[votingprawn]

The SR-71 took off with a small fuel load to reduce stress and improve engine-out performance, that is why it was refuelled immediately after take-off.

The mental image of it leaking so much fuel on the ground such that it needed immediate refuelling is a myth propagated extensively on the internet.

[jpitz]

The characterization is/was also propagated by people who actually refuelled them, so I'm somewhat unconvinced it is a myth.

[votingprawn]

I believe if you look for primary sources like pilots and ground handlers/mechanics you'll mostly find reference to small leaks (I think they were referred to as drips caught in cookie trays).

I had a quick search and found the KC135 chap who says the refuelling was needed due to leaks, but without being rude to him I'm not sure he's really a qualified source for that information. It sounds more to me like 2+2=5.

I'm procrastinating so lets do some napkin maths, the claim is

1) a significant amount of fuel is leaking out of the expansion gaps,

2) climbing up to 25,000 ft at 300 knots would heat the airframe enough to seal those gaps,

3) there would still be sufficient expansion room to allow for travelling at M3.2

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Ignoring that no engineer would be happy with 1.

For 2...Total Air Temp = Static Air Temp + Ram Rise. At 25,000 ft the static air temperature is about -35 C. Ram rise for a true airspeed of 300 kts:

RamRise = V^2 / 87^2 = 300^2 / 87^2 = 12 degrees.

So skin temperature at typical refuelling altitude would be -23 C

Titanium has an expansion coefficient of 9E-69 meters per meter-kelvin. So approximating rather grossly, assuming a s tarting temperature of 20 C over the 33 m length of the plane there would be a contraction of about 1 cm.

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And for 3:

The aircraft then accelerates up to 1900 knots. RamRise = 1900^2 / 87^2 = 470 degrees

Static air temp at over 60000 ft is roughly -55 C. So skin temperature would be 420 C. So assuming the same length and starting temp, the plane would expand by around 11 cm

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So to summarise: According to the claims, at ground level and temperature the expansion gaps were large enough to significantly leak fuel. After take-off the aircraft needs to be refuelled immediately. Assuming this is done so (i.e. take-off, climb to 20,000, refuel) then the skin temperature is lower than ground level, and the expansion gaps should have grown ever so slightly. The aircraft then climbs up to its M3.2 cruise point and everything expands significantly "as designed" and the gaps disappear.

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Perhaps the anecdote we'll see on the internet now is that the SR-71 had to take off and go supersonic to rapidly heat up the skin before briefly decelerating to refuel, but the refuel had to be done super fast to stop the skin cooling down too far...