Why is the Delta IV Heavy taller than the Delta IV Medium?

Question

Looking at below picture, the Delta IV Heavy is clearly much taller than the Medium. Is there any specific reason why?

score 33 · Accepted Answer · answered Feb 23 '23 at 19:20

33

The fairing is bigger (Taller) for larger payloads.

answered Feb 23 '23 at 19:20

geoffc

16

To elaborate a little, it's a more powerful rocket than the DIVM, capable of carrying larger payloads - so it needs a bigger fairing space to put them in. – Simon Geard Feb 24 '23 at 04:00
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...though there are exceptions, like the Parker Solar Probe which needed the Ⅳheavy not because of its payload capability but simply to get enough Δv. It actually looked a big hilarious, such a small probe sitting in that giant fairing. https://en.wikipedia.org/wiki/Parker_Solar_Probe#/media/File:Parker_fairing.jpg – leftaroundabout Feb 24 '23 at 17:42
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@leftaroundabout: next question in this series is then probably why the fairing wasn't smaller.... – PlasmaHH Feb 24 '23 at 21:36
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@PlasmaHH there's https://space.stackexchange.com/questions/26725/why-is-falcon-9s-fairing-so-big/26726#26726 about a case where the fairing was similarly oversized for the payload. – leftaroundabout Feb 24 '23 at 21:42
@PlasmaHH in the case of the PSP, I suspect that qualifying Delta IVH to use either the shorter 5m fairing, or the narrower DCSS and 4m fairing would have exceeded the potential cost savings from using them by a large amount. ULA produces highly reliable rockets, but they have a very expensive cost structure. – Dan Is Fiddling By Firelight Feb 26 '23 at 01:52

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