Forum designs new rocket to replace the SLS

[sevenperforce]

23 minutes ago, shynung said:

Forgot. I agree that hydrolox boosters aren't really efficient for strap-on boosters. Much better off going the SRB route.

Though, I do question whether we can do away with a detachable skirt-mounted engines by substituting more SRBs. Basically, let the SRBs handle the launch TWR, and have the core be some sort of sustainer motor, much like how the old Space Shuttle stack was.

So the booster configurations would primarily be Core + Upper stage, adding SRBs as necessary.

The SLS Core Main Tank is so large that 4-5 RS25s isn't enough to get it off the ground, so Core + Upper won't work; you need at least six RS-25s or you need SRBs. That's why I propose the engine skirt.

With that route, you have a modular capability the current SLS just doesn't have. You can use Core + Skirt for smaller payloads, SRBs + Core for larger payloads, and SRBs + Core + Skirt for the largest payloads. These can be swapped in and out based on mission requirements, whereas the current SLS will depend on block upgrades to improve its overall performance.

I'm still doing the math to determine whether an upper stage makes sense.

Edited May 25, 2017 by sevenperforce

[sevenperforce]

Whoa, guys (and gals). I just realized something really, really crazy.

The current SLS Block 1 comprises:

• Two five-segment SRBs (each with dry mass 102 tonnes, propellant 623 tonnes, isp 269 sec, thrust 16,000 kN)
• ET-derived core stage with four RS-25s (dry mass 85.3 tonnes, propellant 894 tonnes, avg isp 430 sec, avg thrust 2070 kN)
• Interim Cryogenic Propulsion Stage with one RL10 (dry mass 3.5 tonnes, propellant 27.2 tonnes, vac isp 462 sec, vac thrust 110 kN)

It is projected to be able to put 70 tonnes of payload in LEO. My estimate using this calculator and the numbers above gives 73 tonnes to LEO (185 x 185 km, 28.5 degrees inclination, Cape launch).

Here's the thing, though.

If I remove the Interim Cryogenic Propulsion Stage completely, launching only the two SRBs and the core, I get 71 tonnes to LEO. And that's not even taking into account that the higher TWR would make the thrust and specific impulse of the RS-25s come up faster, meaning that the addition of the ICPS could actually decrease the payload.

Why on Earth would we add a whole second stage if we don't actually need it?

[TheEpicSquared]

1 minute ago, sevenperforce said:

Whoa, guys (and gals). I just realized something really, really crazy.

The current SLS Block 1 comprises:

• Two five-segment SRBs (each with dry mass 102 tonnes, propellant 623 tonnes, isp 269 sec, thrust 16,000 kN)
• ET-derived core stage with four RS-25s (dry mass 85.3 tonnes, propellant 894 tonnes, avg isp 430 sec, avg thrust 2070 kN)
• Interim Cryogenic Propulsion Stage with one RL10 (dry mass 3.5 tonnes, propellant 27.2 tonnes, vac isp 462 sec, vac thrust 110 kN)

It is projected to be able to put 70 tonnes of payload in LEO. My estimate using this calculator and the numbers above gives 73 tonnes to LEO (185 x 185 km, 28.5 degrees inclination, Cape launch).

Here's the thing, though.

If I remove the Interim Cryogenic Propulsion Stage completely, launching only the two SRBs and the core, I get 71 tonnes to LEO. And that's not even taking into account that the higher TWR would make the thrust and specific impulse of the RS-25s come up faster, meaning that the addition of the ICPS could actually decrease the payload.

Why on Earth would we add a whole second stage if we don't actually need it?

Hmm. Makes sense, considering that the SLS first stage is supposed to separate with a barely negative perigee. Could the second stage be for extended in-space operations (higher payload capacity to GTO, TLI, etc)?

[sevenperforce]

2 minutes ago, TheEpicSquared said:

Hmm. Makes sense, considering that the SLS first stage is supposed to separate with a barely negative perigee. Could the second stage be for extended in-space operations (higher payload capacity to GTO, TLI, etc)?

The ICPS does have restart capability, yes. But for 70-tonne LEO payloads, it is completely expended during the circularization burn.

The SSMEs are very altitude-sensitive; as you'll recall, they were designed to function from SL through to outer space. Apparently, the added mass of the ICPS keeps them low in the atmosphere for longer, making them so much less efficient that separation occurs roughly 1.4 km/s below orbital velocity. Without the ICPS, they would ascend so much more quickly that their increased efficiency would allow them to carry the exact same payload all the way to orbit.

[tater]

Wait, then why take icps in the first place? I assumed for em-1 it was the TLI stage.

Edited May 25, 2017 by tater

[TheEpicSquared]

25 minutes ago, sevenperforce said:

 

• ET-derived core stage with four RS-25s (dry mass 85.3 tonnes, propellant 894 tonnes, avg isp 430 sec, avg thrust 2070 kN)

Hold on, the wikipedia page for the SLS says the first stage thrust is 7,440kN.

 

8 minutes ago, tater said:

Wait, then why take icps in the first place? I assumed for em-1 it was the TLI stage.

I would assume so. But then shouldn't they make the second stage optional, so to speak? Include it for beyond LEO/GTO payloads, take it out for lighter LEO payloads, put it back in for heavy LEO payloads. 

[sevenperforce]

5 minutes ago, tater said:

Wait, then why take icps in the first place? I assumed for em-1 it was the TLI stage.

Because the SSMEs are not restartable.

For EM-1, the payload will be fairly low (roughly 25.8 tonnes including Orion, the Service Module, and the SM's propellant), so the SLS Core will be able to carry the ICPS and Orion almost all the way to orbit. The ICPS will ignite once to circularize, followed by time in LEO, then ignite a second time for the TLI burn. This is similar to what the Saturn V's third stage did for Apollo 8-17.

But if the SLS can take up to 70 tonnes to LEO, then yeah -- why bother with the ICPS at all? Just add more propellant to the Orion's Service Module and be done with it. The SM's main engine, an old Shuttle OMS engine, has 316 seconds of vacuum isp and it currently flies with 9.3 tonnes of propellant. If you can put 70 tonnes in LEO with the Core and boosters alone, you can add up to 44 tonnes of propellant to the Service Module (probably using drop tanks), giving Orion almost 4.5 km/s of delta-V.

7 minutes ago, TheEpicSquared said:

36 minutes ago, sevenperforce said:

ET-derived core stage with four RS-25s (dry mass 85.3 tonnes, propellant 894 tonnes, avg isp 430 sec, avg thrust 2070 kN)

Hold on, the wikipedia page for the SLS says the first stage thrust is 7,440kN.

The RS-25s develop 1,860 kN of thrust at sea level, giving the SLS first stage a liftoff thrust of 7,440 kN (1,860 x 4). The RS-25s go up to 2,279 kN of vacuum thrust, so I took average thrust as (2279 + 1860)/2.

13 minutes ago, TheEpicSquared said:

But then shouldn't they make the second stage optional, so to speak? Include it for beyond LEO/GTO payloads, take it out for lighter LEO payloads, put it back in for heavy LEO payloads. 

Honestly, I'm not even sure that the ICPS actually increases LEO payload at all.

Think about it. The Shuttle, with 4-segment boosters and three RS-25s, could lift 27.5 tonnes to LEO, inside the 69-tonne orbiter. It also carried the 27-tonne external tank to the very edge of space; if the OMS engines and fuel had been removed, the SSMEs could have carried the tank all the way through orbital insertion. That's a gross total of roughly 123.5 tonnes to LEO.

With the five-segment boosters of the SLS and its four RS-25s with its 73-tonne tank, we are looking at a gross total of 155 tonnes to LEO without any upper stage at all. A second stage doesn't really make much sense.

[RCgothic]

Presumably the ICPS does make a difference for less than full payloads to beyond LEO.

But that is ridiculous. As if we needed more reason to diss the SLS.

 

[sevenperforce]

7 minutes ago, RCgothic said:

Presumably the ICPS does make a difference for less than full payloads to beyond LEO.

But that is ridiculous. As if we needed more reason to diss the SLS.

Anything the ICPS can do, the core can do on direct ascent. The only thing the core can't do is restart.

Launch TWR with a 70-tonne payload is 1.59 with the ICPS, 1.61 without. So it's hardly even a significant difference in initial ascent profile.

Does anyone have a more detailed/exhaustive launch performance calculator than the one I linked to above? I want to double-check these numbers.

[tater]

I assumed the 70 tons included the ICPS and remaining propellant.

Your other post made me think that the ICPS would entirely expend propellant to get to LEO---and ICPS is only going to be used the one time on EM-1, never to be used again, right?

[sevenperforce]

9 minutes ago, tater said:

I assumed the 70 tons included the ICPS and remaining propellant.

Your other post made me think that the ICPS would entirely expend propellant to get to LEO---and ICPS is only going to be used the one time on EM-1, never to be used again, right?

Hmm...after checking the mission schedule, it does look like EM-1 will be the only flight of the SLS Block 1 with ICPS; all other missions will have the Block 1B upgrade and fly with the EUS. 

So the "70 tonnes to LEO" capability of the SLS is meaningless, since it will only ever be lifting a 28.5 tonne payload (the Orion with SM), and it will be sending it to the moon. 70 tonnes to LEO is what the SLS+ICPS could do, fully expending the ICPS, but since the core and boosters can take 70 tonnes to LEO without needing the ICPS, the ICPS would never be useful for LEO launches.

I still think it would be better to give the Orion's Service Module some drop tanks and skip the ICPS entirely for EM-1, but what do I know?

[tater]

It'll be enough of an accomplishment for them to provide the SM as it is, adding a novel technology at this point seems like a non-starter to me.

[shynung]

11 hours ago, sevenperforce said:

I still think it would be better to give the Orion's Service Module some drop tanks and skip the ICPS entirely for EM-1, but what do I know?

I think it's easier to add an upper stage instead of modifying the SM to hold more propellant/add drop tanks. The latter requires a significant redesign of the SM to make space for a bigger propellant tank, or strengthened side-mounting points and plumbing connections for the drop tanks to bolt onto. In comparison, the former simply means stacking the SM onto the aforementioned upper stage using existing mounting points.

[magnemoe]

49 minutes ago, shynung said:

I think it's easier to add an upper stage instead of modifying the SM to hold more propellant/add drop tanks. The latter requires a significant redesign of the SM to make space for a bigger propellant tank, or strengthened side-mounting points and plumbing connections for the drop tanks to bolt onto. In comparison, the former simply means stacking the SM onto the aforementioned upper stage using existing mounting points.

Also you want storable propellant in then SM and high isp in upper stage. 
its not like KSP there my standard early Mun landers tend to have two 6-8 drop tanks, first set(s) is from circulate to start of landing, last two hold the legs and instruments and is used from landing getting into orbit. 

[sevenperforce]

14 hours ago, tater said:

It'll be enough of an accomplishment for them to provide the SM as it is, adding a novel technology at this point seems like a non-starter to me.

 

3 hours ago, shynung said:

I think it's easier to add an upper stage instead of modifying the SM to hold more propellant/add drop tanks. The latter requires a significant redesign of the SM to make space for a bigger propellant tank, or strengthened side-mounting points and plumbing connections for the drop tanks to bolt onto. In comparison, the former simply means stacking the SM onto the aforementioned upper stage using existing mounting points.

 

2 hours ago, magnemoe said:

Also you want storable propellant in then SM and high isp in upper stage. 
its not like KSP there my standard early Mun landers tend to have two 6-8 drop tanks, first set(s) is from circulate to start of landing, last two hold the legs and instruments and is used from landing getting into orbit. 

So upon review, I think the "70 tonnes to LEO" capacity of SLS Block 1 actually includes the ICPS and Orion+SM. The ICPS has a wet mass of 30.7 tonnes; added to the mass of Orion and its SM, that's a total of just 57 tonnes. So the ICPS is included in the payload; the "payload" uses its TLI engine to raise perigee.

It's just a coincidence that the low TWR of the ICPS (0.2 gees) means it would be incapable of lifting more than around 70 tonnes to LEO, if it were used as an LEO lifter rather than a BLEO departure stage. Hence the confusion.

Edited May 26, 2017 by sevenperforce

[Bill Phil]

3 hours ago, sevenperforce said:

 

 

So upon review, I think the "70 tonnes to LEO" capacity of SLS Block 1 actually includes the ICPS and Orion+SM. The ICPS has a wet mass of 30.7 tonnes; added to the mass of Orion and its SM, that's a total of just 57 tonnes. So the ICPS is included in the payload; the "payload" uses its TLI engine to raise perigee.

It's just a coincidence that the low TWR of the ICPS (0.2 gees) means it would be incapable of lifting more than around 70 tonnes to LEO, if it were used as an LEO lifter rather than a BLEO departure stage. Hence the confusion.

The icps is not injected into LEO, but a highly eccentric orbit that would ditch the core and still leave the stack in orbit.

[sevenperforce]

1 minute ago, Bill Phil said:

The icps is not injected into LEO, but a highly eccentric orbit that would ditch the core and still leave the stack in orbit.

Tomato tomatoe. The core has the capacity to inject the ICPS + Orion + SM into a circular LEO, but to avoid leaving the core in orbit, MECO occurs in, essentially, an 1800 km x -10 km orbit. 

[sevenperforce]

Ran some numbers once more (apparently this is all I do) after I realized that the dry mass of the SLS core is rather higher than it ought to be. The Space Shuttle's Super Light Weight Tank held 733.5 tonnes of propellant with a dry mass of 26.5 tonnes; the SLS core carries 22% more propellant but masses three times as much. Add it up and it's about 40 tonnes of extra mass, not counting the weight of the four RS-25s.

That 40 tonnes is divided between two things: the interstage assembly, to support the upper stage, and the engine mount and thrust structure, to support the engines and distribute their thrust, and to support the thrust of the SRBs. Can't do anything about the interstage, but if we went with a super-Atlas configuration with an engine skirt, the majority of the thrust structure could be jettisoned. Conservatively, I'll say 30 tonnes is jettisonable thrust structure and 10 tonnes is interstage and non-jettisonable thrust support structure.

If I run the numbers with that, dropping the skirt structure along with the engines, then a core+skirt configuration can send 50.6 tonnes to LEO with no SRBs at all (jettison at 68% propellant consumption). If the core is launched with SRBs and no skirt, payload to LEO increases to 109.7 tonnes. If the launch uses the core, the skirt, and the SRBs, payload to LEO goes up marginally, to 112.2 tonnes (jettison at 71% propellant consumption).

Of course, it may be that the skirt thrust structure is required for the SRB launches, making the "Core + SRB" configuration infeasible.

However, the "Core + Skirt" configuration is still quite promising. It would be able to deliver ICPS + Orion + SM to the planned staging altitude and velocity for EM-1 without needing SRBs.

[monophonic]

21 hours ago, sevenperforce said:

That 40 tonnes is divided between two things: the interstage assembly, to support the upper stage, and the engine mount and thrust structure, to support the engines and distribute their thrust, and to support the thrust of the SRBs.

Third thing: beefier structure overall to carry the weight of the payload and shroud, in addition to that of the tank and propellant. All the tank had to carry in the shuttle was itself and its contents, the cargo was in the orbiter.

[magnemoe]

1 hour ago, monophonic said:

Third thing: beefier structure overall to carry the weight of the payload and shroud, in addition to that of the tank and propellant. All the tank had to carry in the shuttle was itself and its contents, the cargo was in the orbiter.

Tank also had to support the SRB, so in the start the launcher was carried by the SRB, on burnout the shuttle carried the tank. You also had very complex aerodynamic. 
SLS is simple compared with this with the traditional rocket design.

[sevenperforce]

19 hours ago, monophonic said:

Third thing: beefier structure overall to carry the weight of the payload and shroud, in addition to that of the tank and propellant. All the tank had to carry in the shuttle was itself and its contents, the cargo was in the orbiter.

"weight of the payload and shroud" is the interstage assembly, as I noted. And, as mentioned by mag, the ET carried the SRBs anyway.

[monophonic]

5 hours ago, sevenperforce said:

"weight of the payload and shroud" is the interstage assembly, as I noted. And, as mentioned by mag, the ET carried the SRBs anyway.

The interstage does not extend from top of the stage down to the thrust structure at the bottom of the stage. Or if it does the construction is much heavier than load bearing tank walls would be.

The shuttle ET never carried any weight above its top nor took any thrust from under its bottom. In fact it was the always the thing being carried: a fully loaded Orbiter had a TWR of almost 5 at liftoff! SRBs served only to lift the tank until Orbiter burned enough fuel to carry the rest up on its own.

The SRBs and Orbiter were attached in the sides on two heights to distribute the load. This is a very different strain from the top-and-bottom squeeze between an (empty) orbiter mass payload and (what is the number of the day?) five RS-25s.

The whole setup is just so different that you simply cannot assume SLS's tanks have as good mass ratio as Shuttle's.

[zeta function]

I think that an alternative to the sls would be not hydrolox fueled on the first stage, hydrogen just takes up tons of space and has relatively low efficiency at the surface of earth. Also, the ssme just costs to much.

[tater]

Step 1: design a whole range of payloads that require a new SLS (design then start building them, and pay for them!).

Step 2: design a LV that fulfills those needs.

Edited July 7, 2017 by tater

[DerekL1963]

1 hour ago, tater said:

Step 1: design a whole range of payloads that require a new SLS (design then start building them, and pay for them!).

Step 2: design a LV that fulfills those needs.

That leaves the payloads in limbo for an extended period.  Doing it the other way around leaves the booster in limbo for an extended period.

There aren't any easy answers really.