Raptored Vulcan Part 1: Background


ULA's Vulcan       Graphic Credit: RUAG

As is common in aerospace, the debut of Vulcan, ULA's new launch vehicle, originally planned to take place in 2019, has been delayed by about three years. Among the reasons for the delay, two stand out:


1. The payloads destined for the first two flights -- Astrobotic's Peregrine lunar lander, and SNC's Dream Chaser cargo spaceplane, aren't ready.


       Peregrine                                  Dream Chaser      Credit: News 13

2. Blue Origin, the supplier of the BE-4 engines that will power Vulcan's first stage, has struggled to get their engines ready on schedule. As of January 2022, ULA is still waiting for flight-ready engines to integrate into Vulcan.

BE-4    Credit: ULA

This and other disappointments from Blue Origin have led many spaceflight fans to second guess ULA's decision to go with Blue Origin as their engine supplier and to wonder if another option might be open to ULA even now.

Two engines tend to come up most frequently in such musings. One is AerojetRocketdyne's AR1 -- one of the two original contenders for Vulcan, and the other is SpaceX's Raptor.

AR1   Credit: Aerojet Rocketdyne

Raptors   Credit


AR1

It's easy to see why fans many wish AR1 had been selected over BE-4 -- it was being developed by an experienced engine supplier famous for the RL10s that power Centaur, the RS-25s that powered the Shuttle and will power SLS, the RS68s used on the Delta IV Heavy, and so on. AerojetRocketdyne had pedigree, and the AR1 used a well-understood propellant combination that ULA had plenty of experience with -- kerosene and oxygen, whereas BE-4 uses methane and oxygen. AR1 was most similar to the Russian RD-180 engines used on the Atlas V rockets ULA was seeking to replace with Vulcan.

But ULA ultimately went with the BE-4, in part because it was going to be cheaper and in part because they it appeared to be further along in development. Likely contributing to the decision was that Blue Origin was planning to develop the BE-4 for its own New Glenn rocket, regardless of whether anyone else bought it, whereas AerojetRocketdyne was an engine supplier unwilling to commit to its design until it had a customer like ULA, who would undoubtedly have to pay for most of the development.

New Glenn    Credit: Blue Origin

While Vulcan was designed with the possibility of both engines in mind, switching this late in the game from a methalox engine to a kerolox engine would not be easy. Perhaps more importantly, while BE-4 is almost ready now, AR1 development has not moved along very fast in the absence of a customer. Still, the possibility might still be open.

Raptor

Raptor and BE-4 are both high performance methalox engines with comparable thrust. BE-4 uses the complex and high-performing oxidizer rich staged combustion cycle, while Raptor uses the even more complex and more performant full flow staged combustion cycle. SpaceX seems to have channeled its Merlin-engine experience to produce an engine that is more compact per unit of thrust, likely has a significantly higher thrust-to-weight ratio, is substantially more efficient, and still manages to cost a lot less than to produce -- probably due in large part to aggressive production process streamlining and a high production rate.

Furthermore, while BE-4 development and production seems to be progressing rather slowly, Raptor engines have been getting produced rapidly and in large quantities. While occasional test failures show Raptor to not quite be ready, either, its failures can often be attributed to a more aggressive testing methodology and a still-evolving design that's rapidly changing in the pursuit of ever greater cost savings and performance. On the whole, Blue Origin's failure to deliver two flight-ready engines in a timely manner has not looked good in comparison to Raptor's rapid progress, high production rate, and successful flight tests.

So the idea of using Raptor as a drop-in replacement for BE-4 is quite appealing at first glance.

This idea got more visibility when Elon Musk joked about bringing / taunted ULA by offering to bring Raptors on a factory visit. (For context, as rival launch providers, ULA and SpaceX have not had a friendly relationship, starting many years ago with lawsuits over anticompetitive business practices as SpaceX sought to break into a market monopolized by ULA, continuing as [the leaders of] both companies try to downplay the capabilities and value the other has to offer.)

It wasn't obvious from the tweet how serious the offer might be. Would Raptors work on Vulcan? Would ULA buy them? And would SpaceX really sell vaunted Raptor engines designed for their extremely ambitious Starship project?

Read more in Part 2: Performance and Part 3: But Why?.

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