Concept: SLS core as monster-ACES, modified for high orbit refilling & vac Start

[Robotbeat]

If you just assume SLS is uncancelable…

SLS looks totally obsolete if the two HLSes work. It’s used as a taxi for Orion, but otherwise nearly everything else is launched commercially. SLS operates at a very low flightrate, is stuck in a very uneconomical low rate manufacturing rut. And then every time it looks like there might be funding available, instead of increasing launch rate, the plan is always to upgrade SLS again.

But what might SLS actually be useful for in a world where large fully reusable launch vehicles exist?

Well, the EUS could be upgraded to work like ACES. But that has some overlap with Blue’s HLS and LM’s cislunar transporter.

The core is the lion’s share of the cost, though, and it seems pretty wasteful to spend a ton of money on an expendable hydrolox stage as a first stage/sustainer where hydrolox isn’t even that cost optimal.

So… SLS core has a pretty good mass ratio, SSME has a decent vacuum Isp in spite of being ground lightable.

Maybe the SLS core could be modified to do vacuum start and be refuelable?

Basically, maybe a path to future relevance for SLS may be as a monster version of ACES, refueled in high orbit or Gateway.

Popcorning is a problem. But seems like a 1000ton propellant hydrolox stage might actually be useful in a future where Starship (and similar vehicles) are a thing.

The low flightrate wouldn’t be a problem as you wouldn’t need more than one or two per year anyway (and you could possibly reuse them in-orbit like ACES).

You’d need vacuum/microgravity start capability for RS-25. (Possibly ullage motors could do some of this.) some sort of start pack. Addressing the insulation issue is also important. But you probably still would need to have it launch to orbit itself, so technically it’d still be launching so the rest of the SLS political supporters can still be satisfied (still needs boosters to get to orbit, etc).

But it’d be relevant again even in a world where Starship is a thing.

Throwing a bone here to the SLS folk.

SLS core as monster ACES.

[Jim]

SLS core as monster ACES.

No, it isn't.  It is first stage.
a.  Bad intertank for an upper stage.  Too much mass and open volume.
b. no ACS
c.  Worse than air start, RS-25 can't space start or restart.   Ullage rockets are not the fix
d.  Wrong type of insulation for space.
e.  Vents are in the wrong places

[Robotbeat]

I don’t disagree, except that the dry mass is not that bad. 987 tonnes of propellant, 88 tonnes dry mass. That’s better than EUS or most centaur variants from what I can tell.

There’s no realistic way to get such a huge volume of a stage to orbit except maybe like this, as a sustainer.

[deltaV]

You’d need vacuum/microgravity start capability for RS-25.

The original design of Ares I used an SSME upper stage but that idea was scrapped in favor of J-2X because modifying SSME to start multiple times in space was found to be too hard. That difficulty seems likely to kill your idea too.

[Todd Martin]

Thanks Robotbeat, I've been mulling over something similar.  If you look at the industry, you see:  ESA workhorse will be Ariane 6 which uses hydrolox.  Japan's workhorse will be the H3 and that uses hydrolox.  In the U.S., the Vulcan rocket will have a hydrolox 2nd stage and SLS has a hydrolox 2nd stage (ICPS and EUS).  India's LVM-3 has a hydrolox 2nd stage.  Any international effort to high energy destinations like the moon or Mars would benefit from orbital refueling.  As transit times and/or landing mass can be substantially increased when using these vehicles if orbital refueling of the second stage is available.

Currently, Gateway is planned to have a refueling module, but it is only going to store hydrazine and Xenon.  It will have the docking space to include a hydrolox fuel depot if funded. 

From what I can research, air-starting an RS25 is difficult while restarting an RS25 engine that has already been used during the flight is practically impossible. 

This would indicate to me that hypothetically, if you used BOLE solid boosters, omitted ICPS/EUS and any payload, a modified SLS core would need an orbital tug to place it into a useful orbit or dock it with Gateway.  That said, would the SLS core be correctly sized for refueling any of the vehicles mentioned above?   No.  It would hold much more fuel than needed.

I think it would be better to consider adapting the EUS as a hydrolox fuel depot than the core tank.  The EUS can do air-start, can do engine re-start, and is smaller in mass to move to Gateway.   

[Robotbeat]

You’d need vacuum/microgravity start capability for RS-25.

The original design of Ares I used an SSME upper stage but that idea was scrapped in favor of J-2X because modifying SSME to start multiple times in space was found to be too hard. That difficulty seems likely to kill your idea too.

A single burn would be sufficient, but yes, it’d need to be modified.

[Robotbeat]

Thanks Robotbeat, I've been mulling over something similar.  If you look at the industry, you see:  ESA workhorse will be Ariane 6 which uses hydrolox.  Japan's workhorse will be the H3 and that uses hydrolox.  In the U.S., the Vulcan rocket will have a hydrolox 2nd stage and SLS has a hydrolox 2nd stage (ICPS and EUS).  India's LVM-3 has a hydrolox 2nd stage.  Any international effort to high energy destinations like the moon or Mars would benefit from orbital refueling.  As transit times and/or landing mass can be substantially increased when using these vehicles if orbital refueling of the second stage is available.

Currently, Gateway is planned to have a refueling module, but it is only going to store hydrazine and Xenon.  It will have the docking space to include a hydrolox fuel depot if funded. 

From what I can research, air-starting an RS25 is difficult while restarting an RS25 engine that has already been used during the flight is practically impossible. 

This would indicate to me that hypothetically, if you used BOLE solid boosters, omitted ICPS/EUS and any payload, a modified SLS core would need an orbital tug to place it into a useful orbit or dock it with Gateway.  That said, would the SLS core be correctly sized for refueling any of the vehicles mentioned above?   No.  It would hold much more fuel than needed.

I think it would be better to consider adapting the EUS as a hydrolox fuel depot than the core tank.  The EUS can do air-start, can do engine re-start, and is smaller in mass to move to Gateway.   

The idea would be a massive stage, too big to be launched as payload. EUS is small, not much larger than Centaur V, and about the same size as some variants of ACES. Cislunar tug or Blue Moon would have similar hydrolox propellant capacity, so the value in EUS is limited.

I just don’t see another realistic way to get a hydrolox stage that big to orbit.

[Robotbeat]

Target availability of sometime in the 2030s.

SSME air-start ran into time limits. They needed it for launching in a few short years. There’s enough time for developing vac-start capability.


(And it feels like whoever determines programmatic SLS objectives loves to spend money on development instead of operations anyway.)

[deltaV]

There’s no realistic way to get such a huge volume of a stage to orbit except maybe like this, as a sustainer.

Starship has a slightly higher propellant mass (1200 tonnes) than SLS core stage does (987 tonnes). Starship admittedly has a smaller volume and worse specific impulse and maybe worse dry mass since it's designed for reusability and atmospheric reentry but the missions that SLS core stage refilled in space could do would probably also be doable by Starship for far lower cost.

We'd be better off assigning SLS to launch a record-breaking fireworks display every July 4th and nothing else and use NASA's remaining money to do stuff in space with a less broken architecture.

I recall reading somewhere that NASA looked into using shuttle tanks in-orbit and determined the foam would fall off and produce space debris. If that's true and applies to SLS as well that's another problem.

[Jim]

SSME air-start ran into time limits. They needed it for launching in a few short years. There’s enough time for developing vac-start capability.

No, it would require a completely different engine design.  RS-25 is a head start engine.  It requires the head pressure from the propellant mass to start moving turbines.   It also had to go through an extended purge to condition the engine for start. 
Air starting for Ares I would have been only 2 minutes after ground disconnect but still would have taken 100's pounds of helium and propellant to get back into the start box.   And it would have been done like Starship with hot staging.   Both of these are not feasible for in space use (restart) along with drying out.
.

[Robotbeat]

 So, the idea of reusing Shuttle external tanks as wetlabs. Did that get as far as folks figuring out what to do about the popcorn problem? Did anyone figure out a way to make the external tank insulation (or a replacement) work for a long duration in space?

[Jim]

So, the idea of reusing Shuttle external tanks as wetlabs. Did that get as far as folks figuring out what to do about the popcorn problem? Did anyone figure out a way to make the external tank insulation (or a replacement) work for a long duration in space?

No. 

[Robotbeat]

Jim from the past (17 years ago???) has a great explanation of what exactly the problems are with high altitude start of SSME:

kraisee - 25/3/2006  3:07 PMWith so many informed people on here, can anyone actually dispel the speculation and rumours as to why the SSME can not be air-started as the ESAS originally planned?Does anyone on the board here know the real technical and/or budgetary reasoning which killed this concept?Ross.

The SSME is a head started engine, meaning there is no start cartridge or turbine spin up system.  Just the pressure (head) in the tanks.  Because of this the start box (temp and pressure tolerances) is very small.  The SSME goes thru 4 purge sequences to bring it to the start conditions.  This are tightly monitored by the ground software before it give the go for main engine start.  As a second stage engine, it could be conditioned until T-0 by ground commodities but during the 2 minute first stage burn, some combination of liquid and cold gasous He, N2, H2 and O2 would have to be used.  This would be non propulsion mass that would have to be carried. 

Additionally, the SSME was designed for a sea level start.  The pressures at altitude may not allow start box conditions to be achieved.  Modification to start at altitude may not be cheap  or actually create a "new" engine.

[TomH]

Stretch the EUS. Replace the engines with J-2X.