Could the SuperHeavy booster be SSTO?

[sevenperforce]

6 hours ago, Exoscientist said:

So just a 10% higher speed gets you full coverage to anywhere on Earth as well as orbit.

"just a 10% higher speed" is not nothing.

[tater]

I have no idea if a 1% payload mass fraction (PMF) is a thing for some SSTO, but let's assume it is. As I said last page, it looks like many of the old SSTO ideas were all guestimating PMF at 2%, but none was ever actually built, and SS/SH as a TSTO is saying 2-3% and they have not demonstrated this yet, so I think 1% is fair for this example. We'll call SS/SH 2%.

So a SH SSTO might get 37t to LEO, vs 100t for SS/SH. Let's give your SSTO a massive perk, and up the PMF to 1.35%. Why? So it gets 50t to LEO, half what a lowballed SS/SH does. Every possible advantage to the SSTO.

So SS is now in LEO, and needs refilling. It needs 1100 tons to be 100% full. That requires 22 of our SSTO flights, vs 11 of SS/SH.

Each SH is ~200t and carries 3500t of props. Each SS is ~100t and carries 1200t of props (5000t on the pad for a SS/SH stack, and 3700 for SH_ssto). Not even charging you for the mass of the SSTO, calling it exactly the same as SH now! More advantage to the SSTO.

We good so far?

No idea what the propellants actually cost, but we're comparing them, so units don't matter.

TSTO refill costs 11*4700=51,700 props used.

SSTO refill costs 22*3500=77,000 props used.

The very optimistic SSTO costs just shy of 50% more to do the same job (and I upped its efficiency by 35% to make the math simpler).

Yes, but it is so much easier to operate than 2 stages! Except it literally has to fly twice as often, that seems to completely eliminate any cost benefit from operations IMO.

So what good is this to SpaceX?

Edited September 9, 2022 by tater

[tater]

PS—if the SSTO gets 2% to LEO, and the TSTO gets 3%, the SSTO costs 59,500t props to refill SS compared to 37,600t for TSTO—58% more!

[Exoscientist]

On 9/8/2022 at 1:51 PM, Terwin said:

It looked to me like the ~2% SSTO payloads involved 0% reuse.

If I remember correctly, getting to orbit with 2% payload on the SH would actually have a negative payload fraction if you subtracted enough fuel for SS to deorbit and land(ie not enough for SH, just the much smaller SS), not even including things like structural reinforcement and heat shielding.

So the current comparison is:

~2% payload fraction for a single use SSTO (which is then space-trash)

OR

~2% payload fraction for a fully reusable TSTO that might have a marginal cost to launch in the low tens of millions for > 100T of cargo.  (perhaps $200/kg to orbit?)

 

 

 No those SSTO's that Tater cited were reusable. Remember the 2-3% payload for the SuperHeavy/StarShip is because of the 50% payload loss on full reusability. That's a large loss in payload. That's a key part of the argument for why reusable SSTO's become competitive with reusable TSTO's. Without the 50% payload loss, the SH/SS payload would be 4-6%.

   Robert Clark 

Edited September 9, 2022 by Exoscientist
clarity

[tater]

2 hours ago, Exoscientist said:

 No those SSTO's that Tater cited were reusable. Remember the 2-3% payload for the SuperHeavy/StarShip is because of the 50% payload loss on full reusability. That's a large loss in payload. That's a key part of the argument for why reusable SSTO's become competitive with reusable TSTO's. Without the 50% payload loss, the SH/SS payload would be 4-6%.

The SSTO lose payload, too. If a reusable TSTO gets 2-3%, an SSTO probably gets half that.

Once a vehicle is truly "operationally reusable" (akin to an aircraft, even if a special use aircraft like an SR-71), then the propellant costs dominate.

[Exoscientist]

On 9/5/2022 at 1:32 PM, Exoscientist said:

 Perhaps our difference in estimates for the tank mass is coming from the fact the tank does not reach all the way to the top of the Starship. The tank length is a little more than half the length of the vehicle. Above it, would only be the empty, lighter fairing.

https://www.humanmars.net/2021/01/cutaway-schematic-of-spacex-starship.html

 Key question: how much mass is that empty payload section above the Starship main tank? I have a suspicion Elon leaves this on when claiming the Starship can't carry much payload as SSTO. In other words, for the SSTO version, instead of looking like the 1st image below  it should look like the 2nd:

 

  Robert Clark

Edited September 11, 2022 by Exoscientist

[magnemoe]

4 hours ago, Exoscientist said:

 Key question: how much mass is that empty payload section above the Starship main tank? I have a suspicion Elon leaves this on when claiming the Starship can't carry much payload as SSTO. In other words, for the SSTO version, instead of looking like the 1st image below  it should look like the 2nd:

 

  Robert Clark

Assume the cargo bay is simply because its the extra fuel the tanker can carry so if the top bulkhead was higher it would be too heavy. 
Shortening the ship would change the aerodynamic would likely not be worth it, its not that much distance and it would complicate things quite a bit. 
 

[tater]

5 hours ago, Exoscientist said:

 Key question: how much mass is that empty payload section above the Starship main tank? I have a suspicion Elon leaves this on when claiming the Starship can't carry much payload as SSTO. In other words, for the SSTO version, instead of looking like the 1st image below  it should look like the 2nd:

The payload numbers don't include the vehicle, yes.

The stubby version is wrong, it has the taper start where the tanks are still rings. There are 6 rings above the last vertical side of the tank, so 9600kg of steel. If SS can SSTO at all, the payload is tiny. You can get the thing to SSTO be internally stretching the tanks, but only a little, because you start seriously impacting the TWR, and then you'd need more engines. But it could possibly be done—assuming SSTO SS can actually fly by itself, the weight distribution is odd flaps, etc. Remember it has to get ~20t to LEO just to be able to return.

Looks like 11 engines, 1600t of props will maybe get ~2t to LEO.

Anyway, the more important analysis is the one I did above looking at the total props spent to deliver 1100t of props to top off a Starship in LEO for a mission (to the Moon, mars, whatever).

An incredibly optimistic SSTO assumption takes 2X as many launches to do the job, and uses ~50% more propellant to do so. How much lower are launch costs for 1 super heavy with no SS on top than one where mechazilla stacks SS? You think it's really half? If so, the entire difference is the propellant costs—in which case your SSTO costs 50% more to do the job SpaceX needs done. If the cost is instead maybe 10% lower then launch costs are 19.8 vs 11—80% higher than the TSTO. There is no way the stacking, etc is that complicated with "stage 0" as we see it. If they actually catch SS... LOL, it's literally already on the pad.

 

[Exoscientist]

On 9/11/2022 at 9:56 AM, tater said:

The payload numbers don't include the vehicle, yes.

The stubby version is wrong, it has the taper start where the tanks are still rings. There are 6 rings above the last vertical side of the tank, so 9600kg of steel. If SS can SSTO at all, the payload is tiny. You can get the thing to SSTO be internally stretching the tanks, but only a little, because you start seriously impacting the TWR, and then you'd need more engines. But it could possibly be done—assuming SSTO SS can actually fly by itself, the weight distribution is odd flaps, etc. Remember it has to get ~20t to LEO just to be able to return.

Looks like 11 engines, 1600t of props will maybe get ~2t to LEO.

Anyway, the more important analysis is the one I did above looking at the total props spent to deliver 1100t of props to top off a Starship in LEO for a mission (to the Moon, mars, whatever).

...

 The scaling in the version with the reduced fairing section is wrong. It makes it look like the tank is being stretched. No, same tank just reduced fairing size. Note for the SSTO version of size 1/4th that of the full two-stage you would expect the payload to be smaller so the fairing should be as well. 

 Since the dry mass is smaller, the thermal protection, landing legs, and flaps mass should be as well. So you should first calculate what the expendable payload is then add on the, smaller, TPS, legs, and flaps mass.

 I redid the image, eyeballing it to make tanks appear same size in both versions.

  Robert Clark

[tater]

21 minutes ago, Exoscientist said:

The scaling in the version with the reduced fairing section is wrong. It makes it look like the tank is being stretched. No, same tank just reduced fairing size. Note for the SSTO version of size 1/4th that of the full two-stage you would expect the payload to be smaller so the fairing should be as well. 

 Since the dry mass is smaller, the thermal protection, landing legs, and flaps mass should be as well. So you should first calculate what the expendable payload is then add on the, smaller, TPS, legs, and flaps mass.

 I redid the image, eyeballing it to make tanks appear same size in both versions.

The image is still wrong, the nose cone intersects the vertical walls of the tank. Nose cone would start where the tank dome starts, not before. Regardless, it probably does not get to orbit. If the mass can be down to 85t, it will get 2 tons of payload to LEO (only 7 engines, so lower TWR than they want)—but it needs ~20t to return, so it needs to have a "dry" mass that includes ~20t of props.

Looks like a Starship might start to become a SSTO with zero payload (able to return) with the dry mass between 100-110 tons—where the dry mass includes 20t of landing props. So the vehicle itself needs to mass 80-90t.

Bottom line is that SpaceX knows the actual numbers better than any of us, and they are not working on this. Are they too dumb, or do they simultaneously know more about it than we do, and also know what their use case is?

As I said above, SSTO for the SpaceX use case accomplishes nothing. If they get half the payload, they need to fly twice as often, using 50% more propellant, obviating all operational benefits.

You have to remember, even at KSC, more launches required is bad, not good. They have to close airspace, and all the other regulatory hurdles for every single launch. Doing 11 launches to top off a vehicle is bad enough, and you think doing 22 is easier?

 

Edited September 12, 2022 by tater

[sevenperforce]

On 9/9/2022 at 1:51 PM, Exoscientist said:

Remember the 2-3% payload for the SuperHeavy/StarShip is because of the 50% payload loss on full reusability. That's a large loss in payload. That's a key part of the argument for why reusable SSTO's become competitive with reusable TSTO's. Without the 50% payload loss, the SH/SS payload would be 4-6%.

You keep using this number. It does not mean what you think it means.

You are also arguing backwards. We know what SH+SS can take to LEO in reusable mode; pointing out that it could take a lot more payload into space in expendable mode does not make the payload of a reusable SSTO any closer to what SH+SS can do.

7 hours ago, Exoscientist said:

Since the dry mass is smaller, the thermal protection, landing legs, and flaps mass should be as well. So you should first calculate what the expendable payload is then add on the, smaller, TPS, legs, and flaps mass.

That's also backwards.

7 hours ago, tater said:

As I said above, SSTO for the SpaceX use case accomplishes nothing. If they get half the payload, they need to fly twice as often, using 50% more propellant, obviating all operational benefits.

And they will definitely not manage to even get half the payload.

[tater]

11 minutes ago, sevenperforce said:

And they will definitely not manage to even get half the payload.

Yeah, I was being as favorable to the SSTO concept as possible just to show that it still makes no sense from SpaceX's perspective.

My actual guess is closer to 0.3% (possibly 1/10 SS/SH).

[Exoscientist]

15 hours ago, tater said:

The image is still wrong, the nose cone intersects the vertical walls of the tank. Nose cone would start where the tank dome starts, not before. Regardless, it probably does not get to orbit. If the mass can be down to 85t, it will get 2 tons of payload to LEO (only 7 engines, so lower TWR than they want)—but it needs ~20t to return, so it needs to have a "dry" mass that includes ~20t of props.…

 The image is only to illustrate the case with much reduced payload section and therefore much reduced dry mass. 

 In actuality to estimate the dry mass for the Starship as SSTO, just as in the SuperHeavy case you first start with stage in this form (sans grid fins), i.e., with no fairing:

 

 Then add on fairing mass. For a smaller rocket need smaller fairing. The Falcon 9 fairing for example is only ~2 tons. 

 So the dry mass would not be 85 tons. That’s the estimated dry mass with the full payload section of the full rocket four times larger. Estimate the dry mass without this payload section. As you said you can do that just counting rings. Without that section, the dry mass is 20 to 30 tons lighter, depending on if you also include lightening the tanks by reducing wall thickness from 4 mm to 3 mm. 
 
 Then add on a much smaller fairing, likely only 2 to 4 tons. 

 As I said, I suspect Elon when saying the Starship can’t carry much payload as an SSTO he’s using the original full size payload section incurring a 20 to 30 ton weight penalty. 
 

 Robert Clark

Edited September 13, 2022 by Exoscientist
Clarity

[tater]

6 hours ago, Exoscientist said:

 Then add on a much smaller fairing, likely only 2 to 4 tons. 

Each full ring is ~1.6t.

SH now is ~200t. even with min engines for single stage, it needs 25 engines. That's 50t on the bottom. it will need to put the header tank at the nose, else it lawn-darts engines first, it's gonna need the high AOA, and it's not quite twice as heavy, with 3X the engine mass aft. It will need more landing propellant mass. Call it 40t.

Chucking it in the calculator at 200t dry mass it gets 69t to LEO. Assume that 200t includes the 40t. 69t to LEO, better than FH. Except it has 1 purpose, this is a tank with a tiny nose cone—it's to deliver residual propellant. It will take 16 trips to take up that SS that got to LEO with 100t props. ~33% more trips than the lowballed TSTO version.

So SSTO uses 56,640t vs 51,700t. So more flights, and more propellants. With a more realistic mass, I think it's going to be much the same as the (very optimistic) SS example and be more like 2X as many flights.

I'm not seeing the point.

[magnemoe]

2 hours ago, tater said:

Each full ring is ~1.6t.

SH now is ~200t. even with min engines for single stage, it needs 25 engines. That's 50t on the bottom. it will need to put the header tank at the nose, else it lawn-darts engines first, it's gonna need the high AOA, and it's not quite twice as heavy, with 3X the engine mass aft. It will need more landing propellant mass. Call it 40t.

Chucking it in the calculator at 200t dry mass it gets 69t to LEO. Assume that 200t includes the 40t. 69t to LEO, better than FH. Except it has 1 purpose, this is a tank with a tiny nose cone—it's to deliver residual propellant. It will take 16 trips to take up that SS that got to LEO with 100t props. ~33% more trips than the lowballed TSTO version.

So SSTO uses 56,640t vs 51,700t. So more flights, and more propellants. With a more realistic mass, I think it's going to be much the same as the (very optimistic) SS example and be more like 2X as many flights.

I'm not seeing the point.

Don't see how they could reduce weight to 200 ton with landing fuel. I rather expect weight to go up as you need the flaps who is larger and tiles over an larger area also the nose who has to hold the larger header tanks but they might get away with less steel as it don't have to hold up the 1300 ton of Starship so that might cancel out the extra dry mass. 
I say 69 ton - 40 ton for landing fuel so 29 ton all depending on the dry mass of this. They has not landed Starship yet. 

Some will make an smaller Starship+ superheavy version as an fully reusable satellite launcher able to put say 20 ton into LEO. 
 

[tater]

18 minutes ago, magnemoe said:

Don't see how they could reduce weight to 200 ton with landing fuel.

Nor do I. I have intentionally used the most ridiculously optimistic numbers for the SSTO version I can imagine just to show it's not operationally viable, even if it worked.

18 minutes ago, magnemoe said:

I rather expect weight to go up as you need the flaps who is larger and tiles over an larger area also the nose who has to hold the larger header tanks but they might get away with less steel as it don't have to hold up the 1300 ton of Starship so that might cancel out the extra dry mass. 
I say 69 ton - 40 ton for landing fuel so 29 ton all depending on the dry mass of this. They has not landed Starship yet. 

Some will make an smaller Starship+ superheavy version as an fully reusable satellite launcher able to put say 20 ton into LEO. 
 

Yeah, and we won't have a benchmark until SpaceX actually manages to have a Starship survive from orbit.

[Exoscientist]

On 7/30/2022 at 8:53 PM, tater said:

I was proposing a tug. Still to be tanked up. If it was 50t lower mass, it gets about that much more to LEO.

 Note that SpaceX does not need the passenger section, or thermal protection, or upper and lower flaps for the lunar lander version of the Starship. In fact it probably can even do without a fairing, with the Orion capsule attached above the tank with an adapter for its smaller diameter.

 SpaceX wants to argue it might only take 4 refuelings for the lunar mission. Then they should acknowledge that this form of the Starship with no passenger section, i.e., with the upper rings above the tank removed, might only mass 50 to 60 tons. But that would mean this format of the Starship could do 40 to 50 tons payload to LEO as an expendable SSTO. Then any reasonable estimate for reusability systems would still allow significant payload as a reusable SSTO.

   Robert Clark

 

[tater]

8 minutes ago, Exoscientist said:

But that would mean this format of the Starship could do 40 to 50 tons payload to LEO as an expendable SSTO.

Why would anyone want to throw a perfectly good rocket away?

What is 40-50t that is worth trashing a rocket over?

 

9 minutes ago, Exoscientist said:

Then any reasonable estimate for reusability systems would still allow significant payload as a reusable SSTO.

Where "significant" is some low tonnage SpaceX doesn't care about.

[sevenperforce]

30 minutes ago, Exoscientist said:

Then they should acknowledge that this form of the Starship with no passenger section, i.e., with the upper rings above the tank removed, might only mass 50 to 60 tons.

Elon already said that a maximally slimmed-down Starship would only be around 40 tonnes.

30 minutes ago, Exoscientist said:

But that would mean this format of the Starship could do 40 to 50 tons payload to LEO as an expendable SSTO.

That's stretching the math.

30 minutes ago, Exoscientist said:

Then any reasonable estimate for reusability systems would still allow significant payload as a reusable SSTO.

No, it would not, because your estimates for "reusability systems" aren't reasonable.

There's already a reusability system for Starship: the heat shield and flaps and forward LOX tank. There's no re-entry configuration for Starship that would achieve meaningful payload at lower dry mass than the existing cargo Starship.

[tater]

1 hour ago, sevenperforce said:

There's already a reusability system for Starship: the heat shield and flaps and forward LOX tank. There's no re-entry configuration for Starship that would achieve meaningful payload at lower dry mass than the existing cargo Starship.

This is the critical bit. What Starship actually masses when they finally recover one—that tells us what is needed, at least to start.

There's every chance they have mass creep upwards after all.

[magnemoe]

On 9/9/2022 at 10:05 PM, tater said:

The SSTO lose payload, too. If a reusable TSTO gets 2-3%, an SSTO probably gets half that.

Once a vehicle is truly "operationally reusable" (akin to an aircraft, even if a special use aircraft like an SR-71), then the propellant costs dominate.

Suspect maintenance was an higher cost than fuel on the SR-71 and probably the B-2. 
And I guess SpaceX know this, and why they want to catch the first stage with the claw and other stage zero focus. 

[tater]

16 minutes ago, magnemoe said:

Suspect maintenance was an higher cost than fuel on the SR-71 and probably the B-2. 
And I guess SpaceX know this, and why they want to catch the first stage with the claw and other stage zero focus. 

True, but the propellant mass to vehicle mass for those is sorta tiny compared to SS/SH 

[Exoscientist]

17 hours ago, tater said:

Why would anyone want to throw a perfectly good rocket away?

What is 40-50t that is worth trashing a rocket over?

Where "significant" is some low tonnage SpaceX doesn't care about.

 The current plan is to not reuse the lander because it would take too much propellant weight to return it.

 The Starship without payload section could get twice the payload as an expendable SSTO as the Falcon 9 as an expendable.

 Here's a more representative image of the Starship as a lunar lander without a crew capsule(Orion):  

 

     Robert Clark

[tater]

2 minutes ago, Exoscientist said:

The current plan is to not reuse the lander because it would take too much propellant weight to return it.

It's reused—in space.

LSS can fly RT from LEO to the lunar surface and back to LEO (propulsively) if slightly stretched, or if a tug gives it a push.

 

2 minutes ago, Exoscientist said:

 The Starship without payload section could get twice the payload as an expendable SSTO as the Falcon 9 as an expendable.

50t is ~2 F9 reusable. Only thing expended is 2 second stages, the bulk of the price being 2 engines. SS expended expends 6-9 Raptor engines. Better to throw away the 2 upper stages of F9.

[sevenperforce]

24 minutes ago, Exoscientist said:

The Starship without payload section could get twice the payload as an expendable SSTO as the Falcon 9 as an expendable.

Again, these numbers aren't right; you need to factor in the additional engines that an expendable Starship Lite SSTO would need to get off the ground.

But even if they were right, what of it? If SpaceX is willing to fly Starship upper stages expendable, then they would just put those expendable upper stages on top of the reusable Superheavy booster. Then they only need to throw away three vacuum engines and they get 175 tonnes to LEO.