Commercial Rockets

STARSHIP — USA, 2021?

Of all the entries on this page, this is the most updated and revised, due to the frequent and public changes to the design of the proposed rocket. And also due to the fact that of all the rockets listed here, this one is the most truly revolutionary. If it works, it will expand human access to the solar system in a fundamental way that no other rocket can match. There may be other big rockets which on paper could launch a hundred ton spaceship, but no others have a way of then loading that ship with a thousand tons of fuel. And once you’re up in space with that much fuel... you can go anywhere.

SpaceX may not have announced anything to compete with the New Glenn, but what they’re working on now is a leap right past it: the most ambitious rocket design ever budgeted. Its goal is no less than to enable colonization of other planets! This rocket would start with an enormously powerful methane-burning reusable booster, far larger than the New Glenn, and on top of it, they plan to put a huge passenger-carrying spacecraft. In their original 2016 proposal, this spacecraft would have been twelve meters wide and 49 long — big enough to swallow the entire first stage booster of a Saturn V (10.1 by 42 meters) except for the finned fairings — but in 2017 they scaled it down to only nine meters wide. Since then that diameter has remained unchanged, though there have been constant changes to the height, the expected weight, the number of engines, the aerodynamic surfaces, and even the construction material. The expected dry mass has varied too; the latest definite number I have is from 2019, when they said they’d shoot for about 110 or 120 dry tons for the ship part. (Their initial rough prototype, which never flew, weighed about 200.) In early flying prototypes the bare fuselage is somewhere around 50 tons, I think, at just under 4 mm thick. Later versions may use thinner metal in selected areas. But areas near the bottom which support a heavy vertical load have to be reinforced with a corrugated inner layer, which more or less doubles the thickness, adding many tons beyond the weight of the skin alone.

This spacecraft would have its own engines, which would act as the second stage, thereby eliminating the last part of a rocket stack which previously had to be expendable, and getting them much closer to the dream of a spacecraft which can be reflown for only the cost of the fuel.

To go to Mars or wherever, this spacecraft would put itself in orbit with passengers aboard, and then dock with a tanker for refueling. The tanker would just be another Starship, but with the partitions between the fuel tanks moved closer to the nose to increase its capacity (though just leaving the front empty might leave nearly the same amount of fuel unburned). The passenger ship’s tanks can hold about twelve hundred tons of propellant (the lox being most of the weight), so it will take many tanker flights to fill up the passenger vessel. At first they claimed the payload would be 150 tons, then they said 100, and now they only say it will end up somewhere between those two numbers. Fewer tanker loads will suffice if you don’t need the maximum delta-V. The tanker could be launched by another flight of the same booster, which is designed to be reused rapidly with minimal maintenance. They’ve talked about eventually doing tanker launches as rapidly as three times a day with one booster, which to me sounds very optimistic, and certainly won’t happen in the short term. One part of this hope is the idea of landing the booster right back onto the launchpad, or close enough that it can be swung back into launching position with a simple crane built into the tower. Obviously this is a risky strategy, as a slip could destroy the launch complex. The latest variant of this idea is that the pad would have a C-shaped support ring near the top which the booster’s grid fins would come to rest on, so the bottom end doesn’t touch the ground.

Once fully fueled, the passenger ship would then cruise to Mars, possibly at speeds high enough so the trip might take as little as three months (but I doubt it). Slowing that flight down might save some tanker loads, but on the other hand it could also increase the passengers’ radiation exposure.

To come back from Mars... well, they haven’t fully worked that out yet. As presently designed, it has enough fuel to land there but not to take off again. They are planning to devise a means for people on the surface of Mars to manufacture their own methane and lox out of carbon dioxide and water (2 H2O + 1 CO2 → 1 CH4 + 2 O2) via the Sabatier process, which involves splitting the water and then reacting the hydrogen with the CO2 via a metallic catalyst such as nickel. They hope to land machines for this purpose on an unmanned flight, and it’s conceivable they might be able to start filling up tanks of return fuel before the human passengers set off... but I think it’s likely that human beings might have to get the machine running after they arrive, as it will be hard to automate a process to dig up the ice it needs, which does exist on Mars but is not abundant or easily accessible, except at the poles.

And not only will they be faced with the daunting prospect of mining huge amounts of ice, but they will also need about a megawatt of electric power to run the process, which will not be easy to generate. Solar cells for that much power would have to be made very thin and lightweight... and therefore very fragile. Plus they would be subject to dust storms. All in all, the plan to get people back to Earth when or if necessary definitely needs work. Some have proposed making a mini-starship to make returns feasible.

It is this ability to refuel in orbit, 100 or more tons at a time, which turns a rocket into a spaceship. Without that, it’s just a second stage with a nonremovable fairing, but with it, it gains a level of delta-V never previously available to heavy deep-space missions — up to about eight km/s depending on payload — making any body in the solar system reachable with a bit of gravity assist. Of course, if you put people in it, voyages beyond Mars are still implausible due to the time it would take, but the ship itself can get there. Give it a smaller rocket as a payload, and it should be no problem to perform feats of delta-V such as soft-landing a probe on the surface of Io or Mercury, or flinging something past Voyager 1.

Though on paper they had what looked like a complete design as early as 2016, first under the name “Interplanetary Transport System” and then the placeholder name “BFR”, very little of this was built or tested until 2019 except for the “Raptor” engines. These are not all that much heavier than their existing Merlins but produce twice the thrust — about three fourths as much as Blue Origin’s far larger BE-4. These new motors get a much better specific impulse from methane than the Merlin now gets from kerosene, thanks in part to an exceptionally high combustion chamber pressure: 250 bar for the near term, and hoping to get past 300 later on. In 2017 they already had a two-thirds scale test engine working well at 200 bar (the pressure of a scuba tank) — already high enough to raise some eyebrows even in an expendable motor. By mid-2020 one test engine had survived 330 bar. (The BE-4’s target is 134 bar, a safer value which might lend itself to a longer reuse lifespan, but still outdoes the Merlin. The Space Shuttle’s main engines, which were the most advanced of the 20th century, reached 206 bar.) The first full-scale Raptor was completed and fired early in 2019, apparently with substantial changes from the smaller version, and later that year they started flying it in a low-altitude test vehicle. That version did have issues with durability, producing what is sometimes called “engine-rich exhaust”, but apparently it fully met its ambitious goals for power and thrust. A year later they had the engine flying with no such issues... except when fuel-pressure problems made it run too lean. (Meanwhile, whatever testing Blue Origin is doing on the BE-4 has been out of sight and we have no idea what progress they have made, but they appear to be satisfied with it, as they are now constructing their engine factory.)

Even as they were building test articles, fundamentals of the rocket’s overall construction kept undergoing deep changes. For instance, the intent had originally been to build the ship out of carbon composites, until in late 2018 Musk suddenly said they were moving toward metal construction. They decided that the most weight-efficient material that could survive re-entry might be, of all things, stainless steel! It can be polished to reflect away most radiant heat (which, astonishingly, scales with the eighth power of speed during reentry), and it can be cooled from the back with piped fluids — or in the tank sections, perhaps just by splashing the cryogenic fluid against the interior walls. At one point they said they would also put tiny pores in it so the coolant fluids (i.e. extra propellant) boil out to produce a gaseous protective layer... but this idea was fraught with risks, so they wisely backed away from it. They then developed a thin ceramic tile to put over the steel, getting it to re-emit as much radiant heat as possible, which means it’s near black in color. Because steel can tolerate far more heat than aluminum or carbon fiber, the tiles don’t have to deflect the entire heat load, which means they can be quite thin and light compared to previously used materials such as the Space Shuttle’s tiles, or a traditional ablative heat shield. They can even have little gaps between them. Any hot gases leaking through will first encounter a thin blanket of insulating fibers, and then the shiny steel surface.

To be more specific, the material they plan to use in mature versions of the vessel is a special new chromium steel which is treated at cryogenic temperatures. Apparently it was the breakthrough which made this new stainless steel available which caused them to change their minds about using carbon fiber. They call it “30X” for now. Besides tolerating heat, this new steel also likes cold: it doesn’t turn all glassy and brittle when chilled to the temperature of liquid oxygen, as many materials do — in fact, it’s actually stronger cold than at room temperature, which for steel is kind of bizarre. Insulation for the tanks will essentially be nonexistent: the idea is to make the tank wall and the outer hull a single piece to avoid redundancy, but since steel is much more thermally conductive than carbon composite is, keeping things cool will be a problem, especially in the atmosphere. It looks like the current plan is to just accept the extra boiloff. Whether this might affect the capacity to keep fuel available on months-long missions may still be an open question, as liquid oxygen can produce a vapor pressure far beyond the capacity of the fuel tanks to contain, long before getting near room temperature.

Even after sending prototypes on hop tests they kept making design changes. Musk’s philosophy is to have no attachment to sunk costs, to have no reluctance to ask other engineers to change their part if it’s making your part difficult, and that if a design turns out to be difficult to implement, that means the design is flawed and should change. The result that this philosophy arrives at may turn out to look amazingly simple and crude, belying the amount of sophistication that went into pursuing complex alternatives. He intends to keep pursuing rapid iteration and revision as they build and launch numerous Starships, so that in a year or two they arrive at something much better than what they’ve got now. They are free to do this because so far, almost nobody is actually paying them to launch on one, so the only thing these test flights will be used for is putting up their own Starlink satellites.

Unlike the Merlin, the Raptor uses “full flow staged combustion”. That enormous 250+ bar chamber pressure is applied to all of the fuel and all of the oxidizer, as each is preburned with a minimal amount of the other to power the pump on its side. (On the methane side, the mostly unburned result is then used as coolant on the outside of the bell and combustion chamber.) This means it doesn’t lose specific impulse by having a separate low-pressure exhaust pipe coming out of the turbopump, as the Merlin does. The BE-4, by contrast, uses oxygen-rich partial staged combustion, which is the same approach preferred by Energomash. This puts it ahead of the Merlin in efficiency, but may actually be less safe, as it requires the turbine parts to withstand hot oxygen at high pressure. Hot oxygen is what makes a cutting torch eat through steel in seconds, so handling this is not easy. The mitigating factor is that it’s not nearly as hot as fully combusted exhaust would be. (Blue Origin’s BE-3U hydrogen engine, by contrast, does have an exhaust pipe... it uses an open expander cycle, dumping cold unburned fuel.) The full-flow cycle shares this issue of hot oxygen but it might allow the heat level to be somewhat more moderate in the turbines. SpaceX originally claimed “long life through... more benign turbine environments”, and said they have a new oxygen-resistant alloy to make the turbine from. But they may be walking that back a bit: Musk’s latest hints imply that instead of keeping the internal conditions benign, they will instead push this new alloy even harder, and raise the pressure in the engine to even more unheard-of levels. And to have 300 bar in the chamber, the pressure in the turbines is at least double that, maybe reaching as high as a thousand bar — a rather inconceivable number. Both Energomash and Aerojet Rocketdyne have experimented with full flow, but no such engine has flown before. Staged combustion of any kind is rare in American engines, the RS-25 Space Shuttle Main Engine (which used a fuel-rich cycle) being the outstanding example.

One possible issue I’ve heard mentioned is that it might be tricky and complicated to start the Raptor up. That’s an issue with partial staged combustion too; it’s why the Space Shuttle’s hydrogen engines had to be started seven seconds before lighting up the solid boosters. The Raptor has multiple turbine stages to produce all that pressure, making the issue even trickier. Even the simpler Merlin has to be started by first running helium through the turbine to get it turning. (Historical note: in the Apollo, the initial flow to get the pumps turning was powered by gravity. The rocket was so tall that even standing still, the fuel entering the engine was under quite a bit of pressure.) I have no idea how they start the Raptor; with (as I understand it) two separate sets of turbines on independent axles, I can only guess that they might have to blow both fuel and oxygen through it unburned, which would risk an explosive “hard start”. But despite this, their test flights have demonstrated that the Raptor can, as claimed, be started up very quickly. This would need to happen if, for instance, the Starship suddenly has to detach from a failing booster. The escape plan in this case is mostly to trust the ship’s steel hull to protect it from the booster’s fireball for a second or two, rather than to try to outrun it completely as you would have to with something like a solid-fuel booster. But even so, you still need thrust right away.

SpaceX also plans to develop reaction-control thrusters that burn gaseous methane and oxygen, replacing the hypergolic “Draco” thrusters that the Dragon capsules are equipped with, as well as the very inefficient compressed-nitrogen jets that they now use to steer Falcon boosters during re-entry. Musk says they’ll have quite a lot of thrust. I presume that these will be turbineless, and use electric pumps or heat to pre-pressurize their fuel so they can start instantly... but reliably igniting a gas mixture when surrounded by vacuum is a tricky challenge in itself, so it’s understandable why most makers of deep space thrusters would rather use hypergolics, or even monopropellant. They also plan to eliminate the helium tank that the Falcon uses to pressurize its kerosene, since methane and oxygen can be pressurized enough to feed the turbopumps (about five bar) just by backfeeding some heat from the engines, if ambient warmth hasn’t already given you all the pressure you can stand. Since they also plan to use spark plugs for ignition (like the Space Shuttle’s main engines), instead of the pyrophoric igniter fluid that the Merlin uses, this would eliminate the need for any other consumable chemical to be supplied — an important consideration if your journey is starting from someplace other than Earth. But this may have to wait for a future version. In today’s version they will, for instance, still use cold nitrogen thrusters, and the first high-altitude flight of a Starship prototype (SN-8) crashed because the autogenous pressurization didn’t work for the small “header” tank that stores fuel for the landing burn. This is positioned between the two main tanks, and the lox header is up in the nose of the ship to balance the weight during reentry, so getting hot gas into them is not so simple. For a short while, they went back to using helium pressurant, but this caused a new failure mode so they switched back. And time will tell whether a spark igniter can be reliable enough for reaction-control thrusters... they’ve had some issues with spark ignition dependability in ground tests of the Raptor.

The number of engines per stage has changed repeatedly. By 2020 they had settled on six engines for the ship, and dropped the number on the booster to 31, saying they might go as low as 28 if they can get the thrust high enough. In mid-2021 the current plan is for 29. This high degree of numeric redundancy means that even if several booster engines have issues at once and shut down, they could still reach orbit just fine. It also means that propulsive landings would be safer, as they could also survive an engine failure. They now plan to burn three engines when landing the ship, and be prepared for any one of the three to fail. (Falcon booster landings traditionally depend on the central engine alone, even if two additional ones were sometimes used for part of the landing burn.) Unlike Blue Origin, they feel that keeping the engine relatively small and (by rocket standards) mass-produced will be the secret to cheapness.

The booster engines will be nongimballed except for a central cluster of nine (originally seven). The fixed outer engines will have higher thrust than the central ones. The outer three engines on the spaceship would have large bells optimized for vacuum, about 2.4 meters in diameter, and likewise be nongimballed. These are the engines they’d use for orbital maneuvers, such as sending the vehicle from Earth orbit toward Mars, and for the lion’s share of second stage boosting in the upper atmosphere. The inner three have small bells, about the same as the engines on the booster, and can gimbal widely and quickly. These would be used for landing, and I think for takeoff from Mars. Those large-belled outer engines (called “R-Vac” for short) would have a claimed specific impulse of 3.7 km/s, possibly even 3.8.

At any diameter, they hope to give these engines the unusual ability to throttle down to just 20% of max thrust. The only current engine I’m aware of with such a range is the BE-3 hydrogen burner in the New Shepard, which needs to go that low in order to hover before landing. One issue is that low throttling at sea level is supposed to be incompatible with even a moderately large bell size. But this low throttling probably only needs to be used in low gravity and little or no atmosphere, not on Earth.

Throughout most of 2017 and 2018, I kept hoping that at some point they’d budget some time and attention to giving this “BFR” project a real name, like Condor or Pterodactyl or something, instead of Big, uh, Falcon Rocket. I took to just calling it “the Beef” for short. This allowed me to ask regularly where it is. Then late in 2018 they decided to name the upper stage “Starship”, and the lower stage “Super Heavy Booster”. These names are disappointing, since “Starship” manages to be both blandly generic and preposterously overblown, and “Super Heavy Booster” is just a category name that could be used by anyone.

Of course, this Mars colonization scheme may go nowhere, but if so, the Raptor should still be a very competitive engine or other uses. They could use it to make a cheaper Falcon with fewer motors, or make a new rocket in the fifty ton payload class, or make a compact model with one Raptor. Musk had said that he hoped that the Raptor can make a reusable second stage possible for the Falcon; the Air Force was helping to fund some development for second stage use, and it sounded for a while like that was probably where the engine would first fly. Maybe that was one reason why they were testing the engine at two thirds size: because this would fit into a Falcon second stage. (There don’t seem to be any exact figures available on what size this smaller engine is; it may be more or less comparable to a Merlin, which weighs about half a ton and, with the sea-level bell, is about the size of a phone booth. The mature Raptor is apparently a little over one ton.) Lots of uses are possible; after all, once you’ve got the engines working, you’ve done two thirds of the development work — the rest of the booster is simpler stuff and takes less time, so all kinds of body designs are possible.

But they have now dropped plans for a raptorized or reusable Falcon second stage. Instead, they announced plans to start making some second stages into small BFR testbeds, so they can try out heat shields and reentry techniques, but even that ended up being dropped, as far as I know. They no longer want to invest any new technology into improving the old Merlin system. Since the Starship has no presold customers except one billionaire who is paying for a flight around the moon, they are tightening their belts and trying to cut as many financial corners as possible until they complete a working rocket. Musk has said that the goal is to get out to the moon as quickly as possible, presumably on the assumption that at this point they’ll start getting paying customers.

But while such uses might all be profitable, Musk is threatening to dedicate his life and fortune to promoting a Mars colonization effort involving thousands of ships, at a hoped-for cost of around $200,000 per colonist. Personally, I don’t see the need to rush. But here’s how seriously he takes the idea: he now says the plan is to replace their entire fleet of Falcons and Dragons with these new rockets, so that astronauts will ride a Beef — er, Starship to the space station, and commercial satellites will be launched from a Starship with a huge cargo bay that opens like an alligator jaw. This makes me envision the thing making stops at a dozen different orbits to drop off payloads, like a mailman doing his rounds... but just as likely, this means that a lot of flights are going to leave the Starship’s payload area mostly empty.

Hey, maybe they could fit the interior with some kind of catapult, to help fling the satellites into different orbits. It might be a good use of all that available space, and would save fuel. Telescoping rails, perhaps? If it can unfold to forty meters long, then with 5G of acceleration it could impart 44.7 m/s of orbital change velocity; with 10G, you’d get 63.2 m/s — enough to save at least one ton of fuel per change of orbit.

Anyway, this easing of space constraints would be a big relief to satellite builders, who grumble about the “tyranny of the fairing”. (One area where the Falcon 9 does lag behind its competitors is in fairing size.) This roomy cargo bay would particularly enable the deployment of big telescopes.

And of course with refueling, a Starship could also take a substantial load to the moon, which is a destination that has been getting a lot of renewed interest lately, and is definitely on Musk’s list. He says they are officially budgeting all post-Falcon R&D to building the Starship and its booster. The idea is that using it for their regular business of orbital launches will allow the entire Mars scheme to pay for itself. Their bet is that using an absurdly oversized rocket to launch ordinary satellites will make economic sense because the 100% reusability will make each flight so cheap... but that might not work unless the market for heavy cargo expands considerably as prices drop. If they can’t do large volume business, these savings may remain elusive... but mass-producing ships out of sheet steel does look like a good start on keeping them inexpensive. But if they pull it off, then maybe once the competition also has high reusability, at that point a smaller launcher would save money again. A Mini-FR that scales this design back down to, say, a thirty ton capacity might someday end up being the even cheaper way to carry satellites and astronauts.

Because of these priorities, Musk’s plan is now to devote most of the development effort to the ship rather than the booster, since it’s a much more complex task and requires far more innovation. They’ll be sending prototype ships on suborbital hops before the booster has even started being built. He says the heat shield is particularly challenging, due to the high velocities it’ll be asked to handle. But I bet that’ll be nothing compared to the million details it’ll take to go from a usable unmanned Starship to one that can carry passengers safely for months. I would not be surprised to see more than five years go by between the first uncrewed commercial launch, and the first crewed one. (Their proposed schedule is to cover this gap in just two years.)

Whereas most super-heavy rockets are exotic expensive beasts that launch only a few times, Musk wants the Starship to be mass-produced. That’s the only way to have a fleet big enough to colonize Mars, after all, and I suppose he hopes that if the flights are cheap enough, it will catalyze a boom in demand for other uses. Even as they build the earliest prototypes (and quite often destroy them in tests, because they have fully embraced the principle of “fail fast”), they are looking at every stage for ways to make the process scalable to high volume assembly line production. Musk is talking about increasing the human race’s orbital lift capacity by about two orders of magnitude within a decade or so, if they really do start making these in large numbers. That could be enough to build whole cities in orbit, and large bases on the Moon. Longer term, a full-sized Mars fleet might make that three orders of magnitude. And he’s considering making future Starships of a size far larger yet.

One factor which Musk has basically ignored — and which NASA seems to be turning a blind eye toward as well — is the possibility that there is already life on Mars. The Viking tests back in the seventies are difficult to explain by other means, and strange color blooms have been seen in certain patches of soil in moist areas near the equator... if such life exists, colonizing on top of it could be a bad look, and possibly even mortally dangerous. The fact that NASA has sent multiple probes after Viking but has never been willing to follow up on biological experiments is a strange omission.

The silliest proposal for the system is to use Starships for rapid transportation here on Earth: build spaceports around major cities (most likely out in the ocean), and fly passengers to the far side of the world in 45 minutes. Can you imagine the regulatory nightmare that would be, and what a tempting target it would be to use one for a terror attack? And as someone once said of this sort of ballistic passenger transportation scheme when it was first dreamed of fifty years ago, “For half of the flight you can’t get to the toilet, and for the other half it’s out of order.” Well, it’s not as unrealistic as Musk’s “Hyperloop” plan.

Since Musk is always thinking about the project after next as well as the next project... what would come after Starship?&ensp:What he’s talked about as a successor is the same thing only bigger — a fat Starship. They’re considering increasing the diameter, perhaps as big as 18 meters. Even if there is little increase in height (and I don’t think raising the height would be easy), that would give us a rocket weighing around twenty thousand tons, with a payload capacity of several thousand cubic meters.

Super Heavy and Starship: mass 4850 t, diam 9 m, thrust 72000 kN, imp 3.5 km/s, type Z2m, payload 100‒150 t (2.0‒3.0%) with reuse, cost maybe $2M/t at first (and less later). Yipe!

(...How does 4850 tons compare to the biggest historical rockets? Apollo was just under 3000 metric tons, N-1 (the failed Soviet moon rocket) was 2750 tons, the Energia was 2400 tons without payload, and the Space Shuttle was 2000 tons. As far as I am aware, those are the only rockets that have ever been bigger than a New Glenn or Falcon Heavy. The SLS will be about 2500 tons in its initial version.)

(The original 2016 BFR design was expected to be 10,500 tons. If you think that was absurdly gigantic, well, it still isn’t as big as the legendary “Sea Dragon” which was designed in the early sixties but never built — a very basic but very large rocket designed to make heavy lifting as cheap as possible for the time. It would be built out of sheet steel in a shipyard, and use a simple pressure-fed motor on each of its two stages. That thing would have weighed eighteen thousand tons, and the two engine bells were well over twenty meters wide — large enough to fit comfortably over my house. And Boeing once proposed a far bigger rocket called the Large Multipurpose Launch Vehicle, with a core stage so huge that it could use ten strap-on solid boosters that were each the size of an Apollo. The heaviest configurations might weigh thirty thousand tons, and lift almost two thousand tons to orbit.)