SLS: Higher payload without second stage

[sevenperforce]

I was running some numbers today and I came across something that I can only describe as startling.

With the current SLS Block 1 configuration, payload to LEO is projected as 70 tonnes. Running the numbers myself with this calculator, I get around 73 tonnes to a standard 185x185, 28.5 degree LEO from the Cape.

But if I drop the ICPS entirely, I get 71 tonnes to the same orbit.

Apparently, the added mass of the ICPS forces the SLS to climb more slowly, not only increasing gravity drag but keeping the RS-25s in the lower atmosphere and preventing them from reaching full efficiency until later in the ascent.

Why on earth would the SLS use a second stage when it doesn't actually meaningfully increase payload? I mean, I get that the RS-25s can't restart, but still....

[ThereIWas3]

Because getting to orbit is only halfway to your final destination?

[sevenperforce]

Because getting to orbit is only halfway to your final destination?

Not if the SLS core alone could do TLI with Orion. With expanded tankage on the Orion Service Module and a direct ascent, EM-1 could skip the ICPS entirely. So...why?

Though I can see why the ICPS will only ever fly once. It's utterly useless for LEO.

[kkattula]

Because the ICPS is not intended for LEO payloads? 

Re-start aside, trying to push a nearly empty, but still heavy, core through TLI would be grossly inefficient. In fact if that whole 70 tonnes was propellant it wouldn't quite be enough to send the 85 tonne core through TLI.

[kkattula]

In contrast, the ICPS will only mass 3.5 tonnes empty, and hold 27 tonnes of propellant.

Edit: Which is still not quite enough to send a fully tanked Orion through TLI. That's why the ICPS missions don't go too deep into the Moon's gravity well.

[sevenperforce]

In contrast, the ICPS will only mass 3.5 tonnes empty, and hold 27 tonnes of propellant.

Edit: Which is still not quite enough to send a fully tanked Orion through TLI. That's why the ICPS missions don't go too deep into the Moon's gravity well.

*ICPS mission

[envy887]

SLS core carries Orion and ICPS to a 975 nautical miles apogee. ICPS does perigee raise and TLI.

Try calculating the highest orbit SLS can bring Orion to without an upper stage...

[Patchouli]

Because getting to orbit is only halfway to your final destination?

True as the core stage probably can't even get to escape velocity with no payload as it's dry mass is somewhere around 80 tons.
 Though I can see SLS being used without an upper stage for some LEO payloads.

[redliox]

There's always the weight of added equipment that factors in and such, so I'm not completely surprised.  However, the only way I could see anyone benefiting from those numbers is if you create a vehicle akin to SpaceX's ITS design.  Bear in mind, I'm not amazing peopleing SpaceX here, I'm simply stating if you're wanting to create something efficient we're talking incorporating the 2nd stage into the payload you wish to deliver.  I'm certain Blue Origins, ULA, and Orbital ATK, and Sierra Nevada could all do this easily.  Heck, I'd even suggest using this logic as a basis for nixing the Orion and opening up a slot for a better vehicle that still rides the SLS.

I don't think anyone aiming to deliver a straightforward satellite or probe would not utilize a disposable 2nd or 3rd stage, but then again the SLS is meant more for HSF and the sporadic flagship probe.

[hkultala]

Because getting to orbit is only halfway to your final destination?

Not if the SLS core alone could do TLI with Orion.

It could not.

Without the upper stage, the payload capacity drops terribly when going to anything higher than LEO.

The upper stage is like a space tug, for going from almost-LEO to the destination trajectory.

With expanded tankage on the Orion Service Module and a direct ascent, EM-1 could skip the ICPS entirely. So...why?

To get to the trajectory needed by the mission. The core stage cannot do that,.

Though I can see why the ICPS will only ever fly once. It's utterly useless for LEO.

Who cares about LEO for SLS?

[Steven Pietrobon]

With the current SLS Block 1 configuration, payload to LEO is projected as 70 tonnes. Running the numbers myself with this calculator, I get around 73 tonnes to a standard 185x185, 28.5 degree LEO from the Cape.

But if I drop the ICPS entirely, I get 71 tonnes to the same orbit.

Apparently, the added mass of the ICPS forces the SLS to climb more slowly, not only increasing gravity drag but keeping the RS-25s in the lower atmosphere and preventing them from reaching full efficiency until later in the ascent.

Try offloading some propellant from ICPS. That might get you even more payload.

[sevenperforce]

SLS core carries Orion and ICPS to a 975 nautical miles apogee. ICPS does perigee raise and TLI.

Try calculating the highest orbit SLS can bring Orion to without an upper stage...

Hmm. Ran the numbers. Looks like the SLS core could send Orion to about 16,500 km, best-case scenario. So yeah, this makes sense.

I think my confusion was because I had erroneously assumed Block 1 would fly ordinary LEO missions. Block 1 with ICPS only has one mission, EM-1, and on that mission the ICPS is essentially part of the payload to LEO (ICPS + Orion + SM is only 57 tonnes). It's just a coincidence that the ICPS's very low TWR (0.2 gees at staging) means it itself could only lift about 70 tonnes to LEO if used as part of the LEO lift vehicle rather than the BLEO injection stage.

Because getting to orbit is only halfway to your final destination?

True as the core stage probably can't even get to escape velocity with no payload as it's dry mass is somewhere around 80 tons.
 Though I can see SLS being used without an upper stage for some LEO payloads.

Yeah, by my numbers the core stage can manage an apogee of about 105,000 km without payload.

And SLS sans upper stage for LEO was the DIRECT plan, IIRC.

Who cares about LEO for SLS?

Try offloading some propellant from ICPS. That might get you even more payload.

I didn't realize that there are literally no planned LEO missions for SLS. All planned missions are BLEO. The 70-130 tonne LEO payload numbers don't actually represent any real missions.

[edkyle99]

I didn't realize that there are literally no planned LEO missions for SLS. All planned missions are BLEO. The 70-130 tonne LEO payload numbers don't actually represent any real missions.

Precisely.  I would love to see these discussions talk about the real SLS payload number - the payload sent beyond low earth orbit.

 - Ed Kyle

[sevenperforce]

I didn't realize that there are literally no planned LEO missions for SLS. All planned missions are BLEO. The 70-130 tonne LEO payload numbers don't actually represent any real missions.

Precisely.  I would love to see these discussions talk about the real SLS payload number - the payload sent beyond low earth orbit.

Personally, I loathe the idea of ginormous flagship missions. That was necessary with Apollo, when we lacked the ability to sustain high launch cadence and reliable orbital rendezvous, but those are solved problems now. Assemble in LEO and then go to wherever you want to go.

If I had the SLS budget to play with, I'd say to make it do one thing and one thing only: launch a single-engine high-energy propulsion bus with onboard gaseous biprop RCS and solar-powered cooling system, with side-mounted automated coupling ports. Contract with commercial providers to launch the actual mission spacecraft and then use the SLS bus to send it wherever you want it to go. Your payload too large for one propulsion bus? Launch two of them and couple them side-to-side. In other words, Kerbal the hell out of it.

[clongton]

The SLS flight avionics are not designed to send a payload into orbit. It is designed to get the upper stage to an optimum altitude, speed and trajectory to finish the job of orbital insertion. To attempt to use the core stage as a SSTO vehicle would require a complete re-write of the software.

[Coastal Ron]

The SLS flight avionics are not designed to send a payload into orbit. It is designed to get the upper stage to an optimum altitude, speed and trajectory to finish the job of orbital insertion. To attempt to use the core stage as a SSTO vehicle would require a complete re-write of the software.

Not that I'm advocating this should be done, but wouldn't re-writing the software just be on the order of $Millions, not $Billions?

And I understand that it's not just one system that would have to be rewritten, but multiple that have to be coordinated. It's just that you'd think the ability to adjust the trajectory would have anticipated the full range of possibilities.

Of course if the top-level design spec never called out for that possibility it makes sense to not build it in. Just one of those things that you'd think would not be hard in our modern age of computers...

[sevenperforce]

The SLS flight avionics are not designed to send a payload into orbit. It is designed to get the upper stage to an optimum altitude, speed and trajectory to finish the job of orbital insertion. To attempt to use the core stage as a SSTO vehicle would require a complete re-write of the software.

Presumably, even SSTO launches would leave the perigee low enough that it would re-enter nominally; the payload could self-circularize.

[clongton]

The SLS flight avionics are not designed to send a payload into orbit. It is designed to get the upper stage to an optimum altitude, speed and trajectory to finish the job of orbital insertion. To attempt to use the core stage as a SSTO vehicle would require a complete re-write of the software.

Not that I'm advocating this should be done, but wouldn't re-writing the software just be on the order of $Millions, not $Billions?

And I understand that it's not just one system that would have to be rewritten, but multiple that have to be coordinated. It's just that you'd think the ability to adjust the trajectory would have anticipated the full range of possibilities.

Of course if the top-level design spec never called out for that possibility it makes sense to not build it in. Just one of those things that you'd think would not be hard in our modern age of computers...

Like I said in another post somewhere, it's not the individual software modules that are difficult - it's the system integration that's hard. individual modules may be beautiful and work exceptionally well but put them all together in a sandbox and they may not play well together. The rocket gods have very weird senses of humor. They like things that cause heartburn.

[sevenperforce]

I was thinking about the SLS, particularly the high dry mass of the core in comparison to the dry mass of the Space Shuttle External Tank, and it reminded me of the Saturn S-1D proposal. You know, use the Saturn V first stage as an SSTO, but place the four external engines on a jettisonable launch skirt that is dropped at 70% propellant consumption.

Five RS-25s is not enough to get a full SLS core off the ground, but six would do the trick.

If the heavy SLS core thrust structure (which I'm estimating at around 30 tonnes) was redesigned as a toroidal, jettisonable skirt that clamped around two additional RS-25s mounted to the base of the tank, and launched without SRBs, then jettisoning the skirt at 68% propellant consumption would allow the core engines + tank to deliver up to 50.6 tonnes to a nominal low earth orbit. That's pretty respectable, and it would be able to deliver ICPS+Orion+SM to the planned EM-1 staging altitude and velocity without needing any SRBs at all.

With SRBs added back on, it would be able to deliver up to 112 tonnes to LEO, more than SLS Block 1B can deliver with the EUS.

[A_M_Swallow]

{snip}
Who cares about LEO for SLS?

Heavy cargo to LEO is probably the SLS's main market.

SLS's payload is too small to send people to Mars on a single launch. A ship yard to build Mars Transfer Vehicles is likely to be in LEO. Boots and flags to the Moon on an expendable lander is a very small market.