The new NASA budget calls for development of a new hydrocarbon engine, similar to or greater in thrust than the existing RD-180, as part of its “Heavy Lift Research” effort. Instantly I’m reminded of RS-84, the large hydrocarbon engine that NASA had funded from 2002-2004 as part of its Space Launch Initiative. After the Vision for Space Exploration was announced, RS-84 lost its funding as it didn’t fit into the lunar return mission. The great opportunity it once presented was wasted, but the industry will pick up where it last left off.
The new engine won’t be exactly the same as RS-84; the old effort aimed at producing a reusable engine in the class of the F-1 that powered the Saturn V. The new engine is targeted at roughly half the thrust.
At the heart of a new hydrocarbon engine will be the complex machinery required for the staged combustion cycle. Past US hydrocarbon (kerosene) engines have used the gas generator cycle; even the F-1 and new Merlin 1e have only produced around 304 seconds of vacuum specific impulse. The Russians, on the other hand, developed staged combustion cycles for the RD-170 family of engines (of which the RD-180 is a member.) This more efficient burning process raises the vacuum Isp to 330 seconds or more. The challenge in perfecting this combustion cycle for hydrocarbon engines is avoiding the “coking” of carbon deposits along engine components where it interferes with operation. Coking is less of a concern for single-use engines than with reusable ones like RS-84, but it still must be mitigated to ensure the engine works reliably during boost.
The history of launch vehicles only validates hydrocarbon engines as a great asset for booster engines. While hydrogen/oxygen propellants have higher specific impulse, the low density of hydrogen means larger, draggier propellant tanks and engines that generally produce lest thrust than their hydrocarbon counterparts. This thrust is extremely important during the first stage of the mission because the rocket is at its heaviest. The thrust needs to be slightly greater than its weight at this point so the vehicle can boost straight up. Thrust becomes less important (relatively speaking) once the vehicle is above the densest layers of the atmosphere and pitches over for the rest of the ascent.
The Space Shuttle used high-thrust solid rocket boosters to overcome the deficiencies of its hydrogen-fueled main propulsion system. Solid rockets can generate large amounts of thrust and have very high density, but their specific impulse is very poor. Hydrocarbon engines seem to "split the difference" between the two extremes of hydrogen and solids. A launcher with a hydrocarbon first stage can clear the launch tower and loft its hydrogen-burning upper stages to an altitude where there's little atmospheric drag, before burning out and falling away.
Since the days of Apollo, the US has largely ceded its lead with hydrocarbon engines. Aside from incremental improvements to the Atlas I/II and Delta main engines, the only significant development has been the Kestrel and Merlin engines developed by SpaceX. The propulsion directorate of Air Force Research laboratory has been working on a powerhead for a kerosene-burning, staged combustion engine for several years; sadly, this effort hasn't been funded at a level that will lead to a flightworthy engine in the near term. The new NASA program will likely build a replacement for the Russian-produced RD-180. America's reliance on Russia for this vital component of the EELV program is a national security vulnerability that will hopefully be corrected in the next decade.
The requirement for an RD-180 sized engine seems to mesh with the heavy-lift options discussed by the Augustine Panel. Jeff Greason's assertion that a heavy lifter should be designed around a 50 tonne payload is borne out by the "Atlas V Phase 2" studies. (Of course, new propellant tanks and new engines means it's not really an Atlas V anymore.) A triple-core vehicle with boosters wider than 5 meters can meet the Greason-defined heavy-lift requirement. Its single-core derivative could launch a 20 tonne payload like the former Orion capsule. Each booster core would require two RD-180 class engines, for six in total on the heavy lifter. NASA had also looked into a bigger, three-stage heavy-lifter based on shuttle ET tooling. Such a rocket would be similar to the Saturn V in performance (possibly even greater) while using only seven RD-180 class engines on its first stage.
I'd expect both Pratt & Whitney-Rocketdyne and SpaceX to compete for NASA funding on the new engine. PWR likely has some knowledge of the RD-180 that can be applied for the new engine. SpaceX has been working on a "Big Falcon Engine" for several years. (I had even suggested at one point in the past that BFE should be designed as a replacement for the RD-180 and for the nine-engine cluster on the Falcon 9.) Whether the new effort will be collaborative or competitive seems unsettled. Also vague at this point is whether the effort will focus on kerosene propellants, or if alternatives like propane and methane are under consideration. (Hank Hill would get excited about the former prospect.) Methane would seem to be better suited for upper-stages and earth-escape stages; it's similar to hydrogen in many ways aside from its higher boiling point.) The involvement of SpaceX begs the question of whether a new engine development program is necessary if SpaceX was going to develop one anyways.
The RS-84 program might have been the best element of the Space Launch Initiative program. It was certainly a step in the right direction, and it ended prematurely after NASA started sacrificing technology development in favor of a single-minded focus on the lunar destination. Hopefully this new effort will receive a much higher funding level and an aggressive schedule. We're back to square one in this effort, but this gives the industry a chance to fix past mistakes and get US booster development on the right track.