Could the SuperHeavy booster be SSTO?

[Exoscientist]

On 7/6/2022 at 12:35 AM, tater said:

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

"It should be under 200t..."

"160-200t"

Propellant residuals are ~20t.

About 10 min in.

 

 Thanks for the link to  the video. In addition to the dry weight range it also gives the propellant load as 3,600 tons, higher than the 3,400 tons I used. Also the Raptor 2 will have greater thrust  to 230 tons at sea level. I’ll update the Silverbird estimator with these numbers. Since the estimator uses the vacuum thrust I’ll estimate it as proportionally larger by the 358s vacuum Isp compared to the 330s sea level Isp. That brings the vacuum thrust to 230*(358/330) = 249.5 tons, so to 8,230 tons for 33 engines. The input to the Silverbird estimator, still using the 140 ton dry mass, is:

 

 And the result for expendable payload would be:

 

 If the dry mass is 160 tons, then the expendable payload according to the estimator would be 155 tons. And if the dry mass is 200 tons then the expendable payload would be 115 tons.  

Using the 10% of dry mass estimate for the payload loss due to reusability, it would still be at 95 tons reusable payload even for the 200 ton dry mass estimate for the SuperHeavy.

 By the way, I wonder if the nozzle exit area can be increased due to the higher chamber pressure. Could the sea level Raptor have the vacuum Isp increased to 372s? Small Isp increases are known to allow large increases in payload. The Silverbird estimator then gives a payload estimate of 208 tons.

  Robert Clark 

Edited July 9, 2022 by Exoscientist

[RCgothic]

53 minutes ago, Exoscientist said:

Using the 10% of dry mass estimate for the payload loss due to reusability, it would still be at 95 tons reusable payload even for the 200 ton dry mass estimate for the SuperHeavy.

No.

Double it and add 50t for propellant.

[tater]

1 hour ago, Exoscientist said:

Using the 10% of dry mass estimate for the payload loss due to reusability, it would still be at 95 tons reusable payload even for the 200 ton dry mass estimate for the SuperHeavy.

From orbit it is nothing like 10% I think.

[RCgothic]

It's maybe 10% for suborbital RTLS 1st stages not counting boostback, entry and landing propellant.

It's double and add 50+t propellant from LEO.

[sevenperforce]

On 7/9/2022 at 3:21 PM, Exoscientist said:

Using the 10% of dry mass estimate for the payload loss due to reusability. . . .

Garbage in, garbage out. The "10% of dry mass estimate" is a garbage number so you get meaningless results. Absolutely meaningless.

On 7/9/2022 at 3:21 PM, Exoscientist said:

 By the way, I wonder if the nozzle exit area can be increased due to the higher chamber pressure. Could the sea level Raptor have the vacuum Isp increased to 372s?

No, there isn't enough space on the back end to fit bigger engines. 33 is crowding it quite a bit already.

On 7/9/2022 at 3:21 PM, Exoscientist said:

Small Isp increases are known to allow large increases in payload. The Silverbird estimator then gives a payload estimate of 208 tons.

Silverbird isn't going to factor in additional Isp losses from a poorly-optimized sea level nozzle.

[Exoscientist]

On 7/1/2022 at 6:06 PM, Exoscientist said:

 The military is considering using the Starship for troop transport:

TECH & SCIENCE

Pentagon Mulls Using Elon Musk's Rockets to Deploy Troops From Space

BY ED BROWNE ON 6/24/22 AT 12:33 PM EDT

https://www.newsweek.com/pentagon-military-elon-musk-starship-deploy-troops-space-1718969

 Clearly for such a use it would be better to have this capability as a single stage. Considering that the military, like NASA, overpays for everything SpaceX could probably get a billion dollar deal for developing this capability.

 Having a SSTO capability would be a great selling point for this purpose.

   

 

 I’m not actually sanguine about the 33 engines on the Superheavy being in common use for passenger flights, either for a TSTO or a SSTO. 

 Better would be to develop the 9-engined Starship either as a first stage booster, with a mini-Starship as upper stage or as an SSTO.

 

 

  Robert Clark

Edited July 12, 2022 by Exoscientist
Included video.

[Shpaget]

On 7/2/2022 at 12:06 AM, Exoscientist said:

Having a SSTO capability would be a great selling point for this purpose.

Why?

Anybody who knows anything about rockets will immediately realize that forcing the single stagedness is wasting potential and sacrificing payload.

[Exoscientist]

1 hour ago, Shpaget said:

Why?

Anybody who knows anything about rockets will immediately realize that forcing the single stagedness is wasting potential and sacrificing payload.

 Operational simplicity for rapid deployment if needed. Also the same vehicle can be used to come back or rescue if not needing first stage left back at the launch site. 
 

 Robert Clark

[Shpaget]

Getting there is one thing, but refueling a Superheavy in a middle of nowhere, somewhere in the middle eastern desert is not going to happen.

Obviously SSTOTWR (single stage to orbit twice without refueling) is just fantasy.

[wumpus]

10 hours ago, Shpaget said:

Why?

Anybody who knows anything about rockets will immediately realize that forcing the single stagedness is wasting potential and sacrificing payload.

There appears to be some folks at NASA willing to throw money at SSTOs on the basis that the logistics and integration benefits outweigh the tiny payload capacity (DC-X is my best example).  I wouldn't be surprised if some of the smaller new space companies scarf up some money to produce powerpoint decks of their boosters being used for SSTO.  Probably not worth it to Spacex to lose focus on something as critical as Superheavy to teach expensive lessons on how stupid SSTO is.

[Exoscientist]

On 7/11/2022 at 8:57 PM, Exoscientist said:

 

 I’m not actually sanguine about the 33 engines on the Superheavy being in common use for passenger flights, either for a TSTO or a SSTO. 

 Better would be to develop the 9-engined Starship either as a first stage booster, with a mini-Starship as upper stage or as an SSTO.

https://twitter.com/NASASpaceflight/status/1546605684070006784

  Robert Clark

Elon seems to be acknowledging the larger number of engines on this vehicle increased the likelihood of this occurring:

 

12:29 AM · Jul 12, 2022·Twitter for iPhone 296 Retweets 52 Quote Tweets 4,583 Likes

 

 

Edited July 13, 2022 by Exoscientist
Typo

[SunlitZelkova]

Just now, Exoscientist said:

Elon seems seems to be acknowledging the larger number of engines on this vehicle increased the likelihood of this occurring:

There are certainly risks with 33 engines but I don’t think this is a good example of them.

Isn’t he referencing an issue with Raptor itself, not the number?

[Exoscientist]

On 7/6/2022 at 12:35 AM, tater said:

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

"It should be under 200t..."

"160-200t"

Propellant residuals are ~20t.

About 10 min in.

 

 Elon  also says in that video the tank mass is at 80 tons. Then if SpaceX can reduce the tank wall thickness from 4 mm to 3 mm as they expect that’s cutting 25% off the tank mass, or 20 tons.  Then the expendable SSTO Superheavy payload could be 175 tons to 135 tons, depending on if the SuperHeavy dry mass, prior to the tank mass saving, were 160 tons to 200 tons.

 The grid fins also can be reduced in weight. Elon was unhappy with their mass in the video at 3 tons each for a total mass of 12 tons. Their mass can be reduced to a fraction of this weight using ceramics, subtracting an additional 10 tons from the dry mass, increasing the expendable payload  to somewhere in the range of 185 tons to 155 tons.

  Robert Clark

Edited July 12, 2022 by Exoscientist
Clarity

[Exoscientist]

On 7/11/2022 at 1:06 PM, sevenperforce said:

Garbage in, garbage out. The "10% of dry mass estimate" is a garbage number so you get meaningless results. Absolutely meaningless.

No, there isn't enough space on the back end to fit bigger engines. 33 is crowding it quite a bit already.

Silverbird isn't going to factor in additional Isp losses from a poorly-optimized sea level nozzle.

 Even Apollo-era TPS was at ~15% of landed mass. Landing gear at about 3%, so < 20% of landed mass for both. Even subtracting off from the payload a mass of 20% of landed mass could still give you a reusable SSTO vehicle at over 100 ton payload.

  Robert Clark 

[tater]

A few here have often pointed out the fallacy that drives many arguments against Starship (Zubrin's ideas, for example). The claim is that it is too big, and a smaller version would make more sense, etc. There are not huge payloads to utilize such a system, etc. The answer is NOT, "build it and they will come" although if prices dropped a couple orders of magnitude maybe that might happen at some point. The mistake is in thinking that the vehicle has a primary purpose that is not going to Mars.

If the tanker made sense as an SSTO, they'd probably do that.

[sevenperforce]

22 hours ago, Exoscientist said:

 Even Apollo-era TPS was at ~15% of landed mass. Landing gear at about 3%, so < 20% of landed mass for both. Even subtracting off from the payload a mass of 20% of landed mass could still give you a reusable SSTO vehicle at over 100 ton payload.

So you smear Apollo-era TPS all over it, add "landing gear", and...what?

Watch it burn to slag as it plummets uncontrollably, tail-first into the atmosphere?

[sevenperforce]

On 7/12/2022 at 7:28 PM, Exoscientist said:

The grid fins also can be reduced in weight. Elon was unhappy with their mass in the video at 3 tons each for a total mass of 12 tons. Their mass can be reduced to a fraction of this weight using ceramics, subtracting an additional 10 tons from the dry mass, increasing the expendable payload  to somewhere in the range of 185 tons to 155 tons.

If it's expendable, why would it need grid fins at all?

That's why this whole notion just seems so silly -- it's like you're not thinking about the vehicle as an actual vehicle with actual meaningful parts.

You're still going to need a fairing for whatever payload you're payloading. Even if the fairing is no longer than the one on Falcon 9 and only as wide as the rest of the vehicle, it's still going to have a mass of at least 5.7 tonnes. You're going to need a mechanism to open and close that fairing, but let's ignore that. You claim that the weight of the grid fins can be reduced by over 80% "using ceramics" but you give absolutely no explanation or citation for these magical featherweight ceramics; aerospace/industrial ceramic parts are typically closer to half the weight of equivalent metal parts.

In the video @tater linked, Elon notes 58 tonnes worth of engines, 80 tonnes tank and structural mass, 20 tonnes for the interstage and grid fins and avionics, and roughly 20 tonnes of residuals after landing. So that would be 178 tonnes right there. Of course we're going up to 33 engines instead of 29 so that brings us up to 186 tonnes. Let's just hypothesize that they can use "ceramics" and other savings to shave 10 tonnes off the interstage and grid fins and avionics. You talked about going to 3mm steel instead of 4mm steel but that's only going to be for the tank walls themselves, not for structural mass. Let's say they can cut 10 tonnes off of tank mass. Add in the weight of the fairing and you're looking at 172 tonnes landed mass. You claim landing gear is 3%, which would be 5.2 tonnes, but I don't know how you're going to manage that; Falcon 9's four landing legs have a total mass of 2.4 tonnes for a vehicle with a dry mass of around 25 tonnes. So 10% would be closer to the right number. Let's be generous and put it at 6% (closer to your 3% number than to F9's 10% number) and peg the landing gear at 10.3 tonnes which takes us up to 182 tonnes.

Let's pretend, somehow, that you could put TPS on the butt end and somehow prevent the engines from burning to slag. Then it can enter rear-first. It's absolutely not possible, but let's wave our hands and pretend it is. If Apollo was 15% let's give Superheavy a 10% landed mass margin. So that's 18.2 tonnes of TPS, bringing our landed mass up to 200 tonnes. I'll ignore the mass growth requirements on the landing gear, etc.

So what, then, does the landing burn look like? We can assume that like Falcon 9, Superheavy will need to initiate its burn in the transonic regime, at around 310 m/s vertical speed. It needs to limit its hoverslam to around 3 gees to avoid damage; the landing burn will thus take at least 11 seconds (probably longer but we're being generous to your SSTO idea). That 11 seconds will be 108 m/s of gravity drag, so the total burn needs to be 418 m/s. At the sea level specific impulse of 330 seconds, that's 27.8 tonnes of landing propellant, bringing our re-entry mass to 228 tonnes. We will again ignore mass growth, here related to TPS.

In space, you'll need roughly 100 m/s of dV to deorbit. We'll ignore ullage and maneuvering RCS and everything else like that. You'll benefit from the higher vacuum specific impulse, so that's nice; it'll cost you a little over 6 tonnes of propellant, bringing our effective dry mass up to 234 tonnes. So let's plug all this into Silverbird and see what it tells us now.

This *is* a restartable stage so I checked yes for that box (starting engines uses extra props). I set residuals at 0% because we are already factoring them into dry mass, and I reduced propellant by the amount of propellant needed for deorbit, landing burn, and residuals.

So yes.

If you can find a way to magically keep the grid fins and butt end of Superheavy from melting to slag on re-entry with 18.2 tonnes of TPS, then you could convert it into a reusable SSTO that delivers ~97 tonnes of payload to LEO.

How you plan to do that, or what 97-tonne payload you plan to launch in a 13x9 meter fairing, is anybody's guess. 

 

[Exoscientist]

On 7/2/2022 at 11:14 AM, tater said:

Not for the Ship in orbit. Ship right now is more like 100t dry (tiles, flaps, etc), possibly more. I think I have seen others do the math showing that Starship could maybe be an SSTO with functionally no payload.

I just don't see the point in SSTOs for most use cases, and for SpaceX, it doesn't get them to Mars anyway. Point to point is another optimization, and the safety/regulatory issues are nontrivial (to put it mildly).

Stage 1 recovery is pretty much a given. They can certainly achieve that, it's just a matter of how crazy they can be in terms of stage recovery—landing on/with the pad/tower, or having to stick legs on it like F9.

 

 You seem to be agreeing with me in this prior comment on the SpaceX discussion thread that the tanker Starship is at 50 tons dry mass not the 100 tons of the passenger Starship:

.

 But that’s the essence of the argument that the tanker Starship can carry significant payload as an SSTO expendable. Then a reasonable estimate of the mass of reusabilty systems  still allows significant payload as a reusable SSTO for the Starship.

 

[tater]

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

[sevenperforce]

22 hours ago, Exoscientist said:

You seem to be agreeing with me in this prior comment on the SpaceX discussion thread that the tanker Starship is at 50 tons dry mass not the 100 tons of the passenger Starship

The words @tater literally said were "no flaps, [no] TPS, and shed the nose to reduce mass" -- that's not even remotely the same as saying that the standard tanker starship is 50 tonnes.

The standard tanker starship is approximately 85 tonnes and it needs 30 tonnes of propellant residuals for deorbit and landing.

[Exoscientist]

 Timeline on perceptions of SSTO’s:

1.)SSTO’s can’t reach orbit.

2.)OK, they can reach orbit but not carry much payload.

3.)OK, they can carry significant payload, but TSTO’s can carry more.

Coming soon:

4.) Yes! SSTO’s are GO!

 

  Bob Clark

Edited September 4, 2022 by Exoscientist
Edited to emphasize the support for SSTO’s once operational.

[tater]

21 minutes ago, Exoscientist said:

Timeline on perceptions of SSTO’s:

1.)SSTO’s can’t reach orbit.

2.)OK, they can reach orbit but not carry much payload.

3.)OK, they can carry significant payload, but TSTO’s can carry more.

Coming soon:

4.) Yeah, SSTO’s are GO!

All that matters is COST.

Forget the assumptions made around reuse of the SSTO vs payload don't close (the mass required for orbital recover is high). A low mass SSTO might well be a thing, and with whatever payload you want to claim. The cost is the entire cost of the vehicle, plus props, because it cannot be reused. The TSTO version proposed has a cost equal to the operational cost plus propellants (and amortized construction of the vehicle). A cheap expendable 747 might be cheap—for a 747—but it still costs more than simply operating a regular 747 for the same payload/trip, reusing it.

If you propose a SSTO with low enough mass penalty for full reuse (make up the numbers)—we can simply stop using the real SS/SH numbers for mass, and use the numbers you come up with for the SSTO, and we get much more to space in the same single launch, because TSTO is always better.

Edited September 4, 2022 by tater

[tater]

On 7/30/2022 at 6:09 PM, Exoscientist said:

 But that’s the essence of the argument that the tanker Starship can carry significant payload as an SSTO expendable. Then a reasonable estimate of the mass of reusabilty systems  still allows significant payload as a reusable SSTO for the Starship.

That part is simply untrue. The reasonable estimate for reusability is what the mass is of those on SS/SH. SS is the benchmark because it has to do orbital EDL. That means the full orbital TPS system. It means props reserved for deorbit, and landing. It means flaps. It means thicker steel to survive orbital EDL and reuse.

So everything SS has, but BIGGER and hence more massive.

Starship is supposed to be able to do EDL, right? What's the dry mass, 100t? 85t? It carries 1200t of props.

Call it 85t. Looks like an SSTO SS, if 85t dry can get 0-15,200kg to LEO! Except that can't come back, we're talking dry mass with no props for EDL. I want to say SS reserves ~20t for EDL—which is more than the upper limit of payload.

 

Edited September 4, 2022 by tater

[wumpus]

17 hours ago, Exoscientist said:

 Timeline on perceptions of SSTO’s:

1.)SSTO’s can’t reach orbit.

2.)OK, they can reach orbit but not carry much payload.

3.)OK, they can carry significant payload, but TSTO’s can carry more.

Coming soon:

4.) Yes! SSTO’s are GO!

 

  Bob Clark

SSTOs can reach orbit.

But you have to subtract the weight of all the launch engines (that provide a TWR>1, so a lot of mass) as well as all fuel tanks that could be discarded in a TSTO.

At least one TSTO rocket can recover 90% of the engines and 90% of the mass of the (empty) rocket.  At least one TSTO rocket in production should reach 100% for both.

SSTO can carry minimal payload (if that).  They certainly don't allow for the mass of a heat shield to cover the entire (massive, if any significant payload is considered) rocket, nor any means of landing.  The "means of landing" is critical: Falcon 9 stages need to be around 10% full for recovery.  Try to find a SSTO that has a payload anywhere near 10% of its total mass, and try replacing it with fuel (and don't forget the mass of the heat shield).

So  SSTO is only considered for expendable rockets where payload is not a consideration, nor the extra cost of the overpowerful engines needed to lift the silly thing into orbit.  And they will inevitably be much more expensive for the same payload as a TSTO.

Sounds like a NOGO  for the foreseeable future.

[Exoscientist]

23 hours ago, tater said:

That part is simply untrue. The reasonable estimate for reusability is what the mass is of those on SS/SH. SS is the benchmark because it has to do orbital EDL. That means the full orbital TPS system. It means props reserved for deorbit, and landing. It means flaps. It means thicker steel to survive orbital EDL and reuse.

So everything SS has, but BIGGER and hence more massive.

Starship is supposed to be able to do EDL, right? What's the dry mass, 100t? 85t? It carries 1200t of props.

Call it 85t. Looks like an SSTO SS, if 85t dry can get 0-15,200kg to LEO! Except that can't come back, we're talking dry mass with no props for EDL. I want to say SS reserves ~20t for EDL—which is more than the upper limit of payload.

 

 

 Actually, SpaceX has not given the dry mass for the tanker version of the Starship, only for the passenger version meant to carry 50 to 100 colonists of a 6 month flight to Mars.

  Bob Clark