A reusable SLS?

[sevenperforce]

1 hour ago, Exoscientist said:

There's nothing *fundamental* in the SSME which makes an in-space restart impossible -- no one-shot parts or anything like that -- but it's a complicated engine which has to be set up exactly right for a successful start, and ground equipment (and gravity!) helps out with that. It would not be difficult to develop a variant which could start itself in space, but there has been no reason to do that.

It is tremendously difficult to contemplate an air-startable RS-25. It requires at least one gee of acceleration for the prop downweight to allow the start mechanism to actually function. It would not only be difficult to create an RS-25 that starts in space; it would be a fundamentally different engine.

And that fundamentally different engine wouldn’t even come CLOSE to being useful for a landing, because the minimum thrust level is an order of magnitude too high to permit a successful landing burn.

And you’d never get to “successful landing burn” territory because the SLS core is already going fast enough that the engines and superstructure melt to slag the moment they get anywhere close to the atmosphere. You’d need a 5 km/s entry/braking burn to even approach survivable speeds. Which requires roughly 200 tonnes of reserve propellant JUST for the braking burn.

[Exoscientist]

On 6/17/2022 at 4:37 PM, sevenperforce said:

The core absolutely cannot be landed. Those engines each have a mass of over 3 tonnes. The center of mass is much too far aft for any sort of controlled biaxial entry.

Bereft of its upper stage and the gimbal on its firing RS-25s, the SLS core is a lawn dart. And the SLS is jettisoned while in an actual elliptical earth orbit; it hits the atmosphere faster than a re-entering Crew Dragon. The engines will melt to slag immediately.

What you could do, though, is give it wings. And control surfaces. That way it can re-enter on its side and shield the engines from re-entry heat. The heat shield will be very heavy though. Oh wait, that’s the Shuttle. 

  Because of the large number of engines on the Falcon 9 and the long length of the stage it’s center of gravity is even further aft yet it is able renter broadside.

  Robert Clark

[RealKerbal3x]

17 minutes ago, Exoscientist said:

  Because of the large number of engines on the Falcon 9 and the long length of the stage it’s center of gravity is even further aft yet it is able renter broadside.

  Robert Clark

Falcon 9 enters the atmosphere more or less vertically. It is able to use body lift and the grid fins to descend with an angle of attack, but left to its own devices it would descend like an arrow.

Edited June 19, 2022 by RealKerbal3x

[tater]

40 minutes ago, Exoscientist said:

  Because of the large number of engines on the Falcon 9 and the long length of the stage it’s center of gravity is even further aft yet it is able renter broadside.

The bulk of the work here is done by the entry burn, after which in thicker air the grid fins can add some angle of attack. After entry burn, F9 is going about 1.4 km/s. That means SLS needs to scrub off about 7 km/s to be in the same situation. Looks like it would need to reserve something like 400t of propellants—meaning the SLS core stage is NOT nearly in orbit, because it never got close to orbit with almost half the propellant unburned. It could not possibly close—and remember, to enter sideways to slow down, it would still need to be covered with TPS, yet more mass. There's just no possible way to save SLS.

[tater]

The only portion of SLS that could likely be reused on Earth would be to replace the SRBs with liquid boosters, then recover those. They'd need 18/19 Merlins or ~7 Raptors for similar thrust (required to get SLS off the pad). We'd want to increase payload, so the upper end of those, or more. 19+ Merlins, or maybe 9 Raptors. The boosters are starting to look more like the core size wise. The existence of man-rated boosters of that capability obviates SLS by itself, just use the cheaper, reusable boosters, don't bother with SLS.

SLS is not useful enough at anything to be worth improving, it's a garbage launch vehicle. I really try not to bash it (a buddy of mine will be working MCC on it as soon as it clears the tower)—but it's just SO easy to bash it.

Edited June 19, 2022 by tater

[RCgothic]

It's just not optimised for anything.

If you optimise for a specific mission, at least you can perform that mission unaided.

If you optimise for LEO, you can always add a departure stage if necessary.

If you optimise for cost, missions are cheap.

If you optimise for cadence, it's likely both cheap and opens up a lot of construction/rendezvous architectures.

SLS sins in every category.

The only thing it's optimised for is politics, and even that will not hold if it's visibly inferior to the competition, which it is.

Edited June 19, 2022 by RCgothic

[sevenperforce]

2 hours ago, Exoscientist said:

Because of the large number of engines on the Falcon 9 and the long length of the stage it’s center of gravity is even further aft yet it is able renter broadside.

Falcon 9 re-enters tail-first. The entry burn does double duty of reducing entry speed and insulating the engines from re-entry heat. Once deep in the atmosphere, the grid fins provide enough control authority to give the booster a 1:1 glide ratio, but that is (a) still not broadside, and (b) at extremely low speeds relative to the 8+ km/s that the SLS core is booking when it hits the atmosphere on the opposite end of the globe. Which is itself an issue. How exactly do you imagine we'll land a 212-foot building off the eastern coast of Australia and then get it back to the Cape?

And grid fins wouldn't work. Not even titanium ones. The only control authority that the grid fins would provide at the SLS core re-entry speeds would be to act as ablative thrusters as they were rapidly vaporized.

1 hour ago, tater said:

The only portion of SLS that could likely be reused on Earth would be to replace the SRBs with liquid boosters, then recover those. They'd need 18/19 Merlins or ~7 Raptors for similar thrust (required to get SLS off the pad).

SMART reuse wouldn't be that terribly difficult.

By which I mean it would be INTENSELY difficult, but within the realm of achievability. Of course it would significantly cut into payload but that's beside the point.

 

[tater]

7 minutes ago, sevenperforce said:

SMART reuse wouldn't be that terribly difficult.

By which I mean it would be INTENSELY difficult, but within the realm of achievability. Of course it would significantly cut into payload but that's beside the point.

No, it could certainly be done, but I would imagine all the disconnect hardware (ignoring the inflatable heatshield, chutes, etc) would require redoing the entire crew rating of the vehicle, right? Another Green Run? We all know that this would be a juicy change order that would cost... billions?

[sevenperforce]

2 minutes ago, tater said:

No, it could certainly be done, but I would imagine all the disconnect hardware (ignoring the inflatable heatshield, chutes, etc) would require redoing the entire crew rating of the vehicle, right? Another Green Run? We all know that this would be a juicy change order that would cost... billions?

And let's also note that the 16-tonne engine compartment would exceed the lift capacity of the Chinook itself. Certainly a far, far greater load than any skyhook recovery ever attempted.

[tater]

Just now, sevenperforce said:

And let's also note that the 16-tonne engine compartment would exceed the lift capacity of the Chinook itself. Certainly a far, far greater load than any skyhook recovery ever attempted.

LOL, I forgot about the catch aspect entirely.

16t plus all the other hardware they need to add.

 

[sevenperforce]

2 minutes ago, tater said:

LOL, I forgot about the catch aspect entirely.

16t plus all the other hardware they need to add.

People underestimate the size of SSMEs. Just one weighs four times as much as the entire (empty) Electron first stage. 

[RCgothic]

1 hour ago, sevenperforce said:

And let's also note that the 16-tonne engine compartment would exceed the lift capacity of the Chinook itself. Certainly a far, far greater load than any skyhook recovery ever attempted.

So in practice it'd need to jettison in at least two sections and have a *two or more* catching operations.

[tater]

8 minutes ago, RCgothic said:

So in practice it'd need to jettison in at least two sections and have a *two or more* catching operations.

They could use a bouncy castle!

[wumpus]

55 minutes ago, tater said:

They could use a bouncy castle!

The shuttle boosters weighed at 91tons while falling to Earth under parachute for recovery and refurbishment.  I wonder if you could do a "parachutes plus bouncy landing gear" for an engine compartment.  Sure, the whole thing might weigh more than an electron booster, but hopefully it has the thrust to eat the rather large mass of the parachute compartment and bouncy house gear.

Granted, the STS boosters hit hard (>100mph from memory) and no idea how much parachute it would take to cut down 16 tons + other stuff + parachute itself + landing gear to the point the landing gear would work.

Postscript: How does this thread last this long?   We've seen what a reusable SLS looks like, it had 100+ flights to orbit.  Something called the "space shuttle".  SLS is the "single use" version of the thing (just with even more costs added on).  There were some "two stage shuttles" on the drawing board in the 1970s, but I can't see them working if you are adding the solid boosters from the shuttle.

Edited June 19, 2022 by wumpus
added the postscript

[magnemoe]

On 6/16/2022 at 10:22 AM, RCgothic said:

There isn't any practical way to make SLS reusable, and if you did it would be so different it wouldn't be SLS anymore.

The boosters are barely worth refurbishing even if they were recovered and the best that can be done with the RS25s is a kind of ULA's SMART reuse where the engines alone are recovered.

Agree and the engine structure on SLS will be much heavier than on Vulcan making it harder to catch and the low launch rate of SLS makes this development cost very questionable. 
SRB is very nice if you discard the rocket as its brings the cost down, you can see this in KSP. 

But if you want reuse you want to avoid SRB as they are hard to reuse and you want to drop first stage so slow it can pretty easy be recovered. 
Neutron takes the first stage reuse, dispose of second stage all the way with the integrated fairing and the hanging second stage. 
You also want a first stage with high TWR especially if you has to RTLS, as you want the second stage to go fast enough without traveling to far down range so you minimize boost back burn. 

 

[Sennalen]

A reusable SLS is the shuttle, basically. I've seen a few concepts with a little side-mount engine pod separate from any crew or cargo. It's possible it could be given wings and wheels to land on a runway.