A few years ago, I did a “Random Thoughts” blog post about synergies between the proposed ACES stage and the proposed SLS upper stage1. Now, I’m still not the world’s biggest SLS fan, and I’m still not a fan of sole-sourcing EUS to Boeing, but I was realizing today that the potential for synergies may be even higher now, and I wanted to throw out an idea for a potential public-private partnership that would benefit both NASA and ULA, and save the taxpayer some money now and in the future.
Here were my thoughts/observations that led to my latest concept (in no particular order):
- The most complex part of an upper stage is typically the bottom of the stage where the propulsion systems are located. The tanks themselves are relatively simple comparatively. Tank stretches have always been considered much, much easier and lower risk than changing the diameter of a stage, because now you usually have to redesign all the structures and plumbing on the back end.
- Both EUS and ACES are looking at using four RL-10 class engines on their stage. The EUS wants to use a different RL-10 variant with a longer extendable nozzle, but not a wildly different engine2.
- The EUS LOX tank diameter has in the past usually been 5.4m diameter, while the ACES stage is now baselined at 5.4m diameter3.
- There’s already some interest on the EUS side in leveraging some of the IVF systems as a way of providing auxiliary power.
- On the EUS, the LOX tank is likely suspended, with the interstage reacting loads into the bottom of the LH2 tank. This means the LOX tank doesn’t have to take compressive loads on the pad unpressurized.
So here’s my crazy thought: What if NASA had Boeing and ULA develop the EUS as a public-private partnership, with the LOX tank and propulsion section for EUS and ACES sharing a high-commonality design?
- Have EUS go with the 5.4m diameter resistance-welded CRES tankage from ACES, with a ~4% longer barrel section to compensate for the 10cm (~4in) smaller diameter.
- Have ACES design its propulsion thrust structure to accommodate both versions of the 4x RL-10 class engines.
- Have EUS keep the current design for the aluminum 8.4m diameter tank and intertank structure, but have ACES stay with the common bulkhead design and 5.4m diameter CRES LH2 tank.
- Have the EUS LOX tank sidewalls and top-dome slighly modified compared to ACES to react the loads to/from the intertank structure, and to eliminate the unneeded common-bulkhead, but keep the bottom dome and engine/equipment rack designs the same between the two.
- If EUS needs more IVF modules, either have ACES leave space and minimal scarring of its structure to allow mounting two more modules 4, or have the EUS LH2 tank designed with IVF mounts (either at the bottom or top, depending on what gives the most bang for the buck).
- Have Boeing focus on overall stage integration, the LH2 tank, the intertank structure, and the interstage structure, and any EUS-specific long-duration hardware (sunshields, cryo-coolers, radiators, solar panels, deep-space comms, etc).
- Have ULA focus on the LOX tank and propulsion system, and have them produced on the same line that would make ACES.
There are some risks–this would work best if Boeing was willing and able to keep interfaces between the two halves simple, and wherever possible let ULA drive the LOX tank and propulsion element design without too much micromanaging. SLS, having much higher launch capacity can probably can afford to have EUS be a tiny bit less optimized if it allows for high-commonality and minimum impact to ACES, as opposed to forcing EUS to be hyperoptimized for SLS at the expense of being suboptimal for ULA.
The benefits I could see to NASA is that this would:
- If done right, potentially save significant development costs by leveraging both outside investment by ULA, and by having at a more commercially-driven design for at least part of the EUS. This might also accelerate when EUS was available.
- If done right, EUS would now share at least some of its fixed and marginal costs with the ACES assembly line, and would benefit from higher production rates on many of the subsystems.
- The core subsystems on EUS would see far more flights this way than they would on SLS alone, and the manufacturing team would stay fresh even if the SLS flight rate is modest.
- The EUS stage would probably end up with at least slightly better dry mass numbers, and would likely have longer duration built-in.
- EUS would be able to leverage at least some of the ongoing enhancements ULA is trying to develop for ACES (refueling/distributed launch, longer duration missions, etc).
ULA would obviously benefit for a few reasons too:
- Most of the complexity of ACES is in the LOX tank and propulsion section. The only other complex ACES part that wouldn’t be needed for EUS is the common bulkhead. The LH2 tank is pretty simple. So, if done right, this could help accelerate the development of most of ACES.
- If done right, this would both lower the cost of fielding ACES (since NASA would be footing part of the cost of the common elements), and would likely accelerate when they could have ACES flying by 1-2 years or more.
- They would effectively have an additional customer stream for ACES hardware.
- This would probably better align the interests of at least one of their parent companies with their interests.
If done right, taxpayers would benefit from decreased development and fixed operating costs compared to the current approach.
Now, I have no idea if Boeing or ULA would even consider this. You’ll notice I used the phrase “if done right” a ton of times, and things being done right are rarely a given when talking about government contracting. But it seems like an intriguing approach in a world where SLS is unlikely to get canceled anytime soon.
[Disclaimer: As the founder of a company that has done some work with ULA on their IVF system, I could potentially stand to financially benefit if NASA took this approach. I can’t claim to be an unbiased, unselfish player in this case. But I still feel it’s worth throwing the idea out there, as I think if done right it would make EUS a better stage, save NASA money, get EUS and ACES flying sooner, and generally make both systems better. I still am skeptical that SLS is worth saving per se, but assuming it isn’t going to go away, this seems like a way to get at least some benefit to the commercial space industry out of it.]
- then called the DUUS or Dual-Use Upper Stage, now EUS or Exploration Upper Stage
- Though to be fair, ACES is still theoretically trading the BE-3U and XCOR 25klbf LOX/LH2 engine, and EUS may still be trading the J-2X
- If I’m remembering correctly
- at the 12 o’clock and 6 o’clock positions if the current ACES IVF mounts are at 3 and 9 o’clock