# Why would rocket bodies use specifically ablative coatings to reduce boil-off of cryogenic propellants? Is ablation important for this?

The end of YouTube video Delta IV Heavy Pad Tour, (with CEO Tory Bruno) - Smarter Every Day 199 points to Destin's second "Smarter" channel's video UP CLOSE Delta IV Heavy Launch Pad Tour (Tory Bruno CEO of ULA) - Smarter Every Day where an extensive discussion of the Delta IV Heavy is given by ULA Pres Tory Bruno.

After about 12:30in the second video, Bruno says (my best efforts at transcription):

(cork is) a very common rocket insulative material because not only is it a thermal insulator, it’s an abaltor which means that it erodes or burns off as it’s trying to heat up which helps to remove the heat before it can conduct in.

[…] And of course where we have our cryogenic propellants… I’ll let you look down here, and you’re going to look for areas that look really rough on the side of the rocket… so that’s an insulative material that we paint or spray on, called SOFI, and that’s a polymer insulator, and it’s very similar. It will be thicker, a couple times thicker than the cork, and that does the same kind of thing, it ablates-off… and that important on these big guys because cryogenic propellants - they’re boiling the whole time…

Question: Are the ablative properties of the insulation at all important to reduce the boiloff rate of the cryogenic propellants? Why would rocket bodies use specifically ablative coatings to reduce boil-off of cryogenic propellants?

cued at 12:31, but the whole video is worth watching!

• I would say ask the man himself. At the end of the second video Dustin said that Bruno is on Twitter and Reddit, and "you can ask him super technical questions and he will answer you on the internet." Ask him, and tell us what he says. (To me it sounded like he was talking generally-- it's very commonly used, and the ablation is a reason for that, not necessarily for this particular application. But don't take my word for it.) – Greg Apr 4 '19 at 20:24
• Well, I signed up to Twitter and asked Bruno. I'll let you know. – Greg Apr 5 '19 at 23:57
• I can see two reasons why someone would want ablation: energy that's "used" to vaporize isn't going to conduct into the tank and the additional separation of the hot air to the tank wall due to the ablation gases is somewhat like the Leidenfrost effect. – Christoph Apr 18 '19 at 14:11

The ablator is more to protect the structure itself than the propellant quality/boil-off. Ablator was used in certain areas of the shuttle External Tank for this reason.

This quote from "A Technical History of the External Tank" (not online) describes the initial design which didn't include the complete covering of the tank with the classic orange foam.

Thus, the Martin proposal called for small amounts of ablator on the tank where high heating was expected, ablator on the intertank where the shock waves from the Orbiter and Solid Rocket Boosters (SRB) would impinge, ablator on the struts and brackets, ablator on the LH2 aft dome, and foam on the LH2 sidewalls and forward dome.

Also from here

The External Tank uses ablators on areas that are subjected to extreme heat, such as the aft dome near the engine exhaust and on protuberances that are exposed to aerodynamic heating, such as the cable trays

The shuttle Solid Rocket Boosters also used significant amounts of ablator to protect the structure from aeroheating. Clearly boiloff / propellant quality was not a concern here.

(Source: Fundamentals of Launch Vehicle Ablative Thermal Protection System (TPS) Materials) <-- although this powerpoint presentation is typically terse, there is a lot of information in it about ablators on boosters (mostly SRBs).

I asked, and Tory actually answered April 6, but Twitter didn't send notices, I wasn't paying close attention, and I wasn't really sure how it worked. Anyway, I found it today, and I don't think he really answered the specific question, but this is what he said.

Tory Bruno Verified account @torybruno Apr 6