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The Delta 4 vehicle is a common booster core (CBC) design, where a single booster design can be used to launch in single stick mode, or in triple stick (Heavy) mode.

The single stick model also has mount points for strap on solid rocket boosters of varying numbers.

While the Delta 4 Heavy has the highest payload to LEO (Falcon Heavy is not active as of time of writing) of all active US boosters, what if they needed a heavier version?

Could the side cores of a Delta 4 Heavy launch with strap on SRBs? Could additional strap on SRB's be mounted on the center core?

Obviously flame trench issues would ensue and require modifications, but since my understanding is that each CBC has all the mount points for both strap ons and side CBC cores, could you use both at once?

If so, how much of a payload boost would that add? (The obvious correlary is how much closer could it come to a Falcon Heavy?)

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    $\begingroup$ While the math doesn't say it's impossible, the contraption increasingly starts to resemble something straight out of Kerbal Space Program, and that should really raise some flags if this is a route worth pursuing. $\endgroup$
    – SF.
    Feb 1, 2016 at 10:19
  • $\begingroup$ The limiting factor is the amount of structural margin in the attachment points between the side CBC and the center CBC. With structural margins in rocketry generally not being very generous, I expect you can't add several hundred tons of thrust pushing the side CBCs. $\endgroup$
    – Hobbes
    Jul 23, 2016 at 19:24

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Here are the projected Delta Heavy upgrades from United Launch Alliance (ULA) in different configurations. You can see that the payload value at the name of the rocket version is not the same as the LEO payload value. I guess the name of the rocket version shows the payload for a low earth orbit, 200 km at 28.5 degrees (probably 28.7), that’s why is different. The low earth orbit (LEO) payload value shown here is for 407 km LEO at 28.5 degrees. This data is from astronautix.com. In my opinion the low earth orbit, 407 km at 28.5 degrees, should be at 51.6 degrees because I guess it is for the ISS (International Space Station) orbit, but anyway I am referring to the website.

Delta IV Heavy Upgrade 30 t: stage 0+1st stage, 3 CBC (RS-68 engines) with 4 GEM-60 solid rocket boosters (SRB) + 2nd stage (RL-10B). LEO Payload: 27,000 kg (59,000 lb) to a 407 km orbit at 28.50 degrees, Earth Escape (EE) Payload: 10,000 kg (22,000 lb).

Delta IV Heavy Upgrade 35 t: stage 0+1st stage, 3 CBC (RS-68B engines) + 2nd stage AUS-60 upper stage (powered by 2 MB-45 or RL-45). LEO Payload: 32,000 kg (70,000 lb), EE Payload: 12,500 kg (27,500 lb).

Delta IV Heavy Upgrade 40 t: stage 0+1st stage, 3 CBC (RS-68 Regen engines) with 4 GEM-60 SRB + 2nd stage (?). LEO Payload: 36,000 kg (79,000 lb), EE Payload: 14,000 kg (30,000 lb).

Delta IV Heavy Upgrade 42 t: stage 0+1st stage, 3 CBC (RS-800 engines) + 2nd stage AUS-60 upper stage (powered by 2 MB-60 or RL-60), aluminum-lithium lightweight alloy in place of the existing aluminum in all stages. LEO Payload: 38,000 kg (83,000 lb), EE Payload: 14,000 kg (30,000 lb).

Delta IV Heavy Upgrade 43 t: stage 0+1st stage, 3 CBC (RS-68 Regen engines) with 4 GEM-60 SRB + 2nd stage AUS-60 upper stage (powered by 1 MB-60 or RL-60). LEO Payload: 39,000 kg (85,000 lb), EE Payload: 14,500 kg (31,900 lb).

Delta IV Heavy Upgrade 45 t: stage 0+1st stage, 3 CBC (RS-68B engines) with 6 GEM-60 SRB + 2nd stage AUS-60 upper stage (powered by 1 MB-60 or RL-60). LEO Payload: 41,000 kg (90,000 lb) to a 407 km orbit at 28.50 degrees, EE Payload: 15,000 kg (33,000 lb).

Delta IV Heavy Upgrade 48 t: stage 0+1st stage, 3 CBC (RS-68 Regen engines) with 4 (or maybe 6) GEM-60 SRB + 2nd stage ? (maybe 1 MB-60 or RL-60), cross-feed between the strap-ons and core. LEO Payload: 43,000 kg (94,000 lb) to a 407 km orbit at 28.50 degrees, EE Payload: 15,800 kg (34,800 lb).

Delta IV Heavy Upgrade 53 t: stage 0+1st stage, 5 CBC (RS-68 engines) + 2nd stage AUS-60 upper stage powered by 2 MB-60 or RL-60 ), aluminum-lithium lightweight alloy stages. LEO Payload: 48,000 kg (105,000 lb), EE Payload: 18,000 kg (39,000 lb).

Delta IV Heavy Upgrade 67 t: stage 0+1st stage, 7 CBC (RS-68 engines) + 2nd stage AUS-60 upper stage (powered by 2 MB-60 or RL-60 ). LEO Payload: 61,000 kg (134,000 lb), EE Payload: 22,500 kg (49,600 lb).

Delta IV Heavy Upgrade 70 t: stage 0+1st stage, 7 CBC (RS-68 engines) + 2nd stage AUS-60 upper stage (powered by 3 MB-60 or RL-60). LEO Payload: 63,000 kg (138,000 lb), EE Payload: 24,000 kg (52,000 lb).

Delta IV Heavy Upgrade 76 t: stage 0+1st stage, 7 CBC (RS-68 engines) + 2nd stage AUS-60 upper stage (powered by 3 MB-60 or RL-60), aluminum-lithium lightweight alloy stages. LEO Payload: 69,000 kg (152,000 lb), EE Payload: 26,000 kg (57,000 lb).

Delta IV Heavy Upgrade 87 t: stage 0+1st stage, 7 CBC (RS-800K engines) + 2nd stage AUS-60 upper stage (powered by 3 (or maybe 4) MB-60 or RL-60). LEO Payload: 79,000 kg (174,000 lb), EE Payload: 30,000 kg (66,000 lb).

Delta IV Heavy Upgrade 94 t: stage 0+1st stage, 7 CBC (RS-800K engines) + 2nd stage AUS-60 upper stage (powered by 4 MB-60 or RL-60), aluminum-lithium lightweight alloy stages. LEO Payload: 85,000 kg (187,000 lb), EE Payload: 32,000 kg (70,000 lb).

All of these configurations use different combinations, as we see: a different number of common core boosters(CBC), different number of strap on solid rocket boosters, different engines at the first stages (RS-68, RS-68B, RS-68 Regen, RS-800, RS-800K), and also different upper stages.

In some of these versions, lightweight aluminum-lithium alloy is used at all rocket stages in place of the existing aluminum (used in the current Delta IV versions and Delta Heavy). In this list are two versions, 70 t and 76 t, both with the same configuration of all stages, but the first uses the existing aluminum at the rocket stages and the second uses aluminum-lithium lightweight alloy. Because of this, the second version of these two can put 6 t more payload at 200 km, 407 km orbits and 2 t more at Earth escape.

The cross-feed system is planned to be used by Falcon Heavy. But even ULA projects have included this system as a possible option. Here in these versions only one configuration uses the cross-feed system, the 48 t version. I am not sure how much the cross-feed system will affect its configuration to make a comparison. I guess that since these configurations correspond with the PearsonArtPhoto image, it could have 6 solid boosters and a 2nd stage powered by 1 MB-60 or RL-60. If it has 4 solid booster as in the link, then it will have the same configuration as 43 t version. So it means that the cross-feed system gives 5 t more payload at 200 km orbit, 4 t more at 407 km orbit and 1.3 t more at earth escape. As I said before these configurations correspond with the PearsonArtPhoto image, but astronautix.com has more accurate payload values in 200 km, 407 km orbits and Earth escape. Some of these Delta Heavy versions could lift to space much more payload than Falcon Heavy.

Note: Configurations using RS-68 Regen engine, have this upgraded engines with regeneratively-cooled nozzles to the core vehicles and cryogenic propellant densification.

Delta IV Upgrades

This image shows some other possible configurations. In this image, Delta IV Heavy Upgrade versions are similar or maybe the same with those described above. Delta IV Super Heavy Derived versions are different configurations where one common core booster (should be larger in diameter than the current ones in use) are 2, 3 or 4 engines (RS-68s family, probably RS-68A). In the 1st versions where each CBC has 2 RS-68s the payload in 407 km orbit at 28.5 degrees is around 65 t. In the 2nd version where each CBC has 3 RS-68s the payload looks more than 105 t, and the 3rd one with 4 RS-68s looks to have a payload of more than 145 t. ULA has projected many different configurations for Delta IV Heavy upgrades, but the chances are that none of them will be launched.

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There are a number of differences between the two systems. One of them is the cross-feed system of the Falcon Heavy. The cross-feed is only expected to be used on the Falcon Heavy with the heaviest loads, over 45 tons. The Delta IV has a LEO capability of about 28 tons. Falcon Heavy's max capacity is 54 tons.

Wikipedia states that a Delta Heavy with cross-feed, extra cores, and solid rocket boosters could lift 100 tons to LEO. There is also a paper about a number of proposed Delta IV Heavy configurations, from which the following comes:

enter image description here

The general thought is to fist increase the number of cores, then attach some solid boosters. The solid boosters don't seem to add much performance wise.

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    $\begingroup$ Your Wikipedia citation states that modifications including up to 6 strap-on CBCs could increase the payload to 100 tons (as your chart indicates). Cross-feed alone wouldn't get you anywhere near that. $\endgroup$
    – Russell Borogove
    Jan 31, 2016 at 23:17
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    $\begingroup$ Good catch. I suppose the link is basically to the picture I included, which shows all of the proposed variations. $\endgroup$
    – PearsonArtPhoto ♦
    Jan 31, 2016 at 23:19
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    $\begingroup$ Sure it is only paper, that is some wild and crazy stuff! Thanks. $\endgroup$
    – geoffc
    Feb 1, 2016 at 5:26
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    $\begingroup$ Those are some endearingly Kerbal designs. $\endgroup$
    – Vikki
    Jun 13, 2018 at 19:59

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