SpaceX Starship/Super Heavy Engineering General Thread 4

[FatBear]

Snatching a live booster out of the air is not something with a lot of flight heritage behind it. Add in that the booster has never flown and planning to ditch it first time around makes sense.

The chances of a successful first landing on dry land seem very low.  And the failed landing will result in a massive explosion which destroys most of the craft.  Landing in the water may or may break the structure, but it will cause much less actual overall damage.  So assuming they can recover from the ocean, I think they will gain a lot more useful design feedback from the salvaged water landing than from the bits and pieces of an exploded dry-land landing.

[wannamoonbase]

Question for the NSF collective.

Will we see venting/disposal of the main tanks before re-entry and landing from both the booster and ship?

Seems to me that they’d want to get rid of the mass from residual propellant that is not on the header tanks.

[sebk]

Question for the NSF collective.

Will we see venting/disposal of the main tanks before re-entry and landing from both the booster and ship?

Seems to me that they’d want to get rid of the mass from residual propellant that is not on the header tanks.

Likely they'd want to retain enough pressure to ensure proper rigidity of the vehicles during re-entry, but they indeed may go quite a bit below ascent pressurization. In the case of SH it could reduce the mass by about 15t. And in the case of Starship it'd be about 5t.

[clongton]

Elon said (paraphrase) "If it [Starship] survives reentry it will attempt a water landing". To those who say I'm wrong, please provide a link to where he changed his mind, because I must have missed that.

[chopsticks]

Elon said (paraphrase) "If it [Starship] survives reentry it will attempt a water landing". To those who say I'm wrong, please provide a link to where he changed his mind, because I must have missed that.

It's on the SpaceX website.

[clongton]

Elon said (paraphrase) "If it [Starship] survives reentry it will attempt a water landing". To those who say I'm wrong, please provide a link to where he changed his mind, because I must have missed that.

It's on the SpaceX website.

Just looked but don't see it. Do you have a direct link?

[kraisee]

Hey Chuck,
Like you, I haven't seen anything to contradict the point.

Unless they've been forced to cut a landing attempt due to an FAA issue we aren't privy to, there's simply no reason NOT to include the already-existing code to attempt a landing over the ocean if the mission unexpectedly gets that far.   Any landing attempt would generate useful data.   That's always going to be more valuable than just watching it go splat.

Ross.

[Herb Schaltegger]

Elon said (paraphrase) "If it [Starship] survives reentry it will attempt a water landing". To those who say I'm wrong, please provide a link to where he changed his mind, because I must have missed that.

It's on the SpaceX website.

Just looked but don't see it. Do you have a direct link?

https://www.spacex.com/launches/mission/?missionId=starship-flight-test

From the narrative description of the flight:

For the first flight test, the team will not attempt a vertical landing of Starship or a catch of the Super Heavy booster.

The infographic on that page also depicts a belly-flop into the water, assuming Starship survives entry intact.

There's a lot of discussion about this in the Launch Discussion thread.

https://forum.nasaspaceflight.com/index.php?topic=53846.0

[clongton]

https://www.spacex.com/launches/mission/?missionId=starship-flight-test

From the narrative description of the flight:

For the first flight test, the team will not attempt a vertical landing of Starship or a catch of the Super Heavy booster.

The infographic on that page also depicts a belly-flop into the water, assuming Starship survives entry intact.

There's a lot of discussion about this in the Launch Discussion thread.

https://forum.nasaspaceflight.com/index.php?topic=53846.0

And there it is. Thank you.
Good grief. How in the world did I not see that thread?

[InterestedEngineer]

Found this interesting tweet regarding a potential stretched variant of the Starship:
https://twitter.com/TylerG1998/status/1644878801770278913

If Musk is to be believed, SpaceX may be in the early stages of conceptualizing a taller Starship version measuring over 420 feet tall, bigger than the Interplanetary Transport System proposal (the initial design study for the Starship).

An extra 10m is an extra 5 segments.  That's a lot more fuel.

Each segment weighs 1.6t, so along with 3x3 = 9t of RaptorVac engines the dry mass increases 17t and with 3t of extra landing fuel means non-payload mass to LEO increases by 20t.

OTOH, instead of 1200t of fuel, at 9m * 4.5² * pi * 0.8t/m³ = 570t of extra fuel to LEO, or 1770t of fuel total.

With 6.5 km/sec deltaV required to get to LEO from MECO and an exhaust velocity of 3.6km/sec that's a mass ratio of 6, so that's 350t to LEO, or about 200t of payload.

Reusable.

So those extra 5 rings and 3 raptor Vacs will double the payload to LEO.

That's going to allows Mars Injection boosts from LEO with only 3 refuelings, and only 9 refuelings to get a full load of fuel.

OR 5 refuelings and more payload.  Per Elon's latest tweet:

https://twitter.com/elonmusk/status/1647096182269784065

That's 1000t of fuel.

4.8km/sec is a decent TMI deltaV for travel times of < 150 days.

That requires a mass ratio of 3.8.

That's 357t of Starship, landing fuel, and payload on its way to Mars.

357t with about 64t of landing fuel, or 700m/sec of landing deltaV with a landed mass of 293t.

293-150 = 143t dry weight of Starship.

That about matches the LEO calcs showing 150t ready-to-land-from-LEO mass.  Plus or minus a bit, the TMI deltaV is a bit of a guess.

But it does generally confirm a LEO payload of 200t and Mars payload increase from 100t to 150t.  All by stretching a bit.

[Twark_Main]

With 6.5 km/sec deltaV required to get to LEO

Since this is held constant, can we assume that Super Heavy also increases in mass by ~50%?

[edzieba]

Somewhere in the last hundred-odd pages of thread were the calculations on Super Heavy (and Starship) tipover survival. In short: If pressurised to flight pressure, and at low (single-digit to low tens m/s) contact velocity, surviving tipover is entirely reasonable for both vehicles. If unpressurised, survival is precluded. If pressure vented after tipover, wave action can destroy the tanks.

Now confirmed by SpaceX as per the WR for the Launch License:

After the landing burn ends, the
flight plan is for Super Heavy to impact the water intact vertically. Then, within several seconds,
Super Heavy would tip over and impact the water horizontally. The landing would impart forces
onto the liquid oxygen (LOX) tank and methane tank; however, the tanks’ structural capabilities
allow it to withstand these forces. Therefore, the tanks would remain intact, and there would be
no resultant interaction between the LOX and methane. Nominally, Super Heavy will remain
intact. Following the landing burn, Super Heavy would sink at an angle (similar to a sinking ship),
during which sea water would flood the tanks through the fill drain valves near the bottom. As
the tanks flood, the vehicle would become waterlogged and sink to the ocean floor.

[InterestedEngineer]

With 6.5 km/sec deltaV required to get to LEO

Since this is held constant, can we assume that Super Heavy also increases in mass by ~50%?

Why would Super Heavy increase by 50%?  As far as we know, they aren't adding any rings.

The 6.5km/sec deltaV for Starship.   Need to add the usual 3-ish deltaV.  They are launching an extra 680t (580t fuel, 100t cargo, 20t dry mass) to LEO.  I suspect that mostly means running engines a tad hotter.

mass at MECO currently is (1200+120+100) + 250 or 1,670t.  That's a mass ratio of 3 for a liftoff weight of 5,000t.  At an average Isp of 340 that's a deltaV of 3.7km/sec.  Feels little like overkill.   Net 9.5km/sec is what is needed for LEO, roughly.

With the extended Starship, that's (1,780+140+200) + 250 or 2,370t.   That's a mass ratio of 2.1, or deltaV of 2.5km/sec.    A little low.

So yes they might need to do something to get expended deltaV at MECO back above 3km/sec.  Doing the  math, the nass ratio needs to be 2.5, so that's 5,900t takeoff mass, of which fuel is 900-600 = 300t of added fuel.

Is there room for another 900t of fuel in the Booster?  I have no idea.  That's about 900m³ and each ring gives about 110t of fuel.  So an extra 3 rings worth of fuel is needed on the Booster for the additional extended-Starship mass.

The rough rule of thumb is that the 200t payload version of Starship requires 10% more of everything on the booster - 10% more thrust, 10% more fuel.

Is SuperHeavy maxed out on fuel right now?  Don't know.

It is maxed out on thrust for current Raptor-2.  And there's no more room for engines.

[InterestedEngineer]

How much propellant mass can Super Heavy truly hold?

Wikipedia claims 3,400t of propellant mass.

However, if one assumes 63m out of 69m of its height is tankage, and at 63.55m³ of volume per linear meter, that's 4,000m³ of tank volume.

At a mix ratio (by mass) of LOX at 78.5% and LCH4 at 21.5%, that's an average density of 1.1t/m³.

So that's 4,400t of fuel capacity.  Not 3,400t.

I suspect there's extra room for additional propellant in the future if they can boost the engines another 10% or so, and thus lift more total Starship mass up to MECO.

Please check the math

[tbellman]

How much propellant mass can Super Heavy truly hold?

Wikipedia claims 3,400t of propellant mass.

However, if one assumes 63m out of 69m of its height is tankage, and at 63.55m³ of volume per linear meter, that's 4,000m³ of tank volume.

At a mix ratio (by mass) of LOX at 78.5% and LCH4 at 21.5%, that's an average density of 1.1t/m³.

So that's 4,400t of fuel capacity.  Not 3,400t.

I suspect there's extra room for additional propellant in the future if they can boost the engines another 10% or so, and thus lift more total Starship mass up to MECO.

Please check the math

Close to their respective freezing points, oxygen and methane have densities of 1300 g/l and 450 g/l, respectively.  At a 78.5 : 21.5 ratio by mass (~3.65:1), that gives an average density of 925 g/l, not 1100.  (I suspect you calculated the average as if the 78.5 : 21.5 ratio was by volume, because then you get about 1118 g/l.  When the ratio is by mass, you need to calculate the two volumes by dividing the mass by the density, sum them, and then divide the total mass by the total volume.)

I calculated an estimate of the tank volume in a different way, but there I get about the same as you.  SuperHeavy is 69 meters tall, and the Raptor engines are 3.1 meter   long, leaving you with 65.9 m.  But the ends are roughly hemispheres with a 4.5 m radius, so the cylindrical part is 56.9 m long, with a volume of 3620 m³.  The ends have a combined volume of about 382 m³, so total tank volume would be about 4002 m³.  Entirely filled with oxygen and methane, in the assumed 78.5 : 21.5 mass ratio, that gives us a propellant mass of 3700 tonnes.

In practice, there's probably a few decimeters more of "margin" at the ends of the SuperHeavy, but on the other hand the 69 meters that Elon has told us might be just a rounded number.

The tanks will however not be 100.0% full with liquid oxygen and liquid methane.  There will be a bit of space in the top filled with gaseous oxygen and methane, the so called ullage space.  How much ullage space, we don't know, but perhaps on the order of 100 m³ per tank (5% of the volume)?  Someone with more knowledge about rockets that can give an estimate of how much ullage space is common?  But subtracting 5% gives us 3515 tonnes instead of 3700 tonnes

Further, we don't know how close to their freezing points the oxygen and methane will be either, and that affects the densities.  I suspect it will be a couple degrees above the freezing point, to have a bit of margin in order to avoid getting a slush.

[mandrewa]

Elon said (paraphrase) "If it [Starship] survives reentry it will attempt a water landing". To those who say I'm wrong, please provide a link to where he changed his mind, because I must have missed that.

It's on the SpaceX website.

Just looked but don't see it. Do you have a direct link?

https://www.spacex.com/launches/mission/?missionId=starship-flight-test

From the narrative description of the flight:

For the first flight test, the team will not attempt a vertical landing of Starship or a catch of the Super Heavy booster.

The infographic on that page also depicts a belly-flop into the water, assuming Starship survives entry intact.

There's a lot of discussion about this in the Launch Discussion thread.

https://forum.nasaspaceflight.com/index.php?topic=53846.0

Except that quote -- "For the first flight test, the team will not attempt a vertical landing of Starship or a catch of the Super Heavy booster." -- is in the context of catching the Starship on the launch tower.

So as I read this, nothing is being said about what the Starship will do if and when it gets close to the Pacific Ocean north of Hawaii.


Here's the fuller quote:

At 146 meters, or nearly 500 feet tall, the launch and catch tower is designed to support vehicle integration, launch, and catch of the Super Heavy rocket booster. For the first flight test, the team will not attempt a vertical landing of Starship or a catch of the Super Heavy booster.

[joek]

Except that quote -- "For the first flight test, the team will not attempt a vertical landing of Starship or a catch of the Super Heavy booster." -- is in the context of catching the Starship on the launch tower.

So as I read this, nothing is being said about what the Starship will do if and when it gets close to the Pacific Ocean north of Hawaii.
...

Starship will do nothing. Review the discussion thread; read the FAA updated PEA WR (published Friday):

Based on Starship’s hardware configuration, for the first launch, SpaceX plans to conduct a passive descent that would result in Starship’s intact impact with the ocean’s surface. Starship’s planned landing location for the first launch is shown in Figure 1 below.
...
Starship: For the first launch, after ascent engine cutoff, Starship would vent residual main tank propellant during the in‐space coast phase of the launch at or above 120 kilometers AGL

[mandrewa]

Except that quote -- "For the first flight test, the team will not attempt a vertical landing of Starship or a catch of the Super Heavy booster." -- is in the context of catching the Starship on the launch tower.

So as I read this, nothing is being said about what the Starship will do if and when it gets close to the Pacific Ocean north of Hawaii.
...

Starship will do nothing. Review the discussion thread; read the FAA updated PEA WR (published Friday):

Based on Starship’s hardware configuration, for the first launch, SpaceX plans to conduct a passive descent that would result in Starship’s intact impact with the ocean’s surface. Starship’s planned landing location for the first launch is shown in Figure 1 below.
...
Starship: For the first launch, after ascent engine cutoff, Starship would vent residual main tank propellant during the in‐space coast phase of the launch at or above 120 kilometers AGL

Yes.  But we get this from the "FAA updated PEA WR" that you are quoting, not from the description of the flight at SpaceX.com

[joek]

Yes.  But we get this from the "FAA updated PEA WR" that you are quoting, not from the description of the flight at SpaceX.com

Please take this over to the discussion thread as it has been extensively discussed there and is not germaine to engineering.

[mclumber1]

Perhaps this has already been discussed, but when/if SpaceX attempts to catch Starship using the chopsticks, how will this be accomplished, given the fact that the ship will be approaching the tower from West to East, but the chopstick arms are not oriented in that general direction?