SPACE LAUNCH SYSTEM (SLS) — USA, 2022

The SLS (as it is usually called, because nobody wants to say “Space Launch System” out loud as if it were a real name) has been unpopular and controversial from the beginning. As different mission ideas were tossed around in the early 2010s, it was called a “rocket to nowhere”, and this name was apparently widespread enough that its origin is unknown — we can only blame Wired and Bloomberg for popularizing it. Once it became clear that certain politicians were pushing it forward mainly as a big-money jobs program for their home states without much caring what it did, people started quite justifiably calling it the “Senate Launch System”. Throw in some specious promises of how little it would cost, followed by the kinds of costly reality checks that always happen whenever anyone tries to cut corners in aerospace, and then add space fans all excited about a future of reusable rockets who think putting reusable shuttle parts into a nonreusable rocket is a miserable step backward, and no other NASA program has gotten the level of denunciation and contempt that has been heaped onto this project.

But it’s here, it works, and for some time longer it’s going to be the only launch vehicle proven capable of sending people safely on lunar missions. So as absurd as it sounds when you compare it to more modern systems that might eventually do its job at a tenth of the cost, you could make the argument that the SLS is the best rocket in the world.

If so, let’s just hope its time in that role is brief.

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 at the time it looked more like a move for creating a dominant monopoly. The SLS reinforced that suspicion, as it is built not by ULA, but by Boeing as the primary contractor. The design is a bit weird, due to its origins. 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 taken 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 reach orbit, stopping just short of it so the upper stage requires only a tiny circularization burn to avoid reentry. This means the upper stage’s fuel supply is fully available for interplanetary destinations. In fact, the SLS can put its upper stage into a starting orbit higher than the Shuttle ever reached. The Artemis 2 launch actually threw the upper stage to an altitude of two thousand kilometers before it did its circularization.

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. Its primary mission is NASA’s Artemis program: the campaign to return the United States to the moon, “this time to stay.”

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’s called “Interim” because it will be used for only the first few SLS flights, after which it’s supposed to be replaced with the “Exploration Upper Stage”, which would be scaled up to a suitable size to fit the big stage under it, and have four engines, maybe the RL-10C3 variant. This “Block 1B” variant of the SLS would 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 RL-10 has very little of, hence the quadrupling of engines. The 130 ton version with the modernized solid boosters would be “Block 2”.

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 the first few flights. For later flights, they will switch to a cheapened non-reusable version of the engine. But that supposedly cheaper version might still be very costly, like 70 million bucks apiece, with the first batches costing $100+ million per engine — a price for which SpaceX could probably assemble a whole Starship with all 39 Raptors — according to the Inspector General’s report for 2023.

Why no reuse? Well, they didn’t plan to launch much more often than once every two years or so. In fact, the idea was that the first few rockets will be built largely from the inventory of spare shuttle parts which have been sitting in warehouses. Aerojet Rocketdyne had 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 ring segments left over from shuttle missions. 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, which they need because the SLS is about 25% heavier than a Shuttle stack. 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, which was the original contractor for the Shuttle boosters. They are the most powerful rocket motors of any type that have ever been built. Later, if the program continues long enough, they’ll switch from the old steel segments to a new single-piece carbon fiber design, which was also meant to be used by the Omega before it was cancelled it by Northrop. That would push the performance even further for Block 2... but when Northrop tested this new booster, it failed, similarly to how an earlier test of the shorter Omega version had failed.

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 have been discussing in louder voices lately, Boeing will do just fine. 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 went into it before anything flew. 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, the interim upper stage, and the Orion spacecraft 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). But as “Hanlon’s Law” points out, we should not attribute choices to malice which may be explained by ignorance, and the fact that the Inspector General’s next report found that the main issue was mainly due to “the lack of a sufficient number of trained and experienced aerospace workers at Boeing” plus “inadequate work order planning and supervision”. That is, the problems come from trying to keep payroll costs down, rather than from any intentional padding, which means it’s part of the same general malaise that’s been afflicting Boeing’s normal airplane business. 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 senators from Alabama such as Shelby, were and are determined to keep the pork barrel rolling at any cost. In fact, that’s how the SLS program started in the first place: the Obama administration cancelled the Constellation program and its Ares rockets, so Congress demanded that a similar rocket be revived, along with the Orion capsule, and that missions be created to make use of them — hence the “Senate Launch System” jibe. 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 couid do the job at far lower cost with far less risk of destructive vibration... but common sense prevailed and the Heavy got the job, though that did force it to take a slower trajectory. Even with all three boosters expended it still cost a fraction of the price of an SLS, and was ready a lot quicker too. Some say that the savings may have been as much as $2 billion, and that the probe might not have even survived an SLS launch.

The cancelled Constellation program had two rocket designs, dubbed Ares I and Ares V. 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 the lunar surface, 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 or forty 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, and a changeover from one contractor to another left NASA with no complete set of plans for what had actually been built. 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 didn’t look like it would cost much more than they already spent on the janky refit of the initial tower, meaning that in hindsight they might 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, and besides, don’t you worry, the costs of the second tower are still rising, and since then have climbed way past the refit expense. If they keep up the pace, the price might yet hit three billion bucks.

Once on the pad, the first SLS was plagued by recurring fuel leaks, mostly being due to the condition of the tower.  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, though with somewhat more erosion than exected on the heat shield.

As Artemis 2 was being prepared, Boeing was also working on the Exploration Upper Stage, which will be needed for assembling the Gateway. And guess what: it’s not only seven years late and way over budget, it’s also been plagued by numerous errors. And guess who was building it... yep, Boeing. NASA has had to send Boeing a long stream of Corrective Action Requests. When the Inspector General looked at that project, they found that the workforce was again lacking the level of skill and experience needed to perform such demanding work correctly. In short, Boeing had been up to their usual corner-cutting fuckery. By the end of 2024, when the rocket for Artemis 2 (a crewed test flight around the moon) was being stacked, rumors started to be broadly reported that maybe this time, now that the estimated per-launch ongoing costs of the program were creeping past $2.5 billion, the SLS might finally get cancelled — in fact, when the second Trump administration came in and started recklessly cutting expenditures right and left without even paying attention to what was being paid for, talk quickly started circulating that the SLS’s time might finally be up. Elon (who at the time was running the reckless “DOGE” effort to cut hundreds of expenditures based on only the most cursory glances at what they were actually paying for) wanted it cut, but Jared Isaacman, Trump’s nominee to head NASA (who probably got the nod on Elon’s recommendation) stands on the side of carrying through the Artemis program — in fact, he wanted to speed up the launches, and then transition facilities like Huntsville to start working on nuclear ion propulsion, in part so Congress would stop clinging to expendable rockets. Trump then decided that Isaacman was not “America first” enough — that is, too prone to independent thought. Sean Duffy was picked as an interim appointment, while continuing as Secretary of Transportation. Trump specifically praised his fine work on air traffic control, though that was one of the administration’s most obvious and tragic failures in its early months, with multiple plane crashes occurring. It took until November for Trump to nominate a permanent administrator... and he went back to Isaacman, who was confirmed a month later. Since he’s almost the only Trump nominee for any department with bipartisan support and no glaring signs of untrustworthiness, we can only be thankful.

Regardless, the White House budget proposal was to fund the SLS through the Artemis 3 launch and then cancel it, along with the Orion, the Gateway, and while they’re at it the Mars Sample Return mission. But the Senate had another opinion. And the first senator to speak up with a counter-proposal to restore funding this stuff was Ted Cruz of Texas, the guy who probably would have been the GOP nominee for the presidency in 2016 if Trump hadn’t won. And when the Senate passed their version of the so-called “big beautiful bill”, they added back Artemis 4 and 5, the Exploration Upper Stage, and the Gateway... plus a little something else that Cruz slipped in: funds to move the space shuttle Discovery from Virginia to Texas — a grab that nobody outside of Texas supported at all before this horrific bill came along, in which an unprecedented expansion of naked kleptocracy is only about its third worst feature. Senator Durbin called this “a heist”, and the Smithsonian says Congress cannot legally take the shuttle back after giving them ownership of it. (And of course, neither faction of the majority party spoke up for all the mundane but far more essential stuff that NASA does in the background, which both the Trumpies and the Cruzers agreed on slashing support for, because showy prestige missions are the only ones they care about. A bill to fix that is developing, but I don’t know whether it’ll have much support.) Cruz has also positioned himself as a vocal defender of the SLS — he’s now becoming the new Shelby, more or less.

The plan was for Artemis missions to fly every two years. That already seemed absurdly slow by modern standards, or in comparison to Apollo, but the time between Artemis 1 and 2 ended up stretching to well over three years. But the second launch was not nearly as troubled and worrying as the first was, despite the launch tower still causing leakage issues bad enough that the rocket had to be sent back to the VAB after the first launch attempt, so progress had been made.

Isaacman’s reset

As Artemis II was getting ready to fly, Jared Isaacman tried his best to salvage a workable program from what was clearly, even without Trump, the most mismanaged campaign in NASA history. With the Chinese planning to set foot on the moon in less than five years, and various stakeholders starting to notice that letting them set up bases ahead of us would not be a good idea, he announced several changes:

• no Exploration Upper Stage for Block 1B, or any Block 2 upgrades — instead, an essentially unmodified Centaur V, as used on the Vulcan, will sub in as a mild upgrade with about twice the fuel of the Interim stage, enabling 30+ tons to lunar orbit, and after that all upgrades would halt so the rocket design is frozen and repeatable
• no second launch tower, despite it being mostly assembled — the Centaur stage was chosen because it could fit the old one with a minor update
• no attempt to land Artemis III — instead use that mission for a lander test in low orbit, like Apollo IX
• in the biggest and arguably most unfortunate change, the Lunar Gateway is cancelled — instead they need to build a base on the surface (which may use some Gateway parts), which is what Congress wants... they’re estimating the cost at $20 billion, and that will obviously go up once they start actually making it
• a faster cadence, with crewed launches hopefully just six months apart, so the schedule won’t slip, the rocket production pipeline can be kept more active so skills don’t get rusty, and we might actually be able to build that base by 2030
• nuclear power will be used, both for the base and for future Mars missions — they want to attach a nuclear generator to the Gateway’s Power and Propulsion Element, and send it there by 2028

This plan pins a lot of hope on SpaceX and Blue Origin being able to speed up their lander development efforts. But those are very complex and difficult projects — the sort of thing where it’s likely that no amount of money and no amount of pressure can make it possible to finish quickly. And needless to say, constantly changing the long term plans for how Artemis is supposed to work, at a time when it should be well into the middle steps of an already established plan, does not bode well for its long term future, or for the odds of the USA doing better than China in establishing a presence at the lunar south pole.

It’s clear that Isaacman does not see the SLS as a long term basis for progress. He wants to get what value we can out of it, until something better comes along, and cut his losses on any grandiose future it might once have had.

alternatives?

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... it seemed unlikely at first that it could be ready for such a role ahead of Starship, but now that’s looking more plausible. 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. 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 with a larger upper stage. True, the Starship is flying now, but it’s quite a long way from being capable of lunar missions yet. That requires it to be flying safely and routinely enough to do ten or so launches per mission, in order to refuel the lunar ship in orbit. If they do get that part of it working, then we’ll finally be able to breathe easy.

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 being a fully reusable single stage, with the crew cabin down near ground level as in the ALPACA, a flattened lox tank on its roof, and the big hydrogen tank on top with solar panel wings on its sides. It weighs about 16 tons dry and 45 fully loaded, and stands 16 meters tall. They say a cargo version could bring down 20 tons if it goes back up empty (a lot more than the ALPACA) or 30 tons one way. Its BE-7 engines use a dual-expander cycle to burn hydrogen, with three around the edges of the cabin near the ground. 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. A big disadvantage is the difficulty of storing it, especially for prolonged periods, due to the extreme cold it requires. Even contact with other cryogenic liquids could make it boil. Blue Origin calls this lander “Blue Moon Mark 2”, where Mark 1 is their one-way lunar cargo lander that uses a single BE-7.

NASA chose this lander over the revised ALPACA (which no longer has drop tanks) and the Lunar Starship for the second phase of landings starting with Artemis 5, knowing, of course, that when they have need of a Starship it would still be available. It does make sense to have two approaches for different needs, one midsize and one heavy, each being a backup for the other... but it certainly does add to the cost. And now that NASA sees that the odds of the lunar Starship is not likely to be ready by 2028, and Isaacman announced a reopening of competition for Artemis 3, the only alternative with a chance of being ready sooner is Blue Moon... but not the completed Mark 2. Apparently they are trying to bodge together a quick “Mark 1.5” stopgap version to try to meet the schedule. They have never been a quick-moving company, and the odds of them being ready any sooner than SpaceX don’t look very promising. At this point the most likely outcome may be that China lands at the south pole before we do.

why?

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 first Trump administration wanted to fund by raiding social programs). Blue Origin’s “Blue Moon” had long been promoted as a forthcoming commercial lander, so that the Glenn could handle this kind of work, presumably with a lot of savings over using the SLS... though the lander concept would need a lot of upgrading to support a crew, or even to become 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.

For some time now it’s appeared that in the struggle between the budget-cutters and the pork barrelers, the LOP-G has been losing out, so the Artemis program won’t be able to use 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 has continually pumped 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 1/1/0 through April 2026.