Thread

NASA asked several former SR-71A staffers to help secret restart

327 points · 403 comments · ilamont

0xbadcafebee · 1790728000 · focus · HN ↗
To give you an idea how ridiculous this is: to start each engine, they made a cart out of two Buick V8 engines tied together - so four V8 engines to start the plane. Those V8 engines were long since out of production, so at one point they went around the country, to every junkyard, buying up every one of those V8s they could find, so they would have spares to start planes up in different places. If an SR71 had an issue and landed at an unusual airport, they had to fly the start cart and other equipment to that airport to start the plane again. Everything about this plane is expensive and one-of-a-kind. It's a hot rod that goes to space.
1. AnimalMuppet · 1790729766 · focus · HN ↗
  Rather than re-engineer the cart to use a different block? I mean, I guess when a part reaches end-of-life, your first move is to buy as many as you can, before you try to re-engineer things so it isn't needed, but... horsepower is horsepower. Torque is torque. It shouldn't be that hard to replace...
  1. jauntywundrkind · 1790734548 · focus · HN ↗
    They did eventually replace the Buicks.
    > The original design was conceived by two Lockheed Skunk Works engineers who had prior race car engine knowledge. In the mid-1970's, the engines of the start carts was changed to Chevrolet LS-7 454's. The Chevrolet big block engines developed 465 horsepower each. Even so, for many years they were still referred to as the Buick's.
    <a href="https://www.thesr71blackbird.com/Aircraft/Engines/starting-the-sr-71-blackbirds-j58-engines-ag330-start-cart" rel="nofollow">https://www.thesr71blackbird.com/Aircraft/Engines/starting-t...
    And eventually the start carts themselves got replaced.
    > In the '80's the AG-330 Start Carts were mothballed and in there place came a Pneumatic Air starting system for the SR-71's. It is true that the start carts were hard to maneuver around. Logistically, a recovery of a Blackbird at another base could be done easier with Pneumatic Air.
    Itanu · 1790750446 · focus · HN ↗
    So they LS swapped the SR71 lmao
    m4rtink · 1790761297 · focus · HN ↗
    Yeah, big car engines is actually not that impressive, rather surprisingly primitive. Modern jets have external APUs for starting, usually via compressed air, that have compact yet powerful gas turbines inside.
    Sure, less impressive than a contraption with a bunch of car engines but much more powerful.
  2. mrheosuper · 1790742254 · focus · HN ↗
    Can the plane just carry....the starter ?, it's another 200-300kg of payload, may affect the max range, etc, but much more convenient.
    joha4270 · 1790753076 · focus · HN ↗
    Most jet planes do carry a starter with them (in the tail usually) so the fact the SR-71 doesn't suggests the tradeoffs wasn't worth it
    m4rtink · 1790761407 · focus · HN ↗
    Military fighter jets quite often had external starters to save weight.
    HelloNurse · 1790762075 · focus · HN ↗
    The weight corresponds to a significant amount of fuel or missiles etc., or significantly better radars or other systems.
    ssl-3 · 1790761731 · focus · HN ↗
    It could have, sure. That would have been easy. But it would have added weight and complexity. That would have reduced speeds and/or altitude and/or range between in-flight refuelings.
    We fixed it the other easy way: In the event that an SR-71 makes an unscheduled stop somewhere that wasn't prepared for it, then we send out a bunch of dudes with a cargo plane and some starter carts. That just takes money, which even easier.
    It also takes time, but time was not a big deal: The plane was going to be there for awhile anyway. It could go over 15,000 miles between takeoff and landing; it wouldn't have landed where it did if it weren't broken somehow to begin with.
    It'd need repaired by the crews who actually knew how to work on it -- who would also need flown in.
    It would also need fuel before it could leave. That's problematic since the JP-7 it required was shared by ~nothing else in the sky, so: More deliveries.
    Waiting for some Buick V8 starter carts to be delivered from the other side of the world wasn't really a roadblock at all.
    scrumper · 1790778910 · focus · HN ↗
    There's a bit in the Skunkworks book that talks about added weight costing significant altitude. I don't have my copy in front of me but it's quite a severe tradeoff: some fairly small amount (100kg perhaps, might even have been less) reduces ceiling by 1,000'.
    pwg · 1790778976 · focus · HN ↗
    For military planes, every bit of weight you can shed up front increases some operational capability of the plane. In this case, not carrying an extra 200-300kg of weight for starters meant 200-300kg more spy equipment /could/ be carried along on the mission.
    0xbadcafebee · 1790787389 · focus · HN ↗
    They would have to make the plane bigger. The SR71 is already an enlarged version of the A-12, made bigger to support more fuel, a heavy camera, and an extra pilot. It needed something like 900lb-ft torque per engine; even modern electric motors would add enough weight that it would be problematic.
2. dboreham · 1790731592 · focus · HN ↗
  Granted there are likely many obstacles to getting this airframe flying again, but the V8s probably isn't one of them. All those V8s did (and I've seen them in action) was turn a shaft. There's an infinite number of alternate options available to turn the same shaft.
  1. topspin · 1790758480 · focus · HN ↗
    > but the V8s probably isn't one of them
    You can credit card as many ready-to-run 9L+ V8 engines as you want[1], built to modern tolerances using high quality tooling, tested on a dyno and equipped with a warranty. Maybe different mounts and/or couplers are needed, but that's a very finite problem.
    I suspect one actually difficult problem would be finding a refiner that can make the special fuel used in the unique engines of the SR-71. Then again, the US has a great many agile refiners at its disposal.
    The SR-71 is a rather awkward thing to operate. Fuel leaks out of the tanks until atmospheric friction heats the aircraft enough to make them seal. It's a big mess, and the cohort of people that were actually capable of making these things work is aged out of the workforce. Assuming any of this real, NASA must have something truly extraordinary in mind to consider this.
    1. <a href="https://blueprintengines.com/" rel="nofollow">https://blueprintengines.com/
    dboreham · 1790776975 · focus · HN ↗
    Aware. I've seen one being fueled, started, and flying.
    topspin · 1790814547 · focus · HN ↗
    The best theory I've heard (Alex Hollings) is that NASA is doing an exterior refurb to get the plane ready for a big reveal of some other exotic aircraft: putting it on stage with the new thing for glamor shots.
  2. 0xbadcafebee · 1790788346 · focus · HN ↗
    It's an example of how weird and custom everything about them is. Not just the plane but the tools too, and the knowledge of how to work on them. It's like reinventing the plane again.

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