Rockets of Today


When I started these pages, I was beginning to wonder if the reason Lockheed and Boeing spun off ULA was so that the parent companies would be insulated from its coming bankruptcy (though since that time, the Vulcan is now looking pretty promising). NASA’s forthcoming Space Launch System reinforced that suspicion, as it is built not by ULA, but by Boeing as the primary contractor. It’s a setup with two huge solid boosters on the sides and four hydrogen-burning engines in the middle, with both the solid boosters and the hydrogen motors being copied from those previously used in the Space Shuttle. Even the main fuel tank of the first stage is based on the shuttle’s external tank, though it’s much taller — so tall that the fuel and lox tanks from all three boosters of a Falcon Heavy could rattle around inside the hydrogen section. As with the Shuttle, that big hydrogen-and-lox core is able to place itself just short of orbit, requiring only a tiny circularization burn for the upper stage to avoid reentry. This leaves most of the upper stage’s fuel available for interplanetary destinations.

It’s intended for strictly noncommercial use, lifting governmental payloads too large for existing rockets. It can lift 70 tons to low Earth orbit in its most basic configuration, with later enhancements planned to bring the capacity up to 105 tons by giving it a much larger upper stage, and then to 130 tons with upgraded side boosters — a figure that pretty much matches the Saturn V. No commercial system is aiming for this capacity, except for SpaceX’s Starship.

The “Interim” upper stage is based on the five meter wide upper stage of a Delta IV, with stretched tanks and some improvements for human rating. It’s a big stage in most contexts, but undersized for the SLS. It has one RL-10B2 engine — a variant of the venerable expansion cycle hydrogen burner which has been used for decades because of its unmatched efficiency. It will be replaced after the first few flights by the “Exploration” upper stage, which will be scaled up to fit the big stage under it, and have four engines, maybe the RL-10C3 variant. This will raise the payload capacity for a lunar transfer orbit from 26 metric tons to over 40. Since the extra weight means the core can’t reach orbital speed on its own anymore, there is now a need for substantial thrust, which the RL10 has very little of, hence the quadrupling of engines.

In contrast to other bold new rockets, the SLS is not intended to be reusable at all. For this reason, they plan to use the classic shuttle-derived main engines, which are reusable, only for a limited number of early flights, and then switch to a cheapened version if the rocket continues in service. But that supposedly cheaper version might still be very costly, like 70 million bucks apiece, with the first batches costing more like $100 million per engine — a price for which SpaceX could probably assemble a whole Starship with all 39 Raptors, according to a report from early 2024.

Why no reuse? Well, they don’t plan to launch much more often than once every two years or so. In fact, the idea is that the first few rockets will be built largely from the inventory of spare shuttle parts which have been sitting in warehouses. Aerojet Rocketdyne has about ten leftover RS-25 main engines, for instance — many of them being ones which have already flown multiple shuttle missions and were swapped out for various reasons. This is a bit awkward as some SLS stages will be made with engines that don’t match each other, with some being of earlier revisions and some being later, with slightly different performance. Even the solid booster will, for the first four flights or so, be made from segments left over from shuttle missions. Later, if the program continues, they’ll switch from the old steel segments to a new single-piece carbon fiber design that was also meant to be used by the cancelled Omega. The shuttle boosters were made of four segments, but the SLS boosters use five segments, meaning they have 25% more propellant. You might think that this just increases burn time, but no, all segments burn at once, so it puts more flow through the same old nozzle, meaning more pressure and more thrust. So this change does reduce safety margins and add risk. These boosters are subcontracted to Northrop Grumman, which bought out Orbital ATK, which bought out Thiokol, the original contractor for the Shuttle boosters.

surprising cost and difficulty

When ULA makes rockets, they have to manage costs to make a profit, which as I said may explain why Boeing was ready to spin them off. But the SLS, being a traditional aerospace contract with no competition, is comparatively risk-free profit. Even if the whole SLS project gets cancelled, which is an option that some of the people in charge of funding it are starting to discuss in louder voices, Boeing will do just fine. Some say that maybe the whole SLS program is just being milked for jobs and kickbacks — why else would something built from spare parts have turned out so expensive? About $14 billion has gone into it so far. SpaceX and Blue Origin are both developing all-new superheavy rockets for less. The difficult part, apparently, has been the core stage, and the launchpad; the boosters and upper stage were ready around the beginning of 2020.

And if money is being milked, it looks like Boeing’s work on the core stage is where most of that has happened (though Senator Richard Shelby’s constant efforts to direct extra make-work to the Huntsville area was probably also a significant piece). The core has been the costliest and the longest delayed of the main components. How could it take six billion dollars just to integrate existing engines onto lengthened tanks? Some say NASA administrator Jim Bridenstine lit a fire under them and got them to finally stop farting around, though of course that doesn’t recover any of the lost money. Bridenstine seems to have earned a decent level respect in his term as head of NASA, which was a surprisingly positive thing to see given his beginnings as a know-nothing reactionary politician who denied global warming.

There are plenty of people who’d like to see the SLS program die, but unfortunately, certain powerful politicians, most notably Shelby and the other senators from Alabama, are determined to keep the pork barrel rolling at any cost. This has led some to dub the project the “Senate Launch System”. More than one administration has tried in vain to staunch the budgetary bleeding, but Congress has always managed to keep the flow going, even as other important parts of the Artemis program which don’t use the SLS go underfunded. They also tried to mandate that the Europa Clipper probe would have to be launched on one, even if the Falcon Heavy can do the job at far lower cost with far less harsh vibration... but common sense prevailed and the Heavy got the job.

NASA had a previous project named Ares, which was cancelled. The SLS is pretty close to what the Ares V would have been, with a bit less power and weight — the core stage of Ares V would have had five shuttle engines instead of four. The $14B figure above does not even include the earlier Ares expenditures. (See Omega for the tale of the Ares I.) What do they plan to use this Saturn-like capacity for? For one thing, a space station out by the moon, called the Lunar Orbital Platform-Gateway, or LOP-G — a base that would allow astronauts to make multiple trips to different parts of the moon, going up and down practically at will... so long as they can be supplied with fuel, which is the difficult part. With the large upper stage, the SLS would have the grunt to move thirty-plus tons at a time to this remote location — enough to get the core of the station in place, at which point others might add further modules to it with smaller rockets. That new station would be where future Mars missions set off from, in NASA’s current plans. But if SpaceX gets to Mars first, or starts offering lunar round trips via Starship, that plan may not hold up.

Once the thing was finally built and started testing, it produced multiple delays and setbacks as the core stage suffered the same sorts of glitches as had been plaguing Boeing’s other troubled space project, the Starliner crew capsule. The green runs and wet rehearsals were never fully successful on the first SLS, “Artemis 1”, and eventually they tried to launch it without having fully resolved these issues, leading to weeks and weeks of scrubs, finally dragging the go date from summer almost to Thanksgiving... late enough that two hurricanes blew through the cape. There was a real worry that after sitting vertical for so long, the solid boosters might be unsafe. Another source of problems besides the core stage was the mobile launch tower, in which a lot of the shuttle-era parts are showing their age. Congress paid for a new tower to support the taller followup SLS versions, but apparently they didn’t budget enough: it looks like the contractor, Bechtel, lowballed the bid and overpromised what they could deliver, and now of course it’s falling behind and they need more. But even with the overrun it probably won’t cost much more than they already spent on the janky refit of the initial tower, meaning that in hindsight they would have been better off using an all-new tower from the beginning, instead of spending a billion apiece on two of them... but that probably would have taken longer. But in the end, this rocket which at times seemed to be living under some kind of curse did finally go up, and the empty Orion capsule on top returned safely.


In the absence of extravagant dream rockets like the Starship and the New Armstrong, could the Gateway be built without the SLS? Could the Falcon Heavy or New Glenn do the job cheaper? It’s tempting to just say “of course”, because loads could be split up: instead of lifting a big habitat to orbit along with a stage to push it to the lunar altitude, just lift them separately and then dock them. Two to four SpaceX flights would certainly be cheaper than one SLS launch... but there’s a hitch — apparently the SLS was already going to split up loads that way. So it may be that some pieces, as currently envisioned, are just too big. Or maybe it would just take a third trip to give it enough propulsion. Since the design is still in flux, I think it could be worked out. And if the Falcon Heavy is insufficient, the New Glenn might be more capable for those distant orbits, especially once they give it a third stage... though it seems unlikely that it could be ready ahead of Starship. I think if the New Glenn does what it promises, it should be sufficient to make the SLS largely unnecessary and obsolete despite being a good deal smaller, even though once the SLS evolves to its final planned upgrade (if that is ever budgeted) maybe even the Starship would fall short, unless it’s used expendably. I don’t think anything in the Artemis program truly needs titanic capacity in a single launch, except for one or maybe two launches for the initial core of the Gateway. I think we will soon regard the entire design of the SLS as a holdover of old-time methods from the nineties which are now obsolete.

But at present, all these alternate modes of transporting the Gateway are hypothetical, and as currently designed, the Gateway’s biggest core piece (the “iHab” which will make it big enough to house four) cannot be moved there by anything smaller than an SLS — in fact, it requires the forthcoming large upper stage.


There are competing designs for what future Artemis missions would use to land astronauts on the lunar surface. One proposal was SpaceX’s “Lunar Starship” — a special version of their enormous fully reusable stainless steel launcher, equipped to land on the moon. It would have big airlocks and winches able to lower heavy loads such as rovers down to the surface, and a set of angled rocket nozzles up on the sides which it would use when close to the surface, to avoid spraying lunar dust and rocks all around. NASA chose the Lunar Starship to use for the first couple of landings because it was the only bid within their budget as set by Congress. They were also cognizant that someday in the future, they would have need of its huge capacity.

A second proposal came from a group calling itself the National Team, which included Blue Origin as lead contractor, plus Lockheed, Northrop, and an outfit called Draper. This was for a LEM-like stack in which the bottom stage — essentially an embiggened Blue Moon — would be left behind on the surface, and an orbital tug (Northrop’s part) would also be expended on each use. Only the upper stage housing the crew (the Lockheed piece) would be reused. Astronauts would get in and out with a tall ladder.

If we want a better small lander, an alternative does exist: the third proposal is lightweight, highly reusable, and has lots of clever ideas, such as maybe putting wheels under the habitable section so it can leave the rocket and drive around. It’s from Dynetics, with contributions by two dozen other companies, prominently including Sierra Nevada. It’s called ALPACA — Autonomous Logistics Platform... forget it, the acronym is too tortured to be worth expanding here. It had eight small methane engines in its original design, and disposable drop tanks as the only non-reusable component. The ALPACA proposal, though it sounded very attractive on paper, was ranked third out of three, with one issue being that the early design was overweight, though apparently that was only due to how quickly they had to put it together. A followup design revision cut a few tons from the weight (with one measure being to use only four engines), and they are continuing to cut more, as well as finding a lot of other opportunitues to do things the right way instead of the expedient way. The less solvable problem is the price, which would be something like $9 billion to develop all the novel components — quite a bit more than the National Team asked, and waaay more than SpaceX asked. Also, though it would only take one Vulcan launch to send ALPACA to the Gateway, it would take more to fuel it for each landing. In the original design it would have been multiple Vulcans of fuel but I think they have now cut that down to one. For now they are only hoping for three to five landings for any one ALPACA, but maybe that could improve.

NASA asked for a second round of lander proposals for later missions, where the emphasis would be more on ongoing sustainable traffic. Blue Origin and some of the old National Team came back with a heavily revised proposal, this time much more reusable, and with the crew cabin down near ground level as in the ALPACA. A lot of detail is still vague in the design but apparently it burns hydrogen, with nozzles around the edges of the cabin. An advantage of hydrogen over methane is that it’s something that we eventually might be able to produce right on the moon by digging up ice. NASA chose this lander over the revised ALPACA (which no longer has drop tanks) and the Lunar Starship for the second phase of Artemis landings, knowing that of course if they still needed a Starship it would be available. It does make sense to have two approaches for different needs, each being a backup for the other... but it certainly does add to the cost.


If you’re wondering why there’s a need to build a lunar space station before NASA can go to Mars... well, you’re not alone. There are good reasons why some are disparaging the idea by calling it the “lunar tollbooth”. Any inquiry into building the Gateway by other means also has to question whether it ought to be built at all, or built this early. If the LOP-G is omitted from the current program, there would be no clear need for a rocket as gigantic as the SLS, and New Glenn will probably be sufficient. And indeed, the latest NASA plans are apparently de-emphasizing the gateway in favor of going directly to the lunar surface for the short term (which the previous administration wanted to fund by raiding social programs). Blue Origin announced a commercial lander named “Blue Moon” so that the Glenn could handle this kind of work, presumably with a lot of savings over using the SLS... though maybe not so much, as the early Blue Moon design was not reusable. And SpaceX is trying to prove that a Starship will be viable for lunar missions on a similarly quick schedule... though again, this is problematic because though it is reusable, its enormous dry mass makes its fuel consumption rather extreme, and would require multiple launches per trip just to fill its fuel tanks. But if we foresee moving heavy payloads on and off the surface, the size makes sense.

It does now appear that in the struggle between the budget-cutters and the pork barrelers, the LOP-G is losing out for the short term, so the Artemis program won’t depend on it for the earlier phases. If the gateway slips further, there goes about half of the justification for building the SLS. The other half would be a NASA manned Mars program, and that is also not within any current budgeting. So what we’re left with is a giant super-expensive rocket whose only stated purpose is a few moon flights, in which role it may well be obsolete within the next few years.

But longer term, the gateway could be very valuable — far more than the SLS, or even the entire Artemis program. It could, in fact, be a key step toward opening up the whole solar system. But Congress does not understand that, and continues to pump up the bloated SLS while cheaping out on the Gateway, the lander, and everything else that would give the SLS a purpose.

The SLS needs a better name. Either “Ares” or “Artemis” would have done nicely.

SLS (base “1A” configuration with small upper stage): mass 2500 t, diam 8.4 m, thrust 37000 kN (core 7400), imp 3.6 km/s, staged combustion (hydrogen) and solid, payload 70 t (2.8%), cost ~$8M/t, record 0/1/0 through 2023.