Could the SuperHeavy booster be SSTO?

[sevenperforce]

22 minutes ago, Exoscientist said:

 I’ve acknowledged the expendable TSTO gets more payload to orbit than the expendable SSTO.

You've acknowledged this, but you don't seem to accept it.

Sam and Tom are the same size. Sam is an SSTO. He can take 10 apples to orbit. Tom is a TSTO. He can take 30 apples to orbit.

Let's suppose that if we reuse Sam, Sam will lose 10% of his payload capacity. Sam can now only take 9 apples to orbit. Let's further suppose that if we reuse Tom, Tom will lose 50% of his payload capacity. Tom can now only take 15 apples to orbit.

Tom still carries more apples than Sam.

See??

Even if TSTO reuse was a 50% payload penalty and SSTO reuse was a 10% payload penalty (which is CATEGORICALLY wrong), you still have to account for the fact that the TSTO is carrying 3X more payload to begin with.

22 minutes ago, Exoscientist said:

The point I’m making is the reusable SSTO meets or exceeds the reusable TSTO payload.

It does not, it will not, and it cannot, and I have painstakingly shown you the math to prove it.

If my math is wrong, show me.

22 minutes ago, Exoscientist said:

Note this unused propellant is doubly disadvantage as far as orbital payload is concerned. First, it adds deadweight on ascent to orbit, and secondly it reduces the propellant load that can be used for that ascent.

Small note, but this is not "doubly disadvantage". Dead weight on ascent and reduction in propellant capacity are two ways of saying the same thing. And keep in mind that thanks to the magic of staging, reduced performance on the first stage has only a small impact on the second stage payload.

22 minutes ago, Exoscientist said:

 Elon has stated this reduction in payload on full reusability numerous times:

Elon Musk

@elonmusk 

Replying to @PPathole and @SpaceX

Optimized, fully-reusable Starship is ~150t to same reference orbit as Saturn V. In expendable mode, Starship payload would be 250t to 300t.

4:41 PM · Mar 9, 2022·Twitter for iPhone 747 Retweets 56 Quote Tweets 12.5K Likes

 

So even by Elon's statement, the number is a range of 40-50%, not the flat 50% you keep claiming.

And this is for a specific architecture using a specific approach to reuse. You can't just handwave and pretend that the numbers for one vehicle are going to apply to all other vehicles and architectures.

22 minutes ago, Exoscientist said:

  In contrast, the SSTO loses less than 10% of the dry mass on adding reusability systems, allowing it to meet or exceed the payload to orbit of the reusable TSTO. 

No.

It does not lose "less than 10% of the dry mass on adding reusability systems." That number is wrong. Realistically, making an orbital vehicle reusable is going to require you to approximately double its dry mass.

And even if the reuse penalty was only 10% of dry mass, that means MORE than a 10% payload loss, because an SSTO dry mass is inevitably going to exceed its payload mass.

And even if the reuse penalty to payload was only 10%, that STILL wouldn't make it exceed the performance of a reusable TSTO, because a TSTO has so much more payload to begin with.

2 minutes ago, RKunze said:

Correct me if I got the basic math wrong, but how can even an expendable Superheavy SSTO (with a payload capacity of around 50t, as mentioned before in this thread) exceed the 150t of the fully reusable Superheavy/Starship TSTO?

Well you would have to scale based on total vehicle mass. Starship+Superheavy is about 40% heavier than Superheavy alone, so it's not a fair comparison.

But you're right, the basic math still doesn't work out. If you scaled up Superheavy by 40% to have the same launch mass as the entire stack, then expendable payload capacity would only increase to 70 tonnes, which is still less than half the payload capacity of a fully reusable Superheavy/Starship TSTO.

35 minutes ago, tater said:

Any assumptions you make about the mass requirements for SSTO recovery also apply exactly to the TSTO version (except the booster would actually require less mass, as it is not doing EDL from orbit).

The booster does require less recovery mass, but for a boostback architecture it requires a lot of reserve propellant, which is treated as "dry mass" for ascent purposes.

However, what @Exoscientist seems to think is that this boostback propellant cuts significantly into the payload of the upper stage, when in reality it does not.

He is conflating recovery mass (which he insists is only 10% even though it's more) with payload penalties, when those are measurements of two completely different things.

He's also ignoring the foundational advantage of the TSTO carrying more payload to begin with.

[RKunze]

3 minutes ago, sevenperforce said:

20 minutes ago, RKunze said:

Correct me if I got the basic math wrong, but how can even an expendable Superheavy SSTO (with a payload capacity of around 50t, as mentioned before in this thread) exceed the 150t of the fully reusable Superheavy/Starship TSTO?

Well you would have to scale based on total vehicle mass. Starship+Superheavy is about 40% heavier than Superheavy alone, so it's not a fair comparison.

But that's kind of the point I'm making - a Superheavy SSTO gets way less paylod to orbit than a Superheavy/Starship TSTO, despite the fact that the Superheavy booster has to lift not only the payload, but the entire wet mass of the Starship second stage...

And I've been just to plain lazy to do the math for an SSTO scaled up to full TSTO mass. Besides, it's silly anyway. If SSTO could get even near TSTO performance, everybody would build them, because two (or more) stages are way more complex than a single stage, and complexity is the enemy....

[RCgothic]

2 hours ago, Exoscientist said:

 I’ve acknowledged the expendable TSTO gets more payload to orbit than the expendable SSTO. The point I’m making is the reusable SSTO meets or exceeds the reusable TSTO payload. The reason is the fully reusable TSTO loses 50% of its payload due to the large amount of propellant that has to remain unused on ascent to orbit in order to cancel out the first stage forward motion and to boost it back to the launch site. Note this unused propellant is doubly disadvantage as far as orbital payload is concerned. First, it adds deadweight on ascent to orbit, and secondly it reduces the propellant load that can be used for that ascent.

No, SSTO never matches TSTO. 

The reuse penalty is *already included* in the 150t to LEO. That's 100% reusable Starship, 100% reusable Superheavy. 

The Starship expendable payload is 250t to 300t. Superheavy is still recovered in this figure.

The *fully expendable* Starship/Superheavy combo is somewhere between 420-500t .

 

Superheavy SSTO can do maybe 50t payload expendably and does not have enough margin for recovery.

[tater]

There are many "what if" concepts that could be discussed WRT Starship/Super Heavy that are in fact very interesting, and plausible uses of this system with, and without reuse, or without reuse on Earth (reusing orbital components), etc. All of them are worth investigating, IMO, depending on the desired mission. SSTOs just aren't useful for almost any use case I can imagine in a world where SS/SH is functioning at any level.

Other worthwhile tangents?

Fully expended SH, and a bespoke upper stage/fairing. No need for a "starship" if expended. No need for 4mm steel, thinner, lighter steel, etc.

Reused SH, expended upper stage.

Reused or expended SH, and an upper stage designed for reuse in space. A tug, or a crew vehicle to ferry humans to and from cislunar space, for example.

 

[StrandedonEarth]

2 hours ago, tater said:

Reused SH, expended upper stage.

It is somewhat surprising that they are going for full re-use from the start, but I guess that's the only way tankers (and missions requiring tankers) make any sort of economic sense. At the target price foe Raptor engines, expending the upper stage shouldn't cost that much, and they can certainly crank them out quick enough.

As to the OP, IIRC most first stage boosters can SSTO with negligible payload. But the gees nearing burnout might be too much for the structure on some of them (the ones that can't throttle down enough)

[sevenperforce]

2 hours ago, StrandedonEarth said:

It is somewhat surprising that they are going for full re-use from the start, but I guess that's the only way tankers (and missions requiring tankers) make any sort of economic sense.

They have expended everything so far. Okay, I mean, technically they did relaunch SN10 once.

2 hours ago, StrandedonEarth said:

As to the OP, IIRC most first stage boosters can SSTO with negligible payload. But the gees nearing burnout might be too much for the structure on some of them (the ones that can't throttle down enough)

Yes, that's something to keep in mind. Let's take the first stage of Atlas V, for example. It can absolutely SSTO without any boosters or upper stage. It can even put a small payload into LEO while doing so. Can it come back to Earth? No.

[mikegarrison]

As I understand it, most modern, high-performance first stages could get to a low orbit with no payload. But why?

[Exoscientist]

19 hours ago, tater said:

No. Not ever. This is honestly incoherent.

Any assumptions you make about the mass requirements for SSTO recovery also apply exactly to the TSTO version (except the booster would actually require less mass, as it is not doing EDL from orbit).

 

 The fully reusable Superheavy/Starship is variously estimated to get from 100 to 150 tons to LEO. The expendable SuperHeavy gets 160 tons to orbit per the Silverbird Astronautics payload estimater. But only ~10% of dry mass used for reusability systems, or ~15 tons for the 150 dry mass SuperHeavy. So reusable payload at ~145 tons for the SSTO SuperHeavy. This is in the range already for the reusable SH/SS. BUT not needing the upper stage its actually better than the reusable TSTO on a per gross mass basis.

  Robert Clark

 

[magnemoe]

11 hours ago, StrandedonEarth said:

It is somewhat surprising that they are going for full re-use from the start, but I guess that's the only way tankers (and missions requiring tankers) make any sort of economic sense. At the target price foe Raptor engines, expending the upper stage shouldn't cost that much, and they can certainly crank them out quick enough.

As to the OP, IIRC most first stage boosters can SSTO with negligible payload. But the gees nearing burnout might be too much for the structure on some of them (the ones that can't throttle down enough)

Kind of think single use starship is an way to do very heavy or high dV deep space missions. You also has to option to refuel in orbit but this require more infrastructure on starship. 

One pet idea of mine is an 3rd stage in the cargo bay who is launched empty but could be filled up in orbit together with starship. Thinkin it would be smart to have piping to the cargo hold anyway so you could convert an standard cargo starship to an tanker. 

[Exoscientist]

19 hours ago, RKunze said:

Correct me if I got the basic math wrong, but how can even an expendable Superheavy SSTO (with a payload capacity of around 50t, as mentioned before in this thread) exceed the 150t of the fully reusable Superheavy/Starship TSTO?

I won't even mention the 250 to 300 t of the expendable TSTO here...

 Rocket engineers regard reducing dry mass and increasing propellant load like gold. Yet by requiring that large amount of propellant to remain onboard the first stage unused during ascent is doing the opposite of both. Losing 50% of payload is a huge loss for payload.

 

19 hours ago, sevenperforce said:

Small note, but this is not "doubly disadvantage". Dead weight on ascent and reduction in propellant capacity are two ways of saying the same thing. And keep in mind that thanks to the magic of staging, reduced performance on the first stage has only a small impact on the second stage payload.

 

 Actually, not. There are lots of ways of having extra deadweight on ascent for a stage. For instance suppose you gave a stage wings for return. Then as far as payload to orbit is concerned that is deadweight. You could still use all the propellant though.

  Robert Clark

Edited July 5, 2022 by Exoscientist

[tater]

1 hour ago, Exoscientist said:

 The fully reusable Superheavy/Starship is variously estimated to get from 100 to 150 tons to LEO. The expendable SuperHeavy gets 160 tons to orbit per the Silverbird Astronautics payload estimater. But only ~10% of dry mass used for reusability systems, or ~15 tons for the 150 dry mass SuperHeavy. So reusable payload at ~145 tons for the SSTO SuperHeavy. This is in the range already for the reusable SH/SS. BUT not needing the upper stage its actually better than the reusable TSTO on a per gross mass basis.

Comparing a 100% reusable TSTO vs expended SSTO makes no sense. You also forget that the 150t of payload does not include the Starship itself at all. If SS with residuals for landing is ~100t+, then it's ~250t to LEO.

Switch the tons to LEO to $/kg.

The reused SS/SH is 100-150,000kg for the cost of the props (we'll call ground costs for both launches a wash for this guestimate).

The expended SSTO is the cost of the vehicle, including ~25 engines, plus the cost of the props (which is nearly as expensive as the props for the reused vehicle by itself).

If you compare an expended SS/SH—the proper comparison to make—then the payload for just a few more engines "spent" is a multiple of that tonnage to LEO.

All that matters is cost.

 

Edited July 5, 2022 by tater

[RCgothic]

Even if an upscaled EXPENDABLE SSTO SUPERHEAVY could put 150t pure payload in orbit, that:

1) DOES NOT match the 250t-300t of a similar expendable Starship/recovered Superheavy can manage whilst throwing away 24 or 27 fewer engines.

2) DOES NOT match the cheapness of a completely reusable Starship/Superheavy that throws nothing away.

Generously, if the dry mass of an upscaled SSTO Superheavy would be around 80-90t, then that'd be 230-240t to orbit total. The dry mass of a RECOVERABLE SSTO SUPERHEAVY would be around 160-180t, and the fuel reserves for landing would be around 50t, so the payload would be maybe 10-30t for basically the same fuel expenditure as the TSTO. So the SSTO recoverable Superheavy (if it even works):

3) COSTS 5-15 X AS MUCH PER KG as a fully recoverable TSTO in terms of the only substantial consumable, propellant.

Seriously, the TSTO performance and price is unassailable by an SSTO, even being generous. If recoverable SSTO Superheavy were preferable to TSTO Starship/Superheavy, that's what SpaceX would be building.

Edited July 5, 2022 by RCgothic

[sevenperforce]

56 minutes ago, Exoscientist said:

The expendable SuperHeavy gets 160 tons to orbit per the Silverbird Astronautics payload estimater.

But it absolutely does not.

If you put garbage numbers in,  you get garbage numbers out.

Superheavy has a dry mass of 200-220 tonnes, not the 136 tonnes you plugged into Silverbird. If you use realistic dry mass numbers and a tiny disposable fairing, expendable Superheavy puts 85-90 tonnes into LEO. 

56 minutes ago, Exoscientist said:

But only ~10% of dry mass used for reusability systems, or ~15 tons for the 150 dry mass SuperHeavy.

We’ve explained repeatedly why this 10% value is nonsense. But even if it was realistic, do the math with the non-garbage numbers. 10% of the mass of Superheavy is 20-22 tonnes, so subtract that from the 85-90 tonnes of actual SSTO payload and you get 65-68 tonnes. 

56 minutes ago, Exoscientist said:

So reusable payload at ~145 tons for the SSTO SuperHeavy. This is in the range already for the reusable SH/SS.

That’s because the quoted payload values for SH+SS are based on realistic, real-world engineering numbers, not made-up numbers that don’t represent reality.

If we plug your numbers (136 tonne SH, 45 tonne SS) for SH+SS into Silverbird, you get a payload capacity of 319 tonnes to LEO. 

If it in fact takes only 10% of dry mass to make an orbital stage fully reusable, as you claim, then making this SS reusable only reduces payload to 304 tonnes. And if your claim that boostback reuse reduces payload by 40-50% was correct (it’s not; that’s not what Elon was talking about), then making this SH reusable would reduce payload to 152-182 tonnes, which is still above the 145 tonnes you claim a reusable SSTO SH could get.

In reality, none of these numbers actually mean anything because they are all bogus to begin with. 

57 minutes ago, Exoscientist said:

Yet by requiring that large amount of propellant to remain onboard the first stage unused during ascent is doing the opposite of both. Losing 50% of payload is a huge loss for payload.

Yet that is not what Elon or anybody else ever said.

[sevenperforce]

What Elon actually said is that an optimized expendable Starship would put 250 tonnes into LEO while a reusable will put 150 tonnes into LEO. But that’s talking about Starship only; Superheavy is recovered in either case.

Using realistic numbers with Silverbird gives you an orbital payload of 308 tonnes, expending both Superheavy and Starship. So the payload penalty for boostback recovery of Superheavy is (308-250)/308 or ~19%.

[tater]

17 minutes ago, sevenperforce said:

Superheavy has a dry mass of 200-220 tonnes, not the 136 tonnes you plugged into Silverbird. If you use realistic dry mass numbers and a tiny disposable fairing, expendable Superheavy puts 85-90 tonnes into LEO. 

So for getting 60-90% of nominal SS/SH payload to LEO (100-150t), you only have to throw away an entire booster, and ~25 bleeding edge rocket engines designed to be reused many times.

Why reuse a rocket when you can throw it away, AND spend more!

 

[magnemoe]

1 hour ago, Exoscientist said:

 Rocket engineers regard reducing dry mass and increasing propellant load like gold. Yet by requiring that large amount of propellant to remain onboard the first stage unused during ascent is doing the opposite of both. Losing 50% of payload is a huge loss for payload.

 Actually, not. There are lots of ways of having extra deadweight on ascent for a stage. For instance suppose you gave a stage wings for return. Then as far as payload to orbit is concerned that is deadweight. You could still use all the propellant though.

  Robert Clark

One major benefit of having extra propellant over dry mass is that its extra propellant you can use if you have to. 
I'm pretty sure falcon 9 first stages are set up so if one engine fails early it will use the fuel set aside for landing to do an longer burn saving the payload but loosing the first stage. 

[tater]

The primary payload by mass for Starship/Super Heavy is... propellant. That's sort of the point. The goal is to refill spacecraft in LEO in a cost effective manner. Musk has flat out stated that the optimization for Starship is COST to LEO. Extra launches don't matter when the cost/launch is mostly propellant, and the propellant is mostly LOX.

Any model that throws engines away at this point is kooky, particularly on the booster, because booster recovery is virtually certain. Yeah, the tower catch might not be a thing, we'll have to see, but they can always stick legs on it like F9, it will just be a payload hit, and then a subsequent operational cost (dealing with the legs as they do with F9).

[sevenperforce]

3 hours ago, magnemoe said:

One major benefit of having extra propellant over dry mass is that its extra propellant you can use if you have to. 
I'm pretty sure falcon 9 first stages are set up so if one engine fails early it will use the fuel set aside for landing to do an longer burn saving the payload but loosing the first stage. 

Indeed.

This whole claim that an SSTO can be made reusable with a payload penalty equal to just 10% of the stage dry mass is ludicrous. If that was the case, we would have reusable upper stages by now, because reusing an upper stage is necessarily easier than reusing an SSTO. The dry mass of the Falcon 9 upper stage is only 4.5 tonnes; there have been PLENTY of missions where a 450-kg payload penalty would have been acceptable. Centaur is even lighter, at 2.25 tonnes; there have been many, many Atlas V missions where the launch vehicle had enough performance reserve to make up for a 225 kg payload penalty.

And we are finally on the cusp of having such a reusable upper stage now, Starship. At 85 tonnes dry, the reusable Starship has 89% mass growth over its notional expendable dry mass of 45 tons; adding in landing propellant reserves brings it to 115 tonnes or 156% mass growth. If it was possible to magically make an SSTO recoverable for only 10% of the vehicle dry mass, then SpaceX would simply apply those magical numbers to the notional 45-tonne expendable Starship and so the fully-reusable SH+SS would deliver 215 tonnes to LEO instead of only 150 tonnes.

There is no mechanism, architecture, or configuration which can safely return a rocket stage from orbit to the launch site, for rapid reuse, for a payload penalty less than 10% of its dry mass. It does not exist. 

[Exoscientist]

On 7/2/2022 at 4:50 PM, sevenperforce said:

I don't see how you're claiming you can make a stripped-down 50-tonne Starship recoverable using only 5 tonnes of recovery mass. You talk about PICA-X being lightweight and everything, which is great, but let's do the math. The Apollo CM heat shield was 1400 kg and covered a surface area of 11.95 square meters. The tank section and skirt of Starship, with no fairing at all, is 28 meters high, so covering one-half of it with TPS would mean 28 meters * pi * 4.5 meters = 396 square meters. Let's suppose PICA-X is half the weight of Apollo-era ablative TPS. 0.5 * 1400 kg * 396 m² / 11.95 m² = 23.2 tonnes. Let's be generous and say you can cut that in half again, both because Starship is fluffier than a crew capsule and because you only need to deal with LEO re-entry and not cislunar re-entry; that's still 11.6 tonnes, which is more than twice your estimate for total recovery mass.

 A fair point. Doing a google search, on “PICA-X”, “density”, the first google result that pops up gives it as .27 gm/cc, 270 kg/m³.  But we need to know the thickness used. This video shows an edge view at about the 1:20 point:

 

 It appears to be about 1 inch thick, 2.5 cm.  Then the volume of the PICA-X to cover only the windward half of the cylindrical vertical side of the Superheavy is:  (1/2)*Pi*9*70*.025 = 24.7 m³. Then the mass is 270*24.7 = 6,700 kg, 4.4% of the dry mass of the SuperHeavy stage.

  Robert Clark

[StrandedonEarth]

12 minutes ago, Exoscientist said:

 A fair point. Doing a google search, on “PICA-X”, “density”, the first google result that pops up gives it as .27 gm/cc, 270 kg/m³.  But we need to know the thickness used. This video shows an edge view at about the 1:20 point:

 

 It appears to be about 1 inch thick, 2.5 cm.  Then the volume of the PICA-X to cover only the windward half of the cylindrical vertical side of the Superheavy is:  (1/2)*Pi*9*70*.025 = 24.7 m³. Then the mass is 270*24.7 = 6,700 kg, 4.4% of the dry mass of the SuperHeavy stage.

  Robert Clark

Now add mounting hardware, felt/blanket backing, grid fins, and landing props. At least it doesn’t need legs if it is being caught. Oh, and beef up the structure to handle the side loads of reentry. 

[Exoscientist]

13 hours ago, sevenperforce said:

Superheavy has a dry mass of 200-220 tonnes, not the 136 tonnes you plugged into Silverbird. If you use realistic dry m

 

 Do you have a reference for the 200 ton dry mass of the SuperHeavy? The earlier ITS incarnation had a 25 to 1 mass ratio for the first stage:

 

  For the ITS first stage at 6,975 ton gross mass and 275 ton dry mass, that’s a mass ratio of 25.4 to 1. For the SuperHeavy at a propellant load of 3,400 tons a 25 to 1 mass ratio gives a dry mass of 140 tons.

  Robert Clark

[tater]

52 minutes ago, Exoscientist said:

 It appears to be about 1 inch thick, 2.5 cm.  Then the volume of the PICA-X to cover only the windward half of the cylindrical vertical side of the Superheavy is:  (1/2)*Pi*9*70*.025 = 24.7 m³. Then the mass is 270*24.7 = 6,700 kg, 4.4% of the dry mass of the SuperHeavy stage.

That entirely defeats the purpose as they don't want an ablative which they would have to reapply every flight.

27 minutes ago, Exoscientist said:

 Do you have a reference for the 200 ton dry mass of the SuperHeavy? The earlier ITS incarnation had a 25 to 1 mass ratio for the first stage:

"It should be under 200t..."

"160-200t"

Propellant residuals are ~20t.

About 10 min in.

[tater]

More to the basic point, @Exoscientist, what do you think Starship/Super Heavy is for?

Serious question.

Musk says it in the video above, and in nearly every other setting. To reduce the cost per kg to LEO, and to the surface of Mars. The latter seems like a joke, but it's not.

An SSTO doesn't get to Mars. Lower payload to LEO means more trips to refill a given mars ship—and anything that makes that harder, like ablative TPS, is off the table. SpaceX is fine with variants for customers that want to pay, expendable upper stages have been explicitly mentioned by Musk, and LSS is a Starship only reused in space. But in general they are not going to dev something major that is not in line with their own goals. Ideally they get others to pay for dev on things that further their goals.

Their goal is Mars.

All of us in this thread can think that's a crazy goal—but they don't.

[sevenperforce]

1 hour ago, Exoscientist said:

A fair point. Doing a google search, on “PICA-X”, “density”, the first google result that pops up gives it as .27 gm/cc, 270 kg/m³.  But we need to know the thickness used. This video shows an edge view

The video shows an edge view of something that is not PICA-X.

1 hour ago, Exoscientist said:

Do you have a reference for the 200 ton dry mass of the SuperHeavy? The earlier ITS incarnation had a 25 to 1 mass ratio for the first stage

The earlier ITS configuration was 33% wider and made of carbon composite instead of stainless steel.

You're once again comparing apples to oranges. Or, like, apples to broomsticks.

[RCgothic]

Making a recoverable SSTO Superheavy would require:

Stretching it to accommodate more fuel.

Adding a payload fairing and cargo door.

The engines are less protected than on Starship so that's additional mass for a wider and more substantial base skirt.

It'd also need fins like starship to prevent it orienting engines-first on re-entry, and they'd need to be a lot bigger than starship's. 

A full-length heat shield.

And structural reinforcements to cope with the horizontal entry and fin loading.

A normal Superheavy starts off at about 160-180t dry. All of the above could easily take it above 300t. The only positive is that there's no longer any need for grid fins.