NASA's Space Launch System (SLS) is not looking good. Occasionally referred to as the "Senate Launch System," or even less "Rocket to Nowhere," the super-heavy-lift booster has long been a hot topic for the industry. Designed as an evolution of Core Space Shuttle technology, the SLS promised to reuse existing infrastructure to provide higher payload capacity and lower operating costs than its infamous winged predecessor. However, given the increased competition from commercial vendors and the proposed budget cuts aimed at future upgrades and enhancements of the core booster, the program with a clear budget and timeline is in a very precarious position.
What the SLS has to say does not look impressive, at least on paper. In its original configuration, this would be the title of the world's most powerful rocket, capable of carrying nearly 105 tonnes in low Earth orbit (LEO), compared to 70 tonnes for SpaceX's Falcon Heavy. It would still miss the 155-tonne of the mighty Saturn V for LEO, but the proposed "block 2" upgrades would increase the SLS payload capacity to 145 meters within the reach of the iconic Apollo repeater. Since the departure of the Space Shuttle in 2011, NASA has insisted that the power of the SLS is the only way to accomplish larger and more ambitious missions to the Moon, Mars, and beyond.
Or at least. On March 13, NASA Administrator Jim Bridenstine told Congress that the agency was investigating the possibility of launching their Orion probes on the moon with a commercial launcher to avoid further delays. The statement shocked many in the aerospace industry, as it seems to question the future of the entire SLS program. If commercial rockets can fulfill the role of SLS, why does the agency need it in some cases?
NASA is preparing a report examining what physical and logistical modifications are required for originally planned missions on SLS; a document that is sure to be scrutinized by SLS supporters and critics. Until the report is published, we can speculate on what this hypothetical moon flight might look like.
A Loss of Replacement
Administrator Bridenstine did not exactly mention which commercial vehicle he was considering during his testimony before the Congress. However, since Falcon Heavy has the highest payload capacity on all launch platforms currently operating, it would almost certainly be the focus of the NASA report. It might seem that the considerably smaller capacity of the Falcon Heavy is a problem, but in fact Orion's proposed test flight to the Moon would never fully exploit the capabilities of the SLS.
During Exploration Mission 1, the SLS is solely responsible for bringing the Orion, its service module and the Interim Cryogenic Propulsion Stage (ICPS) into orbit; it actually separates from the spacecraft at a height of only 157 kilometers. From then on Orion works independently, and the ICPS actually propels the ship towards the moon.
The Orion and the service module together weigh 57,000 pounds, and the ICPS itself weighs around 66,000 pounds. At least theoretically, any booster capable of orbiting about 125,000 pounds should be able to engage in this first mission for the SLS. That's fine with the performance of the Falcon Heavy, though we need a few thousand extra pounds for attachments and the adapters needed to screw everything together. A new fairing would have to be designed and manufactured to encapsulate such a large payload, but all in all, it seems a fairly straightforward process. Interestingly, some of the quick retractions of turnover mathematics seem to indicate that there is another option on the table.
With the Falcon Express
Assuming that NASA is prepared to deviate significantly from the original mission parameters, it seems as if the Falcon Heavy alone could move the Orion toward the Moon without using the ICPS at all. This would not only facilitate the integration of Orion hardware into the Falcon Heavy, but would also save the non-reusable ICPS for a future mission.
According to SpaceX, the Falcon Heavy can lift slightly more than 58,000 lbs into geosynchronous transfer orbit (GTO). As you may recall from our last lesson in "Beginner Orbital Mechanics", a GTO orbit is the first half of an orbital transfer from low earth orbit to the geosynchronous equatorial orbit (GEO) elevation of 35,786 kilometers. If the spacecraft was placed in the GTO from the top tier of the Falcon, the on-board propulsion would lift the perigee to orbit the orbit and complete the transfer.
This means that the Falcon Heavy only should have the option Orion and its service module in GTO. Of course, that does not bring you to the moon, but it's not that far away either. To reach GTO from Earth orbit, a spacecraft needs to increase its speed by about 2.5 kilometers per second. For comparison, it has to be accelerated by about 3.2 km / s for the lunar injection. How do we get the last 700 meters of acceleration per second? From the Orion itself.
According to design specifications, the Orion Service Module can deliver 1.8 km / s Delta-V. Theoretically, it should be able to complete the trans-lunar injection maneuver with enough propellant in reserve to make the necessary course corrections. In this theoretical scenario, there may not be enough propellant to slow the lunar orbit and enter it as originally intended. In this case, Orion could simply loop around the other side of the moon and then return to Earth. This "free return flight" path was formerly used as a contingency for the early Apollo missions. If the spaceship had not been able to perform the moonlit launch maneuver, it would have allowed the crew to "drift" essentially home.
NASA Plays it Safe
These are interesting thought experiments, but reality is likely to be a little less exciting. Some believe that the statements made before the Congress are essentially a threat to Boeing, the prime contractor of the Space Launch System. A not so subtle signal that the agency is fed up with the delays and cost overruns. Just two days after his statement, administrator Bridenstine has told Tweete that Boeing teams are working to accelerate the SLS schedule.
Even if the Orion flies with a commercial missile, administrator Bridenstine thinks the agency believes the mission is best served by using two launches and assembly of the spacecraft in orbit. Until the conclusion of the final report, it is not clear why NASA would choose such a complicated path to the moon. Especially considering that Orion currently has no provision for orbital assembly, and this technology needs to be integrated into the vehicle at an extremely short notice to reach the June 2020 launch date.
If we allow one last time conspiracy theory, some members of the community have voiced the idea that NASA is proposing the most expensive and complex way to launch Orion with a commercial missile to support the idea that the SLS is indispensable , If the world sees Orion Exploration Mission 1 completed on the back of a single Falcon Heavy and at one-tenth the cost of flying with the SLS, that would probably be more humiliating to the Agency than simply abolishing the mission altogether.