Navigating the twists and turns steering SLS Development

[Steven Pietrobon]

Dr Steven: if NASA had kept the 10 meter diameter core stage from Ares; how many RS-25s would we be talking about for SLS, 6x or 7x engines?

I estimate a minimum of 4*(10/8.4)² = 5.67 or six engines would be needed. Make it seven and you could make the stage reusable. :-)

And if so, would a 2x J-2X upper stage be best or a bigger cluster of RL-10s than the current EUS concept?

6xRL-10 could be used, but 2x J-2X will give a much higher payload.

EDIT: And seeing how we are going to be stuck with the 8.4 meter, 4x RS-25E corestage; in the spirit of NOT 'Rocket Legos' how can we see SLS being optimized to get the best performance to L.E.O. and BLEO, with the available engine, vehicle height and propellant load options?

SLS Block IB is a pretty good configuration. It has the lowest development cost, provided that new boosters are not developed. Lunar missions can be performed with two launches.

[MATTBLAK]

Block 1B? Yes, I suppose so. They'd have to keep the Lunar Lander under 40 metric tons though and have the EUS able to decelerate the Lander or Orion into LLO.

[Proponent]

Block 1B? Yes, I suppose so. They'd have to keep the Lunar Lander under 40 metric tons though and have the EUS able to decelerate the Lander or Orion into LLO.

And Orion would need redesign, because it can't handle the thermal load in LLO.

EDIT:  woods170, below, presents a paper indicating that Orion can handle the thermal load in LLO.

[MATTBLAK]

Orion for Lagrange Point 1, then! I wonder how it was going to cope during Constellation?!

[Mark S]

I still don't understand why NASA went with a four-engine core stage, when all previous studies (going at least back to ESAS) used a five-engine core stage for a stretched inline SDLV using an 8.4m core stage. Surely it wasn't on the basis of getting four launches using the (mostly) on-hand engines, instead of just three. Surely not.

Was there ever a public justification for the current design? Is there a reasonable non-public explanation behind the public one? I really don't see an economic justification, based on the extremely low projected flight rate (once a year). While the cost of a single RS-25 is not chicken feed, it is small compared to the total cost of a launch (should one ever happen ).

I understand the engine section on a five or six engine core would be more complex than what they are doing now. However.... some unmentionable company manages to cram nine engines under their first stage, and they seem to be doing okay.

The four-engine SLS core just seems... un-optimized. It doesn't seem to get significantly more payload to orbit than the much-smaller Shuttle stack (about 100 tonnes). And that's with 5-segment boosters and running the engines at a higher thrust than Shuttle used. So what gives?

I know this has  been hashed and re-hashed since SLS was announced. Maybe I'm just being dense. For whatever reason, it's just not sinking in. Mods, feel free to delete if too far OT.

Thanks.

[MATTBLAK]

Yeah: using 4× engines per launch instead of 5 would save - what? 40 or 50 million dollars per launch for a billion-plus cost anyway? And would designing for 5 versus 4 engines have increased the development costs any significant amount? Colour me skeptical.

[envy887]

The four-engine SLS core just seems... un-optimized. It doesn't seem to get significantly more payload to orbit than the much-smaller Shuttle stack (about 100 tonnes). And that's with 5-segment boosters and running the engines at a higher thrust than Shuttle used. So what gives?

What gives is that the core stage is much heavier than the elements that it replaces: the ET+orbiter engine section.

SLS without an upper stage will place over 180 tonnes in LEO, compared to STS at about 140 tonnes counting the orbiter, payload, and ET. SLS is considerably more powerful than STS, it's just less efficient if you count the orbiter as useful payload.

[Proponent]

Orion for Lagrange Point 1, then! I wonder how it was going to cope during Constellation?!

I presume the design has changed since Constellation.

[MATTBLAK]

Not for the better, it seems. Honestly; if the Orion can't orbit the Moon in LLO or even have enough delta-v to both burn into and out of lunar orbit; then what the heck is it for?! All the hype about it being a 'deep space spaceship' is then just that - hype

EDIT: It's nearly 50 years on and they will be having a spacecraft that in may ways is less capable than the Apollo CSM?! How is it I'm only now really starting to put all this together? I used to say that when it came to Project Constellation and the Ares launchers etc, Mike Griffin was 'The Emporer With No Clothes'. But he was/is a smart man - and so are many of us. Where did we go wrong? How could we/I have been in such denial? Can the Orion be upgraded into usefulness without taking endless more years and billions with a 'B'? It's thermal rejection systems need upgrading/fixing for close proximity lunar operations. And it needs far more than the relatively paltry 9 tons of propellants it has to move around fully in Cislunar space. And it probably needs more endurance than three weeks if it will go on to support relatively long crewed missions to the lunar surface.

How much will it then mass? Up from 26 metric tons to more than 35?! (Apollo 17 CSM was more than 30 metric tons)
We've been asking these and similar questions on this website and forum since when it was founded. How much longer will we have to do this? So many questions...

[Coastal Ron]

I still don't understand why NASA went with a four-engine core stage, when all previous studies (going at least back to ESAS) used a five-engine core stage for a stretched inline SDLV using an 8.4m core stage. Surely it wasn't on the basis of getting four launches using the (mostly) on-hand engines, instead of just three. Surely not.

Was there ever a public justification for the current design?

I'm not an SLS design expert, but here are some points to remember:

- Congress mandated that the SLS be built, and even specified it's capabilities. Without NASA participation.

- NASA did not have a design for the SLS that Congress used, so once Congress told NASA to build the SLS NASA and Boeing had to reverse engineer the SLS to fit what Congress wanted.

- There were no defined payloads or missions for the SLS when it was mandated to be built, so NASA and Boeing did have to make many guesses about initial flight rates, initial capabilities, and even what NASA launch assets would be available to support SLS launches.

- There was also no defined budget for the SLS, but NASA (and Boeing) have assumed that the budget they would get would be essentially flat from year to year, which is not a good way to build something new.

- Virtually nothing on the SLS is being used "as is" from prior programs, and conversely the SLS is not a "clean-sheet" design, so there are lots of compromises - and I'm not implying safety is affected, just that engineers had less options to design with.

Bottom line is that there were many variables NASA and Boeing had to consider, and over time not only of them were the right guesses - which is not unusual. But that is why there are mismatches between what they are building and what they can do.

My $0.02

[MATTBLAK]

Whether SLS gets something like the existing EUS that is capable of sending 39 tons beyond Earth orbit or a new, far more capable design stage that can send up to 50 metric tons BLEO... Without a new stage, SLS would no longer be able to send a co-manifested payload of about 10 tons BLEO along with the heavier, more capable Orion. But if the Orion grew in (useful propellant) mass to more than 35 metric tons, then the old stage would be good enough because the Orion would now be able to brake itself into and out of low lunar orbit.

A two launch lunar mission architecture could use the existing EUS to send a 40 ton Lunar Lander to the Moon. It should have enough delta-v to brake itself into LLO and await the crewed Orion that would follow it in another launch window (2 or 3 months later?). The Lander should have enough propellant to land most anywhere on the Moon and then return to the Orion to send the crew homewards. Not enough room to get into different architecture options here - but I'm assuming that the Lander would be reusable. After it delivers the crew back to the Orion, it could wait in LLO for a Tanker to send propellants to it during another launch window. It would be cheaper if the propellant delivery was a commercial contract. New Glenn? Falcon Heavy? It would be too expensive to send the fuel by yet another SLS. So many questions - so many ways to approach this mission/s...

[Rocket Science]

If one takes off the "engineering googles" and puts on the "committee goggles" all can be seen with clarity... What should have been a "clean sheet design" with all the time/effort/money invested we have a "compromise design" that keep chasing it's tail... All due to the constraints placed upon it and "mission creep"...
I can not help remembering the aspersions towards the DIRECT team's proposal and their design as "defying the laws of physics", now with what are they trying to do now and the position they find themselves in...

[woods170]

Block 1B? Yes, I suppose so. They'd have to keep the Lunar Lander under 40 metric tons though and have the EUS able to decelerate the Lander or Orion into LLO.

And Orion would need redesign, because it can't handle the thermal load in LLO.

Interesting. How did the Apollo CSM handle this? Because it flew in LLO as well, for days at end.

[MATTBLAK]

The Apollo SM had pretty big radiators.

[ncb1397]

Block 1B? Yes, I suppose so. They'd have to keep the Lunar Lander under 40 metric tons though and have the EUS able to decelerate the Lander or Orion into LLO.

And Orion would need redesign, because it can't handle the thermal load in LLO.

Interesting. How did the Apollo CSM handle this? Because it flew in LLO as well, for days at end.

Temperature control is provided by heat  rejection  from radiators  and a water evaporator.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720012252.pdf

So, essentially sweating whenever the radiators don't reject enough heat.

The Apollo SM had pretty big radiators.

As does Orion. See picture.

[MATTBLAK]

Seems to - though the amount of acreage on the Orion SM may be (?) insufficient for the job, it seems. The Apollo SM was pretty big.

[woods170]

Seems to - though the amount of acreage on the Orion SM may be (?) insufficient for the job, it seems. The Apollo SM was pretty big.

The Whitley paper seems to be incorrect. See the attached document.
It specifies the Orion Thermal Control System.

Low Lunar Orbit environment is in the requirements for the TCS operational range.

Meaning: Orion, as currently designed (and being built) is very much capable of handling the thermal environment of Low Lunar Orbit.

[edkyle99]

The four-engine SLS core just seems... un-optimized. It doesn't seem to get significantly more payload to orbit than the much-smaller Shuttle stack (about 100 tonnes). And that's with 5-segment boosters and running the engines at a higher thrust than Shuttle used. So what gives?

My guess is this.  The reason designers specified four RS-25D engines is because that was enough.  A fifth engine would not have added much capability.  They are nearly at T/W of 1 at SRB sep with four engines, which is good enough.  It isn't the thrust, it is the specific impulse that really matters.  Core isn't a booster, it is a sustainer.  Think balloon tank Atlas.  Atlas IIA sustainer only made 12.5% of the total liftoff thrust, but produced just enough thrust after BECO to put its higher ISP to work.

Also, you can't directly compare a Shuttle Orbiter with an SLS payload.  Orbiter carried a lot of mass that doesn't count as payload on SLS.  Those engines, for example, and their feedlines, and their thrust structure, etc.  Shuttle's "real" payload capability was 24-something tonnes LEO (its heaviest actual payload was IUS/Chandra at 22.753 tonnes), plus whatever mass you might want to add for crew support and reentry.

 - Ed Kyle

[envy887]

The four-engine SLS core just seems... un-optimized. It doesn't seem to get significantly more payload to orbit than the much-smaller Shuttle stack (about 100 tonnes). And that's with 5-segment boosters and running the engines at a higher thrust than Shuttle used. So what gives?

My guess is this.  The reason designers specified four RS-25D engines is because that was enough.  A fifth engine would not have added much capability.  They are nearly at T/W of 1 at SRB sep with four engines, which is good enough.  It isn't the thrust, it is the specific impulse that really matters.  Core isn't a booster, it is a sustainer.  Think balloon tank Atlas.  Atlas IIA sustainer only made 12.5% of the total liftoff thrust, but produced just enough thrust after BECO to put its higher ISP to work.

Also, you can't directly compare a Shuttle Orbiter with an SLS payload.  Orbiter carried a lot of mass that doesn't count as payload on SLS.  Those engines, for example, and their feedlines, and their thrust structure, etc.  Shuttle's "real" payload capability was 24-something tonnes LEO (its heaviest actual payload was IUS/Chandra at 22.753 tonnes), plus whatever mass you might want to add for crew support and reentry.

 - Ed Kyle

"good enough" doesn't mean that it can't be further optimized, if there's a need. Compared to other systems with full boost stages, Shuttle and SLS drop the solids while still travelling really slow, around 1.3 km/s. That's far slower than Saturn V, which dropped the S-IC at around 2.4 km/s. It's even much slower than Falcon 9 on a RTLS mission, which is around 1.7 km/s.

More thrust at staging would absolute improve the performance of the SLS core, even if that thrust comes from low TWR RS-25. Better boosters also improve performance, even without more core stage thrust, because they move the core faster at staging. In the extreme case, a pair of New Glenn boosters would nearly double the velocity at staging, and also nearly double the payload to LEO with no upper stage.

The real question is whether it has a mission that needs more performance.

[envy887]

Well, I think the OIG report explains why EUS development is paused.