Rockets of Today

DELTA — USA, 1960

Delta IV Medium

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 booster was small for easy transport and the upper stages were so skinny that the rocket looked like an electric toothbrush... but they quickly improved its capabilities, and compared to other early rockets, it was exceptionally reliable despite its frequent updates and modifications. 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 was derived from the Rocketdyne H-1 used in the Saturn 1B. It has no gimballing and has to rely on small built-in vernier engines for steering, just as were used in the original Thor. It put the kerosene in the top of the first stage and the lox at the bottom, which would not be worth mentioning except that this arrangement is far more unusual than you might guess. Most rockets, including the Delta IV, put the lox on top of the first stage, whether the fuel is as dense as kerosene or as light as hydrogen, because in either case the oxygen is the majority of the mass, and they want the center of mass to be forward for stability. Maybe it also helps with starting the engines, as the earliest stages of spin-up prefer to move only oxygen with no fuel in order to avoid an external fire, and having extra gravity behind it gives the oxygen some initial pump pressure. (I do know that the very tall Saturn V used gravity pressure alone for starting its turbines.)

The II 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.

Delta IV

The Delta IV is a far bigger rocket than the II. 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 also leaks through leaks which don’t count as a leak to any other substance. It also needs insulation, which you can get along without on a lox tank if you choose to, as its cryogenic temperature is much less extreme than what hydrogen needs. This insulation is orange, giving the rocket a distinctive look. (The Space Shuttle’s external tank was orange for the same reason.)

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 took inspiration from the Space Shuttle Main Engine, but is much simplified to reduce cost, and is also substantially bigger. Despite the many corners that it cuts, it remains one of the least cost-competitive large motors in the business, but its power is impressive, making the Delta IV the only rocket ever able to reach orbit using nothing but hydrogen for fuel (though Japan’s H3 may do likewise soon). The second stage also burns hydrogen, using a variant of the same RL-10 expansion-cycle engine found on the Centaur stage used atop the Atlas. That classic engine was originally made by Pratt and Whitney, before they sold that part of the company to Aerojet-Rocketdyne. The version used for the Delta IV, unlike the Centaur engine, has a telescoping bell to keep it compact before stage separation. In the upper stage, the lox is on the bottom, which is typical for upper stages even though it’s rare for first stages. Due to its cost the Delta IV is used almost exclusively for governmental launches, many being of a military or classified nature. And the RL-10 upper stage engine does contribute significantly to that cost, as prior to the advent of 3D printing, the dense network of internal cooling tubes was quite difficult to manufacture.

Delta IV Heavy

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 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 the SLS is far beyond that. Other new rockets are coming along which are also going to be more powerful: 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, though the overlap appears to be pretty small. For a while, this merger 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 is now retiring entire Delta line, replacing both it and the Atlas with the Vulcan.

Delta III

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 is in turn being handed down for use by early versions of the SLS, where it is again undersized for its booster, and will 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 (eight foot) 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 switching to hydrogen for the first stage, the IV eliminated the last remnant of the Thor heritage. It really should have had a new name, like Zeta or Odin or Jabba the Hutt, instead of Delta.

If you count all the pre-Shuttle versions, the Thor and its descendants are America’s most used and most versatile rocket family, edging out the Atlas family in both quantity and overall success rate. They’ve been our best answer to Russia’s immortal R-7 family.

Delta II 7320 (three added boosters): mass 152 t, diam 2.44 m, thrust 2225 kN, imp 3.0 km/s, gas generator (kerosene) and solid fuel, payload 2.8 t (1.6%), cost $20M/t, record 153/0/2 (final) — for legacy versions about 375/0/46.
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, gas generator (hydrogen), 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, gas generator (hydrogen), payload 28.8 t (3.9%), cost $12M/t, record 14/1/0 through 2023.
[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 expander
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