellant flow.

3 hours ago, tater said:

3 hours ago, GuessingEveryDay said:

He said minimum. Considering pictures of the thrust plate for Superheavy, looks like they only need 4 for an orbital attempt. TWR would be low, carrying Starship, though. 

No way it's 4. 4 could be for a lightly fueled hop. (just SH, nothing on top)

I'd think something like 20 SL Raptors is the bare min. Maybe fewer if the fixed ones generate more thrust.

Starship Mk1 was 200 tonnes and their goal is to get down to 120 tonnes, so I'm guessing that SN20 will be somewhere in the 140-150 tonne range.

Assuming it launches fully loaded with propellant and all six engines (averaging 370 s specific impulse), but no payload at all for the initial orbital test, it will boast an impressive 7.3 km/s of dV. That's a full 1 km/s of dV that Superheavy doesn't have to provide. The actual effective dV of a fully fueled Superheavy with a full Starship on top is around 3.95 km/s, so it should be able to launch with about a 75% propellant load if there is no payload on Starship and Starship provides more of its own boost to orbit.

A full stack would typically be just over 5000 tonnes so reducing Superheavy's load by 25% would put the full stack at 4200 tonnes or about 83% of the normal loaded weight. Probably a little bit more, though, since Superheavy will be a touch overweight at this point. So you need 24 Raptors minimum to match the intended TWR of a full stack.

3 hours ago, SpaceFace545 said:

Cost aside but why don’t they just develop a booster engine that is tailored for SH instead of repurposing the same engine everywhere?

The booster engine is tailored for Superheavy, not for Starship.

1 hour ago, RealKerbal3x said:

Insane 4K views from Cosmic Perspective

That was just incredible.

It looks like they did a three-engine kick but then cut the engine just as rotation was canceled. It also looks like they are now essentially beginning the landing burn during the kick-flip. 

It boggles my mind how it can come down at an angle like that until just a few meters above the pad and then land upright.

[kerbiloid]

1 hour ago, tater said:

Why should a rocket engine necessarily cost a lot of money relative to the price of a house, or the price of a supercar?

Basically, parts get expensive due to variability in their parameters after productions.
So, you should choose 1 part of 100 manufactured ,which is exactly 0.15+/0.01, not 0.148, not 0.152, and scrap others or sell them below the cost.
Or buy more precise and expensive equipment to get 1 of 10 similar details enough good.

And in the rocket engine there are thousands of precise details, so the things are worse.

So, if they can have a cheap and appropriately precise manufacturing equipment to scrap just 4 details of made 5, the engines will get cheaper.