Commercial Rockets

DELTA — USA, 1960

The Delta started as a three-stage variation of the Thor midrange ICBM made by Douglas Aircraft, and launched some of America’s first satellites, such as Echo 1A on its maiden flight, and Telstar soon after. The original Thor variants could barely eke the tiniest of payloads into the lowest of orbits, as the upper stages were so skinny that the rocket looked like an electric toothbrush... but they quickly improved its capabilities. It launched many satellites until 1981, when it was retired in favor of the space shuttle. But the shuttle’s expense brought it back as the larger Delta II in 1989. The II was made by McDonnell-Douglas, who were then assimilated by Boeing. They have now phased it out, with the final launch occurring in 2018. It does have a respectable history, having sent quite a few probes to distant destinations such as Mars and the asteroid belt.

The II used a single Aerojet Rocketdyne RS-27A kerosene engine, which has no gimballing and has to rely on small vernier engines for steering, just as were used in the original Thor. It used a hypergolic second stage, as the Thor-Delta did — in fact, the name “Delta” originally referred to the second stage only. The optional third stage for high orbits or interplanetary missions was solid fueled for some reason — again, like the Thor-Delta. At least three strap-on solid boosters (made by Thiokol and then by Orbital ATK) were necessary for the rocket to get off the ground, and they can cram up to nine of them around the first stage. These boosters were upgraded a couple of times. But even using all nine boosters only got its payload capacity up to six tons, and on one occasion in 1997 when they wedged all nine on there, one of them exploded the whole rocket, raining fiery debris on employees’ cars. But since then, it flew for its remaining 21 years without a single failure.

The Delta IV is a far bigger rocket. It retains little of the old design. It has a single large gimballed nozzle for the first stage. The first stage fuel is now liquid hydrogen instead of kerosene — a fuel which gives the best possible specific impulse, but as a tradeoff needs an enormous oversized tank which has to be super carefully made, because hydrogen not only has very low density, it will leak through leaks which don’t count as a leak to any other substance. The RS-68A engine, built by Aerojet Rocketdyne, is expensive and not the least bit reusable, as it has an ablatively cooled nozzle. This motor is basically an enlarged and cheapened-down version of the Space Shuttle Main Engine. Despite the many corners that it cuts, it remains one of the least cost-competitive large motors in the business. The second stage also burns hydrogen.

They make a version of the Delta IV with a triple first stage for the heftiest payloads, which is known as “Delta IV Heavy”. This has been used for a test flight of the new Orion space capsule being developed for NASA. For loads larger than the basic payload but smaller than the triple booster’s capacity, they can strap two or four solid rockets onto the sides of the first stage. Depending on the payload and on how high an orbit you need, it can be put together with either two or three stages, and the second stage has narrow (4 meter) and wide (5 meter) variants, the former being older. ULA liked to boast that the Heavy was the most powerful rocket available, but if so, the margin over some competitors such as the Long March 5 was thin... now, of course, the Falcon Heavy has left it far behind, and other new rockets are coming along which are going to be even more powerful: the SLS, the New Glenn, the Starship, and eventually the Long March 9 and the Yenisei if those get built.

Delta and Atlas were each other’s main competitors for governmental launches since 1960. Finally, Lockheed and Boeing got tired of competing with each other, and in 2006 formed a consortium called United Launch Alliance which sells both Deltas and Atlases. Though blatantly anticompetitive, the government decided to permit this because it helped rein in the spiralling costs of both rockets. Some parts can now be shared between them. (Both already used very similar motors in the second stage: variants of the Aerojet Rocketdyne RL10, originally made by Pratt & Whitney. When used on the Delta, this engine has a telescoping nozzle to keep it compact before stage separation.) For a while, this sharing arrangement led to the US government giving ULA a monopoly on military launches, but this was opened up in 2015 by SpaceX. (They had tried suing in 2005, but until their own system was mature this did no good). ULA hopes to retire the entire Delta line in a few more years, in favor of the Vulcan.

You may wonder what happened to the Delta III. Well, while the II has a run of 100 successful launches over the last twenty years, and the IV has never failed except for a too-low orbit on the first test flight of the Heavy with a dummy payload, the III failed on its first three flights, with a different problem each time, and was retired without orbiting a single satellite (though Boeing claimed that one flight counted as a success). The one piece of it still in use is the hydrogen burning second stage with the telescoping bell, which was carried forward to the IV. This is why the diameter did not match the IV’s booster at first... when they wanted more from it, widening the diameter to match was an easy way to get there. That bigger version will in turn be handed down for use by early versions of the SLS, where it will again be undersized for its booster, and be replaced later with something full-sized.

The III was a goofy-looking rocket. The bottom end was like the Delta II, with the same engine and the same 2.44 meter LOX tank, this diameter having been inherited from the original Thor. But the kerosene tank above it was 4 meters wide, like the upper stage, overhanging the nine solid boosters (all nine were mandatory). They did this to avoid making the III taller than the II. Little good that did them... in the IV, the first stage alone was as tall as the entire II or III. By replacing the first stage, the IV eliminated the last remnant of the Thor heritage.

Delta II 7320 (three added boosters): mass 152 t diam 2.44 m, thrust 2225 kN, imp 3.0 km/s, type Gk+S, payload 2.8 t (1.6%), cost $20M/t, record 153/0/2 (final).
Stage name GEM-40 Thor/Delta XLT Delta K PAM-D
Role (pos) count booster (S) ×3|4|9 core (1) upper (2) kick (3), opt
Diameter (m)   1.02   2.44   2.44   1.25
Liftoff mass (t) 13.2 104.4   6.9  2.1
Empty mass (t)  1.4  8.8  1.0  0.1
Fuel mass (t) ~3.5 ~29.4  ~2.0 ~0.6
Oxidizer mass (t) ~8.3 ~66.2  ~3.9 ~1.4
Fuel type HTPB kerosene UDMH+hydrazine HTPB
Engine GEM-40 RS-27A AJ10 Star 48B
Power cycle solid gas gen pressure-fed solid
Chamber pres. (bar) 48   ~40   
Ox./fuel ratio   2.3?   2.25   1.90   2.3?
Thrust, vac max (kN) 640    1090     44   66  
Thrust, SL initial (kN) 420    890   
Spec. imp, vac (km/s)   2.69   2.96   3.13   2.80
Total imp, vac (t·km/s) 32.7 283    18.9  5.8
Delta IV Medium (narrow second stage, no extra boosters): mass 257 t, diam 5 m, thrust 3140 kN, imp 4.0 km/s, type Gh(+S), payload 9.4 t (3.7%) [14.1 t (3.5%) with 4 boosters], cost $17M/t, record 30/0/0 (final).
Delta IV Heavy: mass 732 t, diam 5 m (15 m wide), thrust 9420 kN, imp 4.0 km/s, type Gh, payload 28.8 t (3.9%), cost $12M/t, record 12/1/0.
[Show stages] (all Delta IV versions)
Stage name GEM-60 CBC DCSS 4m DCSS 5m
Role (pos) count booster (S) ×0|2|4 core (1|S) ×1|3 upper (2) upper (2)
Diameter (m)   1.52   5.09   3.99   5.09
Liftoff mass (t) 33.8 226.4  24.2 30.7
Empty mass (t)  8.2 26.8  3.7  3.5
Fuel mass (t) ~9   29.5 ~3.0 ~4.0
Oxidizer mass (t) ~21    172.5  ~17.5  ~23.2 
Fuel type HTPB hydrogen hydrogen hydrogen
Engine GEM-60 RS-68A RL-10B-2-1 RL-10B-2-1
Power cycle solid gas gen expander (EC) expander (EC)
Chamber pres. (bar) 90   103    44   44  
Ox./fuel ratio   2.3?   5.97   5.88   5.88
Thrust, vac max (kN) 1370?    3560     110.1  110.1 
Thrust, SL initial (kN) 1155     3137    
Spec. imp, vac (km/s)   2.70   4.04   4.56   4.56
Total imp, vac (t·km/s) 79.8 811    93.4 124.1