In 2014 the Orion spacecraft was launched on top of the Delta IV Heavy for the first time. Why do they build another rocket, the SLS, instead of launching the Orion on the Delta also with humans on board? It surely would be fewer costs to rebuild the launch pad to fit a Delta Heavy with the Orion instead of building the costly new rocket. What's the reason that they don't use the Delta Heavy for crewed flights to the Moon and beyond?

The following link provides answers why another rocket, the Falcon Heavy, is not used: Falcon Heavy question on SE

I think the proposal automatically came up since I didn't edit my question since then (I edited it before).

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    $\begingroup$ Does this answer your question? Is the SLS irrelevant if the Falcon Heavy launch is successful this year? $\endgroup$ Apr 26, 2020 at 11:15
  • $\begingroup$ The link answers partially indeed: why the Falcon Heavy is not used, but not the Delta Heavy. $\endgroup$
    – user35272
    Apr 26, 2020 at 12:04
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    $\begingroup$ Because Boeing would not get billions of taxpayer dollars to develop the SLS in that case, much of which will have been spent in Senator Shelby's state. $\endgroup$ Apr 26, 2020 at 12:10
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    $\begingroup$ @OrganicMarble Why Shelby and why Alabama? Is the SLS being built there? $\endgroup$
    – user35272
    Apr 26, 2020 at 13:02
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    $\begingroup$ Yes, SLS is being built in Alabama. Shelby is the senior Alabama state senator and chairman of the Senate Appropriations Committee, and has become well known for shoveling funding into the SLS project despite its overruns and delays while underfunding possible threats such as Commercial Cargo and Commercial Crew, and outright blocking alternatives such as depots (specifically threatening to cancel the entire Space Technology program if they didn't drop depots). $\endgroup$ Apr 26, 2020 at 14:29

2 Answers 2


What's the reason that they don't use the Delta Heavy for crewed flights to the Moon and beyond?

The Delta IV Heavy isn't powerful enough to send Orion beyond low Earth orbit. Orion masses about 26.5 tons in its lunar-mission configuration, which is just within Delta Heavy's LEO launch capability.

SLS is much more powerful, and can launch both Orion and a "transfer stage" capable of sending Orion to the moon.

During the Apollo program, two different launchers were used with the Apollo spacecraft. The Saturn IB was used to put the Apollo spacecraft into LEO for shakedown flights not headed for the moon; the much larger Saturn V was used for the lunar missions. Delta IV Heavy and SLS fill somewhat analogous roles for the Orion spacecraft.

  • $\begingroup$ The Apollo program used both the Saturn I and IB for LEO flights (except Apollo 9) but it was evident that the Saturn V was needed to reach the Moon. The other two Saturn rockets where 2-stage-rockets and they didn't have the SV's F1 engines as yet, right? The Delta Heavy on the other hand looks like perfectly usable for interplanetary flight, being similar to the Falcon Heavy and the upper stage seems like it might be rebuilt to host a lunar lander. Didn't they launch interplanetary probes with the Delta Heavy ever? $\endgroup$
    – user35272
    Apr 26, 2020 at 16:20
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    $\begingroup$ They launched the Parker Solar Probe on a Delta IV Heavy. But the Orion is much more massive of course. $\endgroup$
    – user35272
    Apr 26, 2020 at 16:23
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    $\begingroup$ Uncrewed probes and satellites are generally much smaller and lighter than crewed spacecraft. D4H can send small payloads beyond Earth orbit, certainly, but its payload to geosynchronous transfer orbit -- still within Earth orbit and not to lunar distance -- is already down to 14 tons. $\endgroup$ Apr 26, 2020 at 16:28
  • $\begingroup$ @user30007 Mass is everything in space launch. For each rocket and each destination there is a limit to the mass of the vehicle it can launch there. For a trajctory to the moon it's something like 10 tons for Delta IV Heavy, 20 for Falcon Heavy and 26-45 tons for the various versions of SLS and about 44tons for Starship & Superheavy (without refueling in orbit). $\endgroup$ Apr 27, 2020 at 15:34

Additional to the mass problem Russell Borogove explained, there is a certification problem:

Putting "manned" or "crewed" in the specification book of Launcher results in having a lot more by number and more strict standards to fulfill. You have to redesign or at least exchange literally every screw used. So you end up in building a new Launcher looking like the old one.

This is one reason, ESA/Arianespace stopped thinking about a manned Ariane 5 very early. Also, this was somehow a big problem for the Space Shuttle Program: Every object started with the Space Shuttle hat to fulfill standards for human space flights making them much more costly. So for the "custumers" it was also cheaper not to fly with a space shuttle.

In the end, it is as so often simply cheaper not to use the Delta IV...

  • $\begingroup$ The Russians launch their R-7 Soyuz rockets several times a year. No clear component of them is reusable but Russia doesn't bother. $\endgroup$
    – user35272
    Apr 28, 2020 at 10:41
  • $\begingroup$ Who are "costumers"? $\endgroup$ Apr 28, 2020 at 11:22
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    $\begingroup$ Organic Marble: Sry, for the Typo, "Customer" of course. @user30007: fir crewed and uncrewed Missions, different Soyus subversion are used. Was maybe easier in the soviet union but I am sure it was rechecked for newer Standards later $\endgroup$
    – CallMeTom
    Apr 28, 2020 at 11:36
  • $\begingroup$ @CallMeTom The Soyuz stages or spacecraft aren't reusable. Soviets built their own Space Shuttle Buran but it was cancelled when the Soviet Union got dissolved. Ironically due to costs. $\endgroup$
    – user35272
    Apr 28, 2020 at 13:01
  • $\begingroup$ Sry, i do not get the point: why are we talking about reusebility? $\endgroup$
    – CallMeTom
    Apr 28, 2020 at 13:14

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