Hard To Believe Some Engineers Came Up With This

[MIflyer]

Back in 1986 I was part of an Independent Readiness Review Team for the first Titan II Space Booster. These were Titan III ICBM's that had been decommissioned and were being converted into space boosters.

One of the limitations on space launches is triboelectric charging. If a rocket is passing through a cloud layer in which any part of the cloud is at the level in the atmosphere where the temperature is below the freezing point of water, it is possible for the ice particles impacting the vehicle to impart an electric charge. This can produce "St. Elmo's Fire" and resultant high voltage buildup can damage electronics in the booster and the spacecraft. At the time the cloud thickness where this was viewed as a hazard was 5000 ft.

One reason this is a problem is that the nose fairings of space boosters have to be insulated against heat and noise to prevent damage to the payload during ascent. The result is that the surface of the fairing will not conduct electricity, allowing charges to build up. For the Titan II space booster Martin Marietta coated the surface of the fairing with a very then conductive layer to enable accumulated charge to bleed off.

But that thin conductive layer was vulnerable to the ice particles in clouds. As a result the maximum cloud thickness under freezing conditions that was allowed was three thousand feet.

So you have a hazard if there is five thousand of cloud containing freezing layer but at three thousand feet your protective layer was damaged, so that became you actual limitation.

To me, this was like having seatbelts in a car that disarmed themselves at 15 mph. I asked if this was really true and the response back was "Yes."

As it turned out the dummies at the Cape later punched off an Atlas Centaur into a thunderstorm and a lighting strike reprogrammed the guidance system and sent it off in a unacceptable direction; fortunately it broke up. But the result was a large science project which led to changing the max allowed cloud thickness with freezing conditions to three thousand feet as well as making other restrictive changes. Never mind that they violated the allowed conditions, the Atlas loss led to the requirements getting more strict. E.G. "Are the surveillance helicopters reporting lightning in the area?" Answer: "We don't know. The helicopters were forced off station by lightning."

 

[buffnut453]

Hmmmm....I'd have looked at this differently.

If the max cloud depth that risks catastrophic damage to the equipment is 5,000ft and you have a protective surface that abrades after 3,000ft then, surely, including the protective surface now gives you an 8,000ft operational cloud depth through which the rocket can pass safely (3,000ft of initial protected flight plus the, now additional, 5,000ft of unprotected flight)?

Am I missing something? I fully accept that the way the policies were written may have been contradictory but the above makes (some) sense to me.

 

[MIflyer]

It does not abrade after 3000 ft. It abrades during 3000 ft. The freezing temp can be anywhere in the cloud layer, from the bottom to the last hundreds of feet before the top. So you can't count on stacking them. I suppose the idea was to protect just the fairing and thus the spacecraft from St Elmo's Fire damage, because the booster was still subject to the 5000 ft limitation even if the fairing was not a problem. For example, the command destruct receivers are still vulnerable via their antennas, and there is no way to get around that. But not bringing the fairing up to the same "robustness" as the rest of the vehicle is a bad approach.

 

[buffnut453]

It does not abrade after 3000 ft. It abrades during 3000 ft. The freezing temp can be anywhere in the cloud layer, from the bottom to the last hundreds of feet before the top. So you can't count on stacking them. I suppose the idea was to protect just the fairing and thus the spacecraft from St Elmo's Fire damage, because the booster was still subject to the 5000 ft limitation even if the fairing was not a problem. For example, the command destruct receivers are still vulnerable via their antennas, and there is no way to get around that. But not bringing the fairing up to the same "robustness" as the rest of the vehicle is a bad approach.

Fair enough. I mistakenly assumed that there was some degree of abrasion to the surface beyond which the application ceased to provide protection...and that it took 3,000ft to reach that degree of abrasion. Mea culpa.