Commercial Rockets

ATLAS — USA, 1957

Like the Soyuz, this rather large rocket was first developed as a long range ICBM, and was also quickly obsoleted from that role as smaller rockets became capable enough. It went on to far more positive uses: it has not only launched plenty of satellites, but it’s sent up manned missions as well during the Mercury program, and launched early interplanetary probes such as Surveyor and Mariner. Today’s version has a somewhat distinctive look due to its bottom end, which has two combustion chambers and two nozzles on a single engine, plus plumbing that bulges out of the rocket’s side. (This double-nozzle design was something once associated with the Titan booster, which was used for the Gemini program and for many historic interplanetary probes. Titan dwarfed the early Atlases but has been left behind by the V. It was discontinued in 2005.)

The fuel is kerosene, or “RP-1” as it’s officially called when refined for rocketry, combined of course with liquid oxygen, which is called “lox” for short. But embarrassingly, the engines used in the current Atlas versions are the Energomash RD-180, and NPO Energomash is majority owned by the Russian government, and furthermore the import company appears to be skimming millions of dollars per engine, which may be going directly to Putin. Naturally, there has been a lot of pressure to find an alternative motor. But no American engine builder has anything handy that can work as a replacement, and at this point, the plan is to just replace the Atlas with a new rocket: the Vulcan.

The original Atlas versions had a single nozzle in the middle, but also had two additional nozzles attached on the side, as small protruding bulges. They were not separate boosters, but extra engines which drew fuel from the main tank, and then dropped off once their additional thrust was no longer needed. Without the extra thrust, it was incapable of liftoff. It also had small vernier thrusters for steering, and unusually, they were mounted up on the sides rather than at the base. The booster’s steel tanks were incapable of supporting their own weight unless pressurized, even when empty. (It’s more common for rockets to require some pressure when full.) This unusual construction made the rocket so lightweight that it could take some satellites to orbit with no upper stage. Mercury capsules were launched this way, in fact... and at the time John Glenn rode one into orbit, the rocket’s failure rate was around fifty percent. They stopped using this “balloon tank” construction in the Atlas V, after having already switched to the RD-180 engine in the Atlas III. One legacy of this design is that today’s Atlas boosters still go to unusual heights and speeds before the second stage has to do any work.

Nowadays, lift can be enhanced for larger payloads by strapping small solid-fuel boosters onto the sides. But because the original rocket wasn’t designed for this, these strap-ons end up placed in odd and asymmetrical ways. They’ve never bothered to re-engineer the first stage’s exterior to correct this, even though the Atlas V is 25% bigger in diameter than earlier models. For most of the rocket’s life these boosters were made by Aerojet Rocketdyne, but ULA recently replaced them with Northrup Grumman’s new “GEM 63” model (developed by Orbital ATK before their merger), which is more powerful and way cheaper. This will pretty much leave Northrop with a monopoly on large solid rockets in the USA.

There’s a choice of “Centaur III” second stages: a classic single-engine model, and a high thrust one with two engines, which is rarely used... it had actually gone unused for many years until they brought it back for launching the Starliner crew capsule in 2020. The Centaur burns liquid hydrogen rather than kerosene, and ignites it electrically so it can start and stop as many times as needed. (Regardless of whether the igniter is chemical or electrical, it is actually quite challenging to ignite a cryogenic-fuel rocket engine in a vacuum, and to this day a lot of rockets have second stages which cannot re-ignite after their initial burn.) Hydrogen has higher efficiency but lower thrust, so it’s not uncommon to see it used on upper stages while using kerosene on the lower one... but the Centaur was the O.G., the first liquid hydrogen orbital rocket stage. It still uses balloon tanks, with the steel being just half a millimeter thick under the insulation. The difference of fuels does largely prevent the lower and upper stages from being able to share any parts. Its RL10 engine uses an expander cycle. The single-engine Centaur’s thrust is quite low at just 99 kilonewtons, only enough for about a third of a G with full tanks and a large payload. This is one reason why the first stage has to go high and fast.

In many ways the Atlas seems archaic and clumsy, but United Launch Alliance (handed down from Lockheed Martin, who took over from General Dynamics, who got it from Convair) managed to continue selling it by keeping the costs reasonable. They’ve even managed to cut prices in response to the competition from SpaceX. As mentioned, they now see the writing on the wall, and are trying to get their replacement ready in the next few years. But don’t laugh at the old-timer: the Atlas V has flown dozens of launches without ever losing a single payload — a record that no other rocket can match. We shall see if the Vulcan does likewise. (Other rockets have had longer runs of consecutive successful launches, after initial failures. For instance, Ariane V is now on a longer run that the Atlas V, with each streak having a single blemish where a satellite reached a mildly incorrect orbit which forced it to expend onboard fuel. Delta II had an even better run with 100 consecutive successes, ending with retirement, and the Falcon 9 also has a long streak going. But only Atlas V includes its very first launch in such a streak.)

What they have lost instead is any excuse to continue buying Russian engines. Congress has been trying for fifteen years to pass laws and policies which would end our dependence on imported engines for national security access to space, and Lockheed and ULA and Aerojet Rocketdyne have repeatedly evaded or simply ignored this mandate. For instance, when told to develop a US-built copy of the RD-180, Aerojet Rocketdyne accepted fat checks for years to spend on the effort, then announced that they didn’t feel like finishing the job. Getting away with that takes some hard-core cronyism.

Now that there is finally a firm schedule in place to move on from the Atlas V and stop buying RD-180 engines, Russia is hoping that some rocket maker in China will buy them. Since no current Chinese engine is even half as powerful, they may find one. They are aware that this may just lead to Chinese copies of the engine being made, but figure that selling some beats selling none. And meanwhile, Aerojet Rocketdyne eventually did finish their replacement, and now they’ve got no customer for it, except one startup named Firefly which has yet to get anything off the ground, but says that when they do, they want to use this engine for the bigger followup rocket they’ll make if the first one is successful.

The Atlas V model has never carried a human being, but the plan is that it will do so in its final year or two if the Starliner capsule can qualify soon, and that’s good, because despite the problematic politics of its construction, the V might well be the safest rocket a person could ride on.

Atlas V 401 (no extra boosters): mass 334 t, diam 3.8 m, thrust 3827 kN, imp 3.3 km/s, type ZOk(+S), payload 9.8 t (2.9%) [18.5 t (3.2%) with 5 boosters], cost $11M/t, record 90/0/0!
Stage name AJ-60A Atlas CCB Centaur
Role (pos) count booster (S) ×0-5 core (1) upper (2)
Diameter (m)   1.58   3.81   3.05
Liftoff mass (t) 46.7 305.1  23.1
Empty mass (t)  2.2 21.1    2.2 *
Fuel mass (t) ~13    ~76.3  ~3.0
Oxidizer mass (t) ~30    ~208     ~17.7 
Fuel type HTPB kerosene hydrogen
Engine Aerojet-Rocketdyne
RL-10C ×1-2
Power cycle solid staged (ZO) expander (EC)
Chamber pres. (bar) 267    24  
Ox./fuel ratio   2.3?   2.72   5.88
Thrust, vac max (kN) 1690     4152      106.3 *
Thrust, SL initial (kN) ~1110      3827    
Spec. imp, vac (km/s)   2.74   3.31   4.41
Total imp, vac (t·km/s) 117    943    ~91