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As a follow up to this answer which identifies the two pointed objects flanking the Titan Transtage's engine in an unusual configuration as its propellant tanks:

enter image description here

How are these protected from the heat of the engine?

Pressure vessels containing rocket fuel typically do not play nicely with heat.

As the exhaust is obviously propelled away from the vehicle (although it will rapidly expand in vacuum conditions as pointed out below), I would imagine that there would be little to no hot exhaust gas impinging directly on the tanks. So I expect that the primary heat transfer to the tanks would be radiative from the engine bell.

Is this heat transfer low enough to prevent the damage, warping and potential failure of the structure of the tanks? Or were the tanks designed with additional shielding?

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    $\begingroup$ For rocket engines running in vacuum, the exhaust will expand in all directions as it leaves the nozzle, so there will be some impingement -- but for upper stage engines, the exhaust leaving the nozzle can be surprisingly cool, so I think you're right that the main issue is radiative. $\endgroup$ – Russell Borogove Jun 28 '18 at 23:24
  • $\begingroup$ Thanks @RussellBorogove, good point. I’ve added to the question $\endgroup$ – Jack Jun 28 '18 at 23:44
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    $\begingroup$ What sort of answer are you looking for? Transtage's configuration is unusual enough that someone must have brought up the heating issue during its development, so they must have done the math and determined that radiative heating would be low enough to avoid failure of the tanks. (There might be an interesting answer here if the heating was actually relied on to maintain pressure in the tanks as fuel was consumed, but that's wild speculation on my part.) $\endgroup$ – Russell Borogove Jun 29 '18 at 0:26
  • $\begingroup$ @RussellBorogove Added a little more to the question. Some document discussing the considerations would be ideal, but any information or calculations would certainly be of interest! $\endgroup$ – Jack Jun 29 '18 at 0:41
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Very little of the heat radiated from the nozzles would be directed toward the shorter oxidizer tank, but I'd guess maybe 10% of the radiant heat would be intercepted by the fuel tank.

The NASA paper "Characterizing GEO Titan IIIC Transtage Fragmentations using Ground-based and Telescopic Measurements" discusses efforts to analyze the materials used on the Transtage in order to characterize space debris from the stage's occasional breakups in orbit.

It has a little bit of information on the tanks:

The asymmetric small-diameter fuel and large-diameter oxidizer tanks were exposed to the engine plume and space environment and used painted and foil insulation for thermal control [during?] the long-duration missions flown.

A diagram in the paper shows both the fuel and oxidizer tanks using "aluminum silicone paint beneath steel net" and "gold plate SS foil beneath steel net".

The analysis was done on a test article, some components of which were not faithful to the stage as flown, but:

...general propulsion module structures, select electronics, the aft close-out/heat shield panel, the main engines and engine functional equipment, including engine bells, and the fuel tank are considered authentic original equipment. For example, the engines display considerable wear from firing, and the fuel tank is correct and displays the accurate types of insulation for this vehicle.

It's not clear to me from the article whether the "gold plate SS foil" and "aluminum silicone paint" were used on the same parts of the fuel tank, or on separate areas.

At any rate, the tanks had some sort of insulating coating, and the sheer mass of propellant in the tanks would also absorb a great deal of heat.

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  • $\begingroup$ Very interesting find! So it would appear that special design considerations were made for tank heating $\endgroup$ – Jack Jun 29 '18 at 8:50

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