Having given some thought to my alternate timeline’s space program recently, I’ve been doing some research on space history in our world. One aspect that stands out to me is how much more capable a super-heavy-lift launcher (like the Saturn V, SLS, or SpaceX Starship) is than more modest systems in terms of truly providing access to space. Sure, there’s the finesse of a Falcon 9 with its reusability driving the costs down, but for many sorts of rather essential missions there’s really no substitute for sheer bulk and power.
Modules too Slender: We can do Better!
We see this reflected in the International Space Station; its modules are sized for the diameter and length of the Proton rocket’s fairing and the Space Shuttle’s cargo bay, resulting in cylinders of rather slender dimensions. Sure, they’re easily enough for a man to stretch himself out and work with colleagues comfortably, but the interior dimensions wouldn’t look out of place as a wide hallway in a rather ordinary house.
NASA photo of Robert Thirsk working at the International Space Station, 2009.
NASA photo of the International Space Station viewed from outside, 2021.
In addition to the dimensions, a rather strict mass limit was imposed by the Space Shuttle‘s payload capacity: 27 tons to low Earth orbit. Skylab, by contrast, benefited greatly from the Saturn V being able to lift a whopping 140 tons to low Earth orbit, and much larger physical dimensions. To wit, Skylab had 40% as much pressurized volume as the entire International Space Station put together, despite Skylab being lofted in just one launch, whereas the ISS took around 40. In addition, the pressurized volume is all in one big piece instead of in a veritable labyrinth of narrow modules, so you can do more with the space. Just with the visuals alone you can really notice the difference in quality.
The best NASA photo I could find of the interior of Skylab showing off its relative vastness.
NASA photo of Skylab’s exterior.
The Shuttle cost us Super-Heavy Lift Capacity…or did it?
Worse yet, there really was no reason to settle for the spaghetti-like architecture of the ISS; the Space Shuttle was premised around providing cheap access to space by dint of reusability, but famously that didn’t pan out, and we ended up with a vehicle that cost as much as the Saturn V did pound-for-pound but was much less capable. Or was it? As I’ve said in a Twitter thread once, the Shuttle launch stack is a perfectly good super-heavy-lift launch system; all one need do is swap out the orbiter for a cargo module. Such a mode was considered off-and-on in real life, starting with the 1980s Shuttle-C concept and encountering its last gasp in the form of the Augustine Commission’s side-mount launcher concept in 2009.
1980s NASA painting of Shuttle-C launching.
2009 rendering of the Augustine Commission’s take on the Shuttle-C.
How much cargo could you haul with this system? The dry mass of the orbiter was 78 tons, so it stands to reason that payloads around this size could be lofted up to low Earth orbit, such as space station modules. Considering that it’s taken 20 years or so for a Shuttle-derived heavy launcher, the Space Launch System, to become operational, it would have been better in retrospect if the more conservative Shuttle-C design had been adopted. Crew could have been flown in a capsule or perhaps a lifting-body spaceplane atop an off-the-shelf Atlas V (more recently a Falcon 9 could have done nicely).
Space Launch System: big Rocketry for small Thinking
Anyway, we now have the Space Launch System, which is capable of lofting 95 tons to low Earth orbit even in its Block 1 configuration. Block 1B will enable 105 tons, and Block 2 will let us haul 130 tons. These numbers are comparable to Saturn V figures, so we should be seeing a next-generation space station with modules about the size of Skylab, right? Er…not really. NASA is planning a next-generation space station in the form of the Lunar Gateway, which uses…modules comparable to the ISS’s! Huh?
Four NASA astronauts and candidates standing inside the LOP-G space station training module, 2019.
NASA rendering of what the Lunar Gateway will look like from the outside.
Now, the Gateway space station is going to be in orbit around the Moon, not Earth, which demands more fuel, so payload capacities shrink accordingly, all other things being equal. For instance, the Saturn V could only loft 43 tons to trans-lunar injection; the Space Launch System in Block 1 configuration can only loft 27 tons, expanding to a somewhat better 46 tons with Block 2.
Nevertheless, it’s striking to me that there’s no plan to take advantage of the sort of capacity a super-heavy-lift launch vehicle provides. Indeed, the first two modules for Gateway are supposed to be lofted aboard a Falcon Heavy, not SLS; Falcon Heavy can only lift 64 tons to low Earth orbit, about half that of SLS Block 2.
Of course Falcon Heavy is also vastly cheaper per launch than SLS is, which is likely the real reason the modules are being designed with that smaller rocket in mind. Once SpaceX Starship becomes operational, however, it will have a payload capacity of 100 tons to low Earth orbit at a far cheaper price than any existing system, opening up a wide variety of possibilities. On the other hand, that’s the same thing they said about the Space Shuttle 50 years ago, and look what happened with that!
Anyway, with regard to Lunar Gateway, it’s puzzling to me that the plan is not to use a super-heavy-lift rocket (be it SLS or Starship’s Super Heavy booster) to loft a Skylab-sized space station module (or modules, plural) into low Earth orbit and then loft a fuel tank so it can boost itself into the appropriate lunar orbit. This is in fact the same sort of technique envisioned for SpaceX Starship; tankers filled with fuel will ensure the payload that reaches low Earth orbit can also make it to Mars or anywhere else in the solar system. It even appears as early as NASA’s infamous “Integrated Program Plan” from the Apollo years that called for “space tugs” to be developed for just this purpose!
The Possibilities for super-heavy Lift Rockets
If you ask me we should have pressed on with the Apollo-Saturn hardware, or, with a later point of divergence in the timeline, Shuttle-C, and built a space station like this, perhaps even a Deep Space Habitat that could also serve well for longer trips. Skylab-sized modules lend themselves better to experiments in artificial gravity. Take just two of the things, tether them together across a good long distance, spin them around, and voila!
NASA photo of the Gemini 11 tethered to the Agena, the two modules spinning around each other for artificial gravity; yes, this technique was tested on a small scale as early as 1966!
Skylab-sized modules are not the ultimate endgame, however. You can do better! As part of the rather interesting “First Lunar Outpost” plan from the early 1990s, a group at NASA proposed a Saturn-derived launcher, the Comet heavy-lift launch vehicle. Some next-generation upgrades to the hardware and using three core stages would have enabled the Comet to loft a whopping 250 tons to low Earth orbit, and 98 tons to trans-lunar injection, double what the Saturn V could! Pieces of space infrastructure bigger still could have been launched in one go.
NASA rendering of the Comet HLLV.
In my alternate timeline I actually use this design as the basis for the first lunar missions, enabling a lander and command module stack twice as heavy as what Apollo used, as well as truly robust cargo capabilities: a habitat big enough to serve as an outpost unto itself could be launched to the Moon in one go.
Toward Ultra-Heavy Lift Rocketry
The Saturn V was planned to be upgraded to use a nuclear-thermal rocket upper stage, the so-called NERVA program, which would have increased its payload capacity to low Earth orbit from 118 tons to 150 tons. It was thought a 77 ton space station, comparable to Skylab’s mass, could be put in orbit around the Moon with NERVA.
Now, imagine if something like the Comet HLLV had a nuclear-thermal upper stage and got a proportional increase in capacity. That means 317 tons to low Earth orbit, and 125 tons to trans-lunar injection. And the latter figure is without in-orbit refueling! Attach a space tug and that 317-ton spacecraft in low Earth orbit (with or without being mated by tether to a counterpart for spin gravity!) could go all the way to Mars and back! Would only take two launches. Wow.
You could do even better than the Comet HLLV. The current version of SpaceX Starship is designed to loft 100 tons to low Earth orbit, but the earlier plans for the Interplanetary Transport System proposed a Starship-like stack that could loft 300 tons to low Earth orbit in a fully reusable configuration, or a whopping 550 tons in an expendable configuration (e.g. if it were lofting a space station module a la Skylab that was to be boosted up later). And that’s without a nuclear upper stage; with it, assuming the same augmentation as NERVA was projected to give the Saturn V, the cargo capacity rises to 381 tons in the fully reusable configuration, or 699 tons (!) in the expendable configuration.
Designs for super-heavy-lift rockets tend to top out at this size for a host of reasons. The only comparable and well-known design fully envisioned, the early 1960s Sea Dragon, also called for lofting 550 tons to low Earth orbit. I wouldn’t be too surprised if fully chemical rockets top out at perhaps this 500 ton or so range in my timeline, with models sporting nuclear thermal upper stages perhaps extending that to 1000 tons. Though as Voltairine of Mastodon.social helpfully pointed out to me, there are always proposals like the Convair Nexus, which was designed to loft 1000-2000 tons to low Earth orbit, which as far as I can tell wasn’t even nuclear-powered, opening up the possibility of extending conventional rockets’ payload range somewhat further upward.
Nuclear Pulse: the Final Boss of Spacecraft Propulsion?
Interestingly, designs for nuclear pulse propulsion called for an orbital test version massing just 300 tons to low Earth orbit, with earlier operational interplanetary models being capable of lofting 1600 tons to low Earth orbit, comparable to what something like the Convair Nexus could achieve. In a timeline like mine, where unlike real life spitting hydrogen bombs out the back of a vehicle raises few eyebrows, I suspect nuclear pulse will make more sense for payloads that reach into four-digit tonnage, especially for payloads that reach really deep into that range, because of the steep economies of scale involved. For an eight-million-ton vehicle, the size of a small city and which could easily be lofted, nuclear pulse propulsion could take a man to low Earth orbit for $6 a trip once advanced but quite achievable levels of mass production are achieved. Which not only makes any rocket look like a rip-off, it’s also competitive with trucks. Let that sink in.
Conclusion
But even if we did have the wherewithal to embrace the full power of nuclear pulse propulsion, chemical rockets, especially combined with nuclear thermal rockets, would likely remain dominant for a long time to come (say, the first century or so of spaceflight), since you need a large amount of outer-space traffic to truly make nuclear-pulse cost-competitive, along with a great deal of development work in mass-producing nuclear explosives and in refining the designs, all of which takes time to work through.
In a timeline like mine the Moon is reached with mere super-heavy chemical rockets, with an even heavier rocket sporting a NERVA-style engine taking us to Mars and beyond, all the while nuclear pulse drive is more or less experimental, or at best used for the very heaviest payloads or the very longest trips. The latter including trips to remote destinations such as Neptune or Pluto; a trip that would take years with nuclear thermal rockets could be shortened to just weeks with even relatively primitive nuclear pulse propulsion. Which itself would likely only come later.
So much the better, since this leaves an ample opening for the more down-to-earth chemical and nuclear thermal rockets to have their heyday, their time in the sun as the ultimate in interplanetary transportation, just as many a space cadet has always dreamed of for our real-world near future. I plan to include such monstrosities, as well as more featherweight approaches, in my alternate timeline, but they may just as well fit in in our real-world future, not being all that alternate after all, if only we have the courage to dream big.