A basic principle of spaceflight is that simpler is preferred to the more complex. The use of a heavy lift launch vehicle (HLV) is “simpler,” in that fewer launches of big rockets creates less risk than an architecture requiring multiple launches of smaller ones. Woven into our current debate about future strategic directions in space are arguments over whether to develop and use an HLV for human missions beyond low Earth orbit, and if so, what type. In brief, the problem flows from the fact that even when using the largest rockets, we arrive in orbit with empty fuel tanks. To go beyond LEO we must carry a fully fueled Earth departure stage, which is very heavy. Thus, the requirement for “heavy lift.”
An alternative to a heavy lift vehicle is the fuel depot, in which caches of rocket fuel are collected and stored over time. When we are ready to leave on a trans-LEO mission we re-fuel our rocket at the depot in space and go. A variant of the depot concept employs separate launch, rendezvous and the assembly in orbit of multiple pre-fueled pieces, all launched within a short period of time. Each approach presents its own technical and fiscal challenges.
The possible use of the promised future Falcon Heavy (FH) launch vehicle to conduct human missions to the Moon was the topic of a recent AmericaSpace article. I found this piece particularly interesting as the debate about the use of FH vs. the NASA Space Launch System (SLS) has some historical resonances with a previous round of lunar exploration, namely when America raced the Soviet Union to the Moon.
Although the fuel depot/HLV debate is one side of the architectural argument, I will set that aside for now and focus here on the type of heavy lift vehicle needed. Based largely on the limited payload capacity of Falcon Heavy (53 tons to LEO) compared to the NASA developed SLS system (initially, 70 tons, expandable up to 130 tons), the article at AmericaSpace questions the capability of the Falcon Heavy to conduct lunar exploration.
The Falcon Heavy consists of three Falcon 9 rockets strapped abreast. The 3 rockets contain nine engines each, resulting in 27 engines firing simultaneously at launch. Although claimed to be tolerant of engine out conditions, obviously if too many failures occur, loss of vehicle would ensue. The FH uses liquid oxygen (LOX) and kerosene, a tried and proven fuel system, containing somewhat lower total energy than the liquid oxygen LOX)-liquid hydrogen (LH2) of the SLS. The specific technical details of Falcon Heavy suggest a parallel with a previous trans-LEO launch vehicle: the Soviet N-1 rocket, the super HLV that the USSR designed and built in their failed bid to beat America to the Moon.
The N-1 rocket contained four stages (30 engines in the first stage), all burning LOX-kerosene (as the Soviets had yet to master the difficulties of developing and flying high-energy cryogenic hydrogen engines and LOX-kerosene is more forgiving). At liftoff, it produced over 11 million pounds of thrust. Keeping such a behemoth stable during launch was a major challenge but unavoidable; the N-1 used carefully balanced throttling and gimbals on each engine to control and steer the vehicle. In contrast, America’s Saturn V produced 7.5 mission pounds of thrust and used LOX-kerosene only in its first stage; both the S-II and S-IVB upper stages used LOX-LH2 fuel. Even with the greater thrust produced by the Soviet N-1, it would have placed only 90 tons into LEO and sent 24 tons to the Moon; the Saturn V put 120 tons in space and delivered 45 tons to translunar injection. Energy is the ability to do work and equals thrust times the length of the burn. Because of its propellant, the N-1 had lower total energy even though its thrust was greater than the Saturn V, resulting in lower delivered payload mass.
The differences between the N-1 and the Saturn V are similar to those between the Falcon Heavy and the SLS core. FH uses 27 LOX-kerosene engines to put 53 tons into LEO; the relatively low specific impulse resulting from the use of these propellants results in a performance of about 12 tons into translunar injection. Thus, a lunar mission mounted with the FH would require multiple launches (at least 3 and possibly as many as 6 to conduct a complete lunar mission). The SLS uses cryogenic hydrogen propellant and puts 70 tons in LEO and can send about 25 tons to the Moon. A complete lunar landing mission could be done with two SLS core vehicle launches. Incidentally, in the argument about which vehicle will be ready for flight first, I note that we could have had a 70 ton HLV flying now if a sane decision to build Shuttle side-mount had been taken before Shuttle retirement. But I digress.
While both the N-1 and FH are comparable in their number of rocket engines, their layout and functioning are different. The N-1 was a single vehicle with propellant tankage, lines and structure all common – feeding the 30 NK-15 engines simultaneously. The ring-like arrangement of the N-1 engines was intended to take advantage of symmetry to handle failures through balanced shutdown of engine pairs. But with thirty engines all firing simultaneously, instabilities (if they developed) would likely to lead to catastrophe. Falcon Heavy is designed to operate in unison in a more linear fashion as three vehicles strapped together, although supposedly cross feeds are envisioned to even out flow rates and variations in thrust. It remains to be seen how FH will handle instabilities.
In terms of historical performance, the Saturn V flew 13 times, with one partial failure on the second (unmanned) mission caused by severe “pogo” (longitudinal oscillations) during second stage firing. The N-1 flew four times, never successfully. A catastrophic explosion of the vehicle during the second N-1 launch killed several high-ranking Soviet officials and their lunar program never recovered. The N-1 design was barely sufficient for its stated purpose and this marginal performance led to the complete collapse of the Soviet lunar program. In contrast, enough mass margin was built into the Saturn V that an augmented, heavier version of the Lunar Module (including a surface rover) was flown on the later missions, thus extending the range and stay time of crew on the Moon, along with a commensurate increase in mission return.
There is a limit to how much even the largest HLV can carry into space. Eventually, we will reach a stage where a single, double or even multiple launch cannot accommodate all of the mass required for a mission. A case in point is the current Design Reference Mission for Mars, which requires not only seven (7) 120 ton HLVs (plus a separate crew launch) but also a nuclear thermal rocket (NTR) for the Earth departure, a piece of technology with which we experimented 50 years ago and will require tens of billions of dollars to develop an operational model. If we cannot develop this NTR, an all-chemical propulsion Mars mission could require as many as a dozen HLV launches. All things considered, the resources and complexities of this mission concept approach the outer edges of what is possible. I leave judgment as to the likelihood of such a mission ever flying to the delicate sensibilities of the reader.
The use of either the FH or SLS launch vehicle for trans-cislunar missions requires multiple launches; SLS will require fewer launches than the FH. The choice of which to use is not exclusively related to cost but also to architectural complexity. In general terms, the fewer launches required for a given mission, the better. But other considerations may drive the mission design to more launches.
In short, although an HLV makes trans-LEO missions possible, future human missions to a variety of Solar System destinations will require us to eventually learn how to assemble and fuel large complex craft in space. The basic requirement for any human Mars mission is about a million pounds in LEO, a value well beyond the capability of any HLV, existing or envisioned. Moreover, I would argue the height of fiscal and technical irresponsibility is undertaking a planetary mission whose principal architectural strategy requires launching (energy) everything (material) we need out of the deepest gravity well (Earth) in the inner Solar System.
The current controversy over HLV or fuel depots is transitory. Eventually, we will assemble and fuel trans-LEO missions in space because that is the inevitable direction in which we must evolve to become “space faring.” Using an HLV in the early stages of cislunar development can jump start a permanent spaceflight capability, including developing the essential skill of using off-planet resources of materials and energy. There will come a time when this debate will seem to our descendants as arcane as medieval arguments about the number of angels that can dance on the head of a pin.
Previous posts on heavy lift: