23
$\begingroup$

(Prompted by this answer):

Do the designers of large rockets* (have to) take the mass of the exterior paint into account?

If so, do we have examples of actual design changes/decisions based on this (and not on other paint properties like color/heat exchange)?

* Saturn 5, the larger Deltas, Falcons, ...

$\endgroup$
  • 9
    $\begingroup$ I remember looking through the documentation for a Cessna 152 (two seat flight trainer) and marvelling at the fact that, not only did they list the weight and center of gravity for the exterior paint, but they also broke it down into separate numbers for the white base coat and for the color stripe. $\endgroup$ – Solomon Slow Sep 19 at 16:32
  • 3
    $\begingroup$ @SolomonSlow Nothing too surprising about that. Aircraft engine manufacturers usually have a "weights department" with a team of specialist engineers whose sole job is to keep track of mass and CG position, literally down to the last nut, bolt, and washer. This often involves physically weighing parts - the "weight calculations" in CAD software are notoriously inaccurate. $\endgroup$ – alephzero Sep 20 at 12:34
  • $\begingroup$ Listing the center of gravity of the surface paint in the shape following the aircraft surface separately is genuinely nerdy. [Imagining the dried paint as separate object in weightlessness] $\endgroup$ – Volker Siegel Sep 22 at 0:21
  • 1
    $\begingroup$ And while we are at it: if there is more than one color used, the center of gravity changes when the viscosity of a paint is changed, nice... $\endgroup$ – Volker Siegel Sep 22 at 0:34
58
$\begingroup$

When the shuttle External Tank stopped being painted white, the weight savings was ~600 lbs (~270 kg).

This is not a tremendous amount from a vehicle standpoint, but the tank was carried almost to orbit, so weight shaved off it was a direct addition to payload capability, and that amount could be significant for payloads.

$\endgroup$
  • 11
    $\begingroup$ I never looked at it from that direction: weight saved = extra payload possible given the same design $\endgroup$ – Jan Doggen Sep 19 at 20:51
  • 1
    $\begingroup$ @JanDoggen don't know about other vehicles, but shuttle's propellant tanks were always filled to the same point, so on a given mission, a weight savings either meant more weight for payloads or performance margin. $\endgroup$ – Organic Marble Sep 19 at 20:53
  • 13
    $\begingroup$ Cool, so the crew could stand to gain 38 kg each and still be within mission parameters. $\endgroup$ – Ingolifs Sep 20 at 1:48
  • $\begingroup$ Why was it painted in the first place then? $\endgroup$ – Thorbjørn Ravn Andersen Sep 22 at 8:22
18
$\begingroup$

WD-40 was designed to protect the Atlas rocket from rust and corrosion. The skin of the Atlas was so thin, to save weight, that from the moment it was welded together it always had to be pressurized so it wouldn't collapse in on itself. It needed that protectant because it was unpainted, to save weight.

https://en.wikipedia.org/wiki/Atlas_(rocket_family)#SM-65_Atlas_missile

$\endgroup$
  • 3
    $\begingroup$ But why needed tanks made from stainless steel protection against rusting? Was it a low quality stainless steel not immune to rust? $\endgroup$ – Uwe Sep 19 at 20:14
  • 7
    $\begingroup$ Good question! I had never seen the alloy mentioned. Looking around... Greg Smith tells us that it's 301, which is stronger but less corrosion-resistant than 304. But I don't know what his authority is. Jeff Deem also mentions the corrosiveness of sea air. groups.io/g/cia-rocketry/topic/mercury_atlas_rocket/… $\endgroup$ – Greg Sep 19 at 20:41
  • 4
    $\begingroup$ My 1964 Spacecraft and Boosters book agrees that its' "a special cold-rolled austenitic steel (A1S1 grade 301) (p.219) $\endgroup$ – Organic Marble Sep 19 at 20:50
  • 4
    $\begingroup$ No common "stainless" steels can be in a marine environment (such as florida) without cover. For that you need specifically (chloride) corrosion resistant steels. more like 314 and 316 and even those can be argued are not sufficient, you'd need the moly-steels. In any case, paints / "coatings" today not only have weight, they also support the structure, in some cases quite substantially. $\endgroup$ – Stian Yttervik Sep 20 at 13:22
  • 3
    $\begingroup$ @Uwe stainless definitely can rust happens in rifle barrels all the time, high quality 416r $\endgroup$ – DatsunZ1 Sep 20 at 16:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.