totm nov 2023 SpaceX Discussion Thread

[sevenperforce]

16 minutes ago, Josh IN SPACE said:

It's gonna be nuts to see that thing belly-flop. I wonder how it would feel to be in that thing when it does the rapid turn at the end of the fall before performing the controlled landing. Seems a bit jarring.

Probably no more jarring than chute opening but I could be wrong.

[CBase]

Anyone has an idea why the hell they build sea level raptors in a second stage ?

I mean for landing burn they showed around 50 m/s dv. Even with a big ISP penality I would have thought that vacuum raptors are less weight with more fuel than carrying around 3 SL engines.

[Ultimate Steve]

1 hour ago, CBase said:

Anyone has an idea why the hell they build sea level raptors in a second stage ?

I mean for landing burn they showed around 50 m/s dv. Even with a big ISP penality I would have thought that vacuum raptors are less weight with more fuel than carrying around 3 SL engines.

If I understand right, you really don't want to fire vacuum engines at sea level because of flow separation.

[CatastrophicFailure]

1 hour ago, CBase said:

Anyone has an idea why the hell they build sea level raptors in a second stage ?

I mean for landing burn they showed around 50 m/s dv. Even with a big ISP penality I would have thought that vacuum raptors are less weight with more fuel than carrying around 3 SL engines.

Because they'll be landing with them at sea level.  Vacuum engines with enormous, overexpanded bells generally don't play nice with sea-level air pressure.

[Xd the great]

3 hours ago, sevenperforce said:

Probably no more jarring than chute opening but I could be wrong.

From seating to lying on your back.

[mikegarrison]

By the way, the discussion about sea level raptors starts to explain why reusable spaceships have not been the norm.

Reusability sounds great, but it carries a LOT of penalties. The shuttle had to bring along wings and landing gear, this ship has to being along sea level engines and landing gear, they both were constrained not to use ablative heat shielding, etc. Getting into orbit is hard, and doing it while carrying a lot of heavy stuff to allow reusability is even harder.

Yes, it can be worth it. But reusability is an investment. It cuts the margins on every trip, although you hope to make that back up again by fewer capital costs.

[AngelLestat]

5 hours ago, tater said:

https://amp.cnn.com/cnn/2019/09/29/business/elon-musk-spacex-mars-starship-cost/index.html

 

She quotes part of the Bridenstine tweet and asks him about it. He says “Did he say ‘commercial crew’ or ‘SLS’?”

 

Incredible that Bridenstine have choose Spacex to start to complaint about delays instead of all the other NASA projects.
In his defense, he is on NASA since the end of 2017, so not much to blame him, but still...   bad play.
 

10 hours ago, sevenperforce said:

The hot-gas thrusters are pretty cool, though. I wonder -- if they put the meth and ox bottles up front, would the hot-gas thrusters be strong enough (and oriented properly) to provide abort capability for the crew cabin?

One thing he didn't discuss in connection to the hot-gas thrusters is that the meth and ox bottles can be refueled with a simple pump from tap-off of the tank pressurant or from thermal vaporization of main-tank props. They seem to have wholly abandoned transpiration cooling plans, but I wonder if they might revisit a regenerative cooling approach. Weld piping along the stagnation path of the hypersonic flow and pipe cryo methane and LOX through to vaporize and thus sap heat. You need to vaporize props to keep the hot-gas thruster bottles full, after all. 

With similar channels on the leeward side, you could even use this system in space for power generation on orbit. Pipe liquid methane through the sunward side to vaporize and run a generator; pipe the methane gas through the leeward side to condense. Solar power without solar panels.

Hearing that make my day. 
I would like to know the ignition choice for those thrusters, sparks or laser ignition?
I read in a paper than laser ignition could be 99.98% accurate and 5 to 10 ms in delay.
I wish as you have said, that the main tanks could refill the pressure vessels for the control thrusters, it does not seem complicate, just a bit of electrical heat and stop in the desired pressure. 
I would avoid the thermal cycle to generate power, PV is always cheaper, weight efficient and less problematic, that particular thermal cycle does not seem very efficient either. 
In any case they can include a fuel cell as backup. 
 

4 hours ago, CBase said:

Anyone has an idea why the hell they build sea level raptors in a second stage ?

I mean for landing burn they showed around 50 m/s dv. Even with a big ISP penality I would have thought that vacuum raptors are less weight with more fuel than carrying around 3 SL engines.

Because they need to land on earth, you get flow separation at sea level with such big bell, additionally the engine gimbals should be more powerful to move such heavy bell. 
Besides, once they are in orbit, they can use just the vacuum engines, they dont lost ISP. 

Well, those where my 2 cents on the presentation.
I cant wait for the upcoming months, it would be epic. 

[ChrisSpace]

14 hours ago, MinimumSky5 said:

How long would it take for Starship to get to Saturn? AFAIK, it would be multiple years. 

https://toughsf.blogspot.com/2019/05/starship-lite-from-rapid-interplanetary.html

TLDR, for the 2018 design with minimal payload:

55 days to Mercury

30 days to Venus

40 days to Mars

under a year to Jupiter

just over 24 months to Saturn

4 years to Uranus

7.5 years to Neptune

 

14 hours ago, ThatGuyWithALongUsername said:

He also mentioned destinations past Mars to be Ceres, Callisto, Ganymede and Titan

When did he say that?

[ThatGuyWithALongUsername]

3 minutes ago, ChrisSpace said:

When did he say that?

Obviously in a bit of a speculative way, of course, but clearly some thought was put into this- refueling should work in all these places.

Edited September 30, 2019 by ThatGuyWithALongUsername

[Guest]

The presentation wasn’t that great, same old stuff from Elon, not to say the message isn’t great! Just not new. But building the rocket in such a short time was pretty amazing and they really knuckled down in the last month or so to get it ready on time! Those guys are also amazing! Sitting on a big shiny oven in the Texas sun doing welding.... they probably pulled some crazy shifts too, they deserve a medal.

The single seam construction in the future should be cool! it would probably be a big reduction in labour, and hopefully look a lot cleaner. 

Life support is something I’m super interested in so touching on that was nice but very little detail, seems like they haven’t really started on that yet. Also his estimates of space per person were funny. 1000m3 divided by 100 = 10m3 per person right. Only what proportion of that space is public areas, washing facilities, exercise space, food and goods storage, crew areas, command and control, airlock etc etc... if you got half of that 10m3 per person figure I’d be amazed. Also his idea of living space being spread over 6 sides of a cube is similarly optimistic. The shape of the space and ergonomics come into play when designing the layout and it will almost certainly need to be designed with functionality predominantly in one orientation (that imposed on you by gravity, since it will be sitting on a planet for extended periods! You can’t have the toilet on the roof for example)

He was right about one thing tho, I think people need to feel good about the future or what’s the point.

Edited September 30, 2019 by Guest

[kerbiloid]

4 hours ago, Dale Christopher said:

Sitting on a big shiny oven in the Texas sun doing welding.

A hotseat playing.

4 hours ago, Dale Christopher said:

The single seam construction in the future should be cool!

Once they get from the sun into a building.

4 hours ago, Dale Christopher said:

1000m3 divided by 100 = 10m3 per person right. Only what proportion of that space is public areas, washing facilities, exercise space, food and goods storage, crew areas, command and control, airlock etc etc... if you got half of that 10m3 per person figure I’d be amazed. Also his idea of living space being spread over 6 sides of a cube is similarly optimistic.

A cylindric room with rotary bed for 12 persons.
If rotate it fast, can be a catapult.
Or a medical centrifuge for 1 g training. Depends on if the door is closed.

Edited September 30, 2019 by kerbiloid

[sevenperforce]

15 hours ago, CBase said:

Anyone has an idea why the hell they build sea level raptors in a second stage ?

I mean for landing burn they showed around 50 m/s dv. Even with a big ISP penality I would have thought that vacuum raptors are less weight with more fuel than carrying around 3 SL engines.

Usually, you cannot safely fire a vacuum engine at sea level, because the overexpansion of the flow will lead to it being "pinched" by atmospheric pressure and separating inside the nozzle. This flow separation is chaotic and so it can produce different forces on different parts of the engine bell. With the amount of force in a rocket engine, even a tiny bit of misalignment on the force vectors will easily rip the thin bell apart.

Elon has suggested the possibility of using a dual-bell nozzle in the vacuum Raptors to allow them to fire safely at sea level. A dual-bell is a means of altitude compensation that puts a tiny convergence crimp part of the way up the bell, creating a defined surface where the flow separation can attach. It's very very slightly less efficient than a proper vacuum expansion bell, and of course the added weight of the outer bell portion is useless at liftoff, but it's a good tradeoff for a sustainer stage and is lighter than something like an aerospike.

Spoiler

The bigger reason is raw thrust. The sea level Raptor has a ridiculously high thrust to area ratio for a booster-class bipropellant liquid rocket engine. It has a nozzle that opens 40% larger than the Merlin 1D but produces 130% more thrust. For example, the BE-4 has a nozzle that opens 46% larger than the Raptor's but produces only 25% more thrust (and may ultimately produce less thrust if Raptor's chamber pressure is upgraded). SL Raptor has a thrust-to-area ratio that is 135% greater than the mighty F-1. In fact, the SL Raptor's thrust to area ratio is a whopping 39% higher than that of the Space Shuttle SRBs themselves. 

But going from a sea level nozzle to a vacuum nozzle is a huge loss of this critical value. Recall that the Merlin 1D Vacuum engine bell takes up almost as much space as all nine lower-stage engines, yet produces only about 1/8 as much vacuum thrust as those nine would  (the Merlin 1D Vacuum's nozzle extension gives it about 12% more thrust in vacuum than the underexpanded SL engines). The vacuum Raptor will only have about 8% more vacuum thrust than a SL Raptor. This is because the chamber pressure is so high that there aren't as significant expansion losses. Even though it's only an 8% higher vacuum thrust, a single vacuum engine would take up essentially the entire space occupied by the three sea level engines (plus substantially more vertical space) with no room to gimbal. It would also have no redundancy; those three gimbaling SL Raptors have enough thrust to land safely even with an engine out, and they have roll control, while a single vacuum raptor would not.

This thrust is important for other reasons, too. The Starship needs a LOT of thrust at staging so the three SL Raptors will actually fire with the vacuum Raptors after Superheavy separation until their added thrust is no longer needed. Swapping the three SL raptors out for a single vacuum Raptor would lower thrust at staging by 31% while only providing a negligible drop in wet or dry mass, with noticeably higher gravity drag losses due to the loss of T/W ratio.

13 hours ago, mikegarrison said:

By the way, the discussion about sea level raptors starts to explain why reusable spaceships have not been the norm.

Reusability sounds great, but it carries a LOT of penalties. The shuttle had to bring along wings and landing gear, this ship has to being along sea level engines and landing gear, they both were constrained not to use ablative heat shielding, etc. Getting into orbit is hard, and doing it while carrying a lot of heavy stuff to allow reusability is even harder.

Yes, it can be worth it. But reusability is an investment. It cuts the margins on every trip, although you hope to make that back up again by fewer capital costs.

Indeed. If reusing orbital stages was easy, then Centaur and Agena and Block D and Briz-M and DCSS and Fregat and Transtage would have all been redesigned for it. It was hard enough to find a way to land Falcon 9's first stage and that was the first reusable orbital-class liquid booster ever.

12 hours ago, AngelLestat said:

Hearing that make my day. 
I would like to know the ignition choice for those thrusters, sparks or laser ignition?

Same dual-redundant spark torches used on Raptor, most likely.

Alternately they could actually just rebuild the spark torches into the hot-gas RCS thrusters themselves. That's what those spark torches for Raptor are, anyway: spark-ignited hot-gas thrusters.

12 hours ago, AngelLestat said:

I wish as you have said, that the main tanks could refill the pressure vessels for the control thrusters, it does not seem complicate, just a bit of electrical heat and stop in the desired pressure. 

I am certain that will be the method.

11 hours ago, ThatGuyWithALongUsername said:

Obviously in a bit of a speculative way, of course, but clearly some thought was put into this- refueling should work in all these places.

I wonder if they would do aerocapture at Titan or try to do aerocapture at Saturn itself.

 

[sh1pman]

7 minutes ago, sevenperforce said:

I wonder if they would do aerocapture at Titan or try to do aerocapture at Saturn itself.

Aerobraking at Saturn would be at 36+ km/s, more than 3x the Earth escape velocity. I don't think there are materials that can take this amount of heating. Maybe it can aerocapture at Saturn with 100m-wide plasma magnetoshell, though.

[Racescort666]

23 hours ago, sevenperforce said:

Welding on the 35,000 nuts will be a pain for sure, though.

It seems a little premature to comment on this but spot welding is on the order of 3 seconds per weld. Granted, spot welds don’t have to be as accurate as weld nuts and they require some fancy setups from an automation standpoint but at 3 seconds/weld, you’re at about 30 hours of fabrication for 1 robot. 

Weld nuts aren’t the only fastening method though, depending on the thickness, they could probably just thread the skin. Or, if they wanted to get really fancy, they could flow drill: 

 

[sevenperforce]

54 minutes ago, sh1pman said:

Aerobraking at Saturn would be at 36+ km/s, more than 3x the Earth escape velocity. I don't think there are materials that can take this amount of heating. Maybe it can aerocapture at Saturn with 100m-wide plasma magnetoshell, though.

Good point. Titan is a much better bet.

I'm wondering about the upcoming flight schedule. In my head, I was imagining that Mk1 will perform at least one or two hover tests to validate terminal descent variables before progressing to the 20 km hop, right? That just made sense. Gradatim felociter and so forth.

Though perhaps not; gradatim felociter is the other egomaniac billionaire rocketman's motto. If someone went wrong during the Mk1 hover test, they'd have no data and an LOV, whereas if something went wrong during the landing after the 20 km hop, they'd have the skydiver-descent validation and mountains of useful data and an LOV. Seems like it is to their advantage to go straight from static fire to the full 20 km hop.

If the touchdown fails and you lose the vehicle, nbd -- you've got the data from the skydive envelope and the turn-and-burn, so you update the landing info for Mk2 and iterate while you complete Mk3. If everything works as planned, then you test Mk1 and Mk2 to destruction on increasingly challenging recovery envelopes (and perhaps successively hotter re-entries) while preparing to do an orbital test with Mk4. Not sure where Mk3 fits if Mk1 is successful, then, except as perhaps a hypersonic L/D test article for substantially hotter entry, or as a lessons-learned artifact from whatever Mk1 and Mk2 are able to teach them.

UPDATE: New SpaceX Starship website! Lots of goodies.

https://www.spacex.com/starship

 

[sevenperforce]

Animation on one of the sub-pages shows self-leveling landing feet descending vertically out of those sheaths for touchdown on lunar regolith:

Evidently still planning on a chomper payload fairing rather than the cargo bay. It makes attachments easier, I suppose. 

9 meters is only 2 meters larger than New Glenn's.

Then again I am "only" just over 2 meters tall, so I guess that is a big difference.

Length-wise, though, I think it is roughly the same as FH.

Edited September 30, 2019 by sevenperforce

[magnemoe]

1 hour ago, sevenperforce said:

Good point. Titan is a much better bet.

I'm wondering about the upcoming flight schedule. In my head, I was imagining that Mk1 will perform at least one or two hover tests to validate terminal descent variables before progressing to the 20 km hop, right? That just made sense. Gradatim felociter and so forth.

Though perhaps not; gradatim felociter is the other egomaniac billionaire rocketman's motto. If someone went wrong during the Mk1 hover test, they'd have no data and an LOV, whereas if something went wrong during the landing after the 20 km hop, they'd have the skydiver-descent validation and mountains of useful data and an LOV. Seems like it is to their advantage to go straight from static fire to the full 20 km hop.

If the touchdown fails and you lose the vehicle, nbd -- you've got the data from the skydive envelope and the turn-and-burn, so you update the landing info for Mk2 and iterate while you complete Mk3. If everything works as planned, then you test Mk1 and Mk2 to destruction on increasingly challenging recovery envelopes (and perhaps successively hotter re-entries) while preparing to do an orbital test with Mk4. Not sure where Mk3 fits if Mk1 is successful, then, except as perhaps a hypersonic L/D test article for substantially hotter entry, or as a lessons-learned artifact from whatever Mk1 and Mk2 are able to teach them.

UPDATE: New SpaceX Starship website! Lots of goodies.

https://www.spacex.com/starship

 

Agree with you here and its indicated that spacex will go directly for 20 km. 
I assume they will follow up this with an falcon 9 first stage like trajectory but might do another atmospheric flight first. Perhaps one more. 
Now you need superheavy and I'm pretty sure they will take Mk1 to orbit unless they find that this will not work. One crazy idea might be an suborbital flight across the Atlantic. 
This will be far more heating than the falcon first stage jump but not as much as orbital but both require superheavy. 
I belive Mk2 will be the real deal as in an useful spaceship, probably as an starlink pump and testing systems like refueling. Perhaps an moon flyby to test that. Mk3 will be operational as in launching satellites. Guess Mk4 will be maned for the dear moon mission. 

[sevenperforce]

14 minutes ago, magnemoe said:

Agree with you here and its indicated that spacex will go directly for 20 km. 
I assume they will follow up this with an falcon 9 first stage like trajectory but might do another atmospheric flight first. Perhaps one more. 
Now you need superheavy and I'm pretty sure they will take Mk1 to orbit unless they find that this will not work. One crazy idea might be an suborbital flight across the Atlantic. 
This will be far more heating than the falcon first stage jump but not as much as orbital but both require superheavy. 
I belive Mk2 will be the real deal as in an useful spaceship, probably as an starlink pump and testing systems like refueling. Perhaps an moon flyby to test that. Mk3 will be operational as in launching satellites. Guess Mk4 will be maned for the dear moon mission. 

Not quite that speedy. On Saturday night Elon said Mk1 and Mk2 were very similar test articles for demonstrating skydive, turn-burn, and landing; Mk4 would likely be the first to orbit. He said they will build Mk1-4 before starting on a Superheavy at either site. I am guessing that Mk3 is the first to mate to Superheavy, but only if they get enough data from Mk1 and Mk2. We would potentially see propellant transfer demonstrations between Mk4 and Mk5...potentially 3 & 4 if they do end up sending Mk3 to orbit after Mk4.

Mk1-2 are built with welded-panel construction; Mk3 and onward will be built from single-seam roll steel rings. The rings for Mk4 are already in place at Florida even though they are not yet done with Mk2.

Elon said that because building with steel is so cheap, they're basically just going to keep building at max speed from now until they have a full fleet. Limiting factor is the Raptors. They are building a few Raptors a week now but need to be at a Raptor per day to get enough for a couple of Superheavy boosters. That's one reason why the build sites are going all the way up to Mk 4 before either starts on a Superheavy.

[tater]

I agree that they will test in a way to gather the most data... they are becoming hardware rich where the hardware is SLS Core Stage sized flight article rockets. LOL.

NASA just barged in their SLS Core Stage test article (a large boilerplate thing to rehearse moving it around) to the VAB. I'd be willing to bet that that boilerplate object cost as much as Starship (minus the engines).

[sevenperforce]

The abort scenarios are starting to worry me. Even with all six Starship engines pushing 2 MN, we'd be looking at 12 MN divided by the stack's 1420 tonnes, which is less than a gee. 

[tater]

4 minutes ago, sevenperforce said:

The abort scenarios are starting to worry me. Even with all six Starship engines pushing 2 MN, we'd be looking at 12 MN divided by the stack's 1420 tonnes, which is less than a gee. 

I'm not seeing LES as a thing. What's the abort scenario on a 777 if a wing falls off?

For crew applications I've always had issues with the BFR/ITS/SS concept. Honestly, if they are still remotely serious about P2P, they need full on "airworthiness" certification.

The stubby legs, too... Yikes.

[mikegarrison]

3 minutes ago, tater said:

I'm not seeing LES as a thing. What's the abort scenario on a 777 if a wing falls off?

There are quite possibly more 777 takeoffs in a week than there will ever be BFR launches. It's a whole different level of reliability.

[sevenperforce]

1 minute ago, tater said:

I'm not seeing LES as a thing. What's the abort scenario on a 777 if a wing falls off?

For crew applications I've always had issues with the BFR/ITS/SS concept. Honestly, if they are still remotely serious about P2P, they need full on "airworthiness" certification.

The stubby legs, too... Yikes.

Elon has always insisted that if there is a problem with Superheavy on ascent, the upper stage can just boost way. But it cannot do that if it cannot pull a gee.

If they bump up from six engines to nine, we could see a high enough TWR to be useful and potentially single-stage P2P.

Shame no one talked to Elon about P2P on Saturday night.

At least the legs will self-adjust.

[tater]

4 minutes ago, mikegarrison said:

There are quite possibly more 777 takeoffs in a week than there will ever be BFR launches. It's a whole different level of reliability.

Yep. That's part of the reason I see P2P as almost comically unlikely (again, unless they can somehow demonstrate airliner levels of safety).

3 minutes ago, sevenperforce said:

Elon has always insisted that if there is a problem with Superheavy on ascent, the upper stage can just boost way. But it cannot do that if it cannot pull a gee.

If they bump up from six engines to nine, we could see a high enough TWR to be useful and potentially single-stage P2P.

I think they'd likely make a specific version for P2P, anyway, no booster.

3 minutes ago, sevenperforce said:

Shame no one talked to Elon about P2P on Saturday night.

P2P is sorta kooky, anyway.

3 minutes ago, sevenperforce said:

At least the legs will self-adjust.

Yeah, but it seems too narrow, and too low for any landing someplace like the Moon.

[tater]

I have to say, best "Starship" lunar landing might be:

Use SS to get to lunar orbit (it has the dv for a lunar orbit and return when refilled). Drop off 100t in LLO. Land cargo.

A 100t (wet) lander with a single Vac Raptor could do a round trip from LLO with 37t of cargo (I'm calling the lander 9t dry). You can drop the cargo mass slightly and take the lander all the way back to Gateway (if anyone cares to do that).

Nominally you'd return to SS, which takes the now 9t Lander back to earth.

This makes way, way more sense to me than SS to the lunar surface. Vacuum space vehicles will always make more sense to stay in that environment.

Edited September 30, 2019 by tater