Avoiding the blaze of glory

A reader writes:

How come NASA spacecraft need all that heat shielding, but SpaceShipOne and Two don't? Does this have something to do with escape velocity - they don't go that fast, so they fall back down when the engines stop and don't have to re-enter. But they do get outside the atmosphere, right? Is there more than one kind of re-entry?


Space Shuttle re-entry trail

There's no clear line where "the atmosphere" stops. By convention, the Kármán line at an altitude of 100 kilometres is treated as the end of the atmosphere; SpaceShipOne made it to 112 kilometres, and SpaceShipTwo is intended to do the same, but with more people on board. But satellites in low orbit well above the hundred-kilometre line need periodic re-boosting to compensate for the drag of the tenuous outer reaches of the atmosphere. Take the International Space Station, for instance; it orbits from 330 to 410 kilometres up, but still needs periodic re-boosting to prevent its orbit decaying. This goes for anything else delivered or serviced by the Space Shuttle, too; inability to reach high orbit was one of the Shuttle's numerous shortcomings.

(The Shuttle carried some satellites that ended up in high orbits, and even space probes that left earth behind entirely, but those payloads needed their own booster rockets for the second part of the trip.)

(Oh, and orbital decay also shows up in umpteen Star Trek episodes as another Acme Mechanically-Assisted Plot-Tensioner, even when the Enterprise seems to be orbiting way above the conceivable atmosphere of any earth-like planet. Presumably they deliberately keep themselves in a super-slow pseudo-orbit by use of engine power, because... tech tech tech.)

You could say that "re-entry" means any trip from orbital altitude back into the atmosphere, but what most people mean when they use the term is a trip from an actual orbit back into the atmosphere. That's where the big difference lies, because orbital velocity is high.

Low-orbital velocity is particularly high, because the closer an orbiting object is to the thing that it's orbiting, the stronger will be the gravitational pull on it, and the higher its orbital speed must be, for it to actually be in orbit and not just fall back down.

The earth, orbiting approximately 150 million kilometres from the (very large mass of the) sun, takes a year to go around it once, travelling at about thirty kilometres per second.

The moon orbits approximately 385,000 kilometres from the earth; if the earth had the mass of the sun then the moon's orbit would be extremely fast at that relatively small distance - Mercury, orbits the sun at an average distance of about 58 million kilometres, and travels at about 48 kilometres per second. But because the earth is much less massive than the sun, the moon takes 27.3 days to go around us once, travelling at only about one kilometre per second relative to us.

The International Space Station's low orbit takes it around the planet in only about ninety minutes; it therefore travels at about 7.7 kilometres per second, more than 22 times the speed of sound at sea level.

Re-entry is still a problem even if you're out in a very distant and slow orbit, though, because you can't just teleport from that distant orbit to the edge of the atmosphere. You have to use something - rockets, or some gravity-assist trick around some other body - to reduce your orbital velocity, putting you on a new orbit that intersects the planet's atmosphere, preferably in a survivable way.

That orbital adjustment reduces your speed relative to the planet, but then your new elliptical path means you fall toward the planet at greater and greater speed. Five hours before splashdown at the end of the Apollo 11 mission, the spacecraft was about 76,000 kilometres from the earth and approaching the planet at less than three kilometres per second. Five and a half hours later, as the spacecraft started to catch some real atmosphere and lose radio contact, they were still about 3,000 kilometres from their splashdown point (including a large diagonal component, since they weren't plunging straight down toward the planet), and were now moving at eleven kilometres per second.

(There's a lot more complexity to orbits and de-orbiting in the real world, of course, not least because many orbits are far from circular, with a slow portion further from the planet and a fast portion closer to it. Such orbits can be rather useful, and various advanced and less-advanced simulators exist to help you get a feel for them.)

So one way or another, a return to the earth from orbit or from a trip to some other part of the solar system involves very high speeds. Such high speeds, in fact, that friction with the air contributes little to the heating effect; it's air piling up in front of you and trying to get out of your way, and being heated by hypersonic compression, that creates the glowing plasma halo and glowing-hot heat shields on re-entering spacecraft.

You can avoid all of this if, like SpaceShipOne and Two and other "sub-orbital" vessels, you never get anywhere near orbital velocity, and just fly up until the sky is black and the earth is curved, then fall back down. When you start to fall there's little air resistance and almost as much gravity as at the surface of the earth (even the International Space Station is close enough to the earth that it's subject to gravity about nine-tenths as strong as at sea level), so you can get up to some moderately impressive speeds by aeroplane standards. But you're a long way from true re-entry speed.

For comparison, the fastest aircraft humans have ever managed to make that truly qualifies as an aircraft - takes off and lands under its own power, can be refuelled and re-used, has enough fuel to fly a reasonable distance, carries living humans and usually keeps them that way - is the Lockheed SR-71 spy plane. Most of the SR-71's technology remains impressive today and was nearly miraculous in 1964, but the thing was such a nuisance to operate (and was largely superseded by satellites and drones) that it's now been retired in favour of its 1950s predecessor, the glider-like U-2, maximum speed only impressive by World War II standards.

Flat out, with its skin hot enough to melt lead and five kilograms of fuel going into the engines per second, the SR-71 could manage about one kilometre per second.

That's nine times the speed of the fastest production car, three times the land speed record, and quite close to the muzzle velocity of the most outrageously fast rifle bullets. But any random piece of dead-satellite or rocket-casing space junk that fireballs its way to destruction in the atmosphere is pretty much certain to beat the SR-71 by a factor of at least ten. Space Shuttle re-entry was carefully controlled to get it under nine kilometres per second before it started really heating up, but you can see why it was such a big deal when Columbia had a hole the size of a saucer in one leading edge.

You can avoid all this, too, if you've got a lot more engine power to play with. Come up with a sci-fi drive that can deliver lots of thrust for long periods of time with little vehicle mass (in technical terms, both large thrust and very high specific impulse; the closest we've managed to come to these goals has been strangely unpopular...), and you can leave the atmosphere as slowly as you like, accelerate to orbital velocity as slowly as you like, and generally Superman your way around the solar system without having to endlessly account for every joule and newton lest you end up drifting to Neptune while your air runs out, or turn into an array of orange streaks across the sky.

This is where "escape velocity" comes in, too. Escape velocity (more correctly, in physics terms, escape speed, since direction is irrelevant) is how fast you need to be going, from wherever you currently are, to break free of the gravity of a given body. If you're at sea level on an earth with a magic spaceship that is not subject to air resistance, then 11.2 kilometres per second is the speed you need. If you shoot off in any direction (even, theoretically, through the planet, if your magic spaceship is also not subject to ground resistance...) at 11.2 kilometres per second, you're not going to come back down.

Escape velocity on the moon (where air resistance really isn't a problem) is only 2.4 kilometres per second, but Alan Shepard's golf balls definitely did come back down. They probably wouldn't have on Phobos or Deimos, though, because those tiny bodies' escape velocities are only 11.3 and 5.6 metres per second, respectively.

Escape velocity isn't of much direct relevance to Earth-launched spacecraft, though, because something shot out of an 11.2-kilometre-per-second cannon at sea level will definitely come back down after atmospheric drag eats most of that speed. The great problem of getting things up out of our atmosphere and gravity well when all you have to propel them are poxy chemical rockets is finding a way to strike a balance between having lots of rocket power, and using most of that power just to launch the fuel and engines that you need to launch the fuel and engines that you need to launch... You get the idea.

A particularly good simulator of this conundrum also exists!

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

14 Responses to “Avoiding the blaze of glory”

  1. OrbGuy Says:

    Orbiter (http://orbit.medphys.ucl.ac.uk/) is also a very nice simulator, so different from typical space sims.

    • dan Says:

      Thank you!

      I knew the things I linked to weren't optimal, but I didn't really know what to link to ahead of them.

      Anybody who has a favourite Space-War-ish game or simulation, the faster-accessible the better, is cordially invited to suggest it here!

  2. TwoHedWlf Says:

    Heh, the whole time reading this I was thinking, "I wonder if he'll mention KSP?" Right at the end, YES!:) Very fun little game, still a work in progress.

    • dan Says:

      KSP is sort of a mini-Minecraft, in the sense of "incredible things created by brilliant people in a game where I can pretty much manage to make a small hovel out of mud / ungainly collection of engines that explodes on the launch pad".

      The most incredible things are usually made out of third-party add-on components, but those only make new kinds of construction POSSIBLE, they don't make it EASY.

      Behold, a rocket-launch amphibious flying aircraft carrier and its little friend:

    • Gridlock Says:

      Well, there goes my weekend.

  3. wetnap Says:

    the shuttle could reach high altitudes, just at the limit of its fuel, for safety reasons they avoided doing it much:P

    why they couldn't add a fuel store module in the huge truck bed they had in the back i dunno, probably budget i guess.

    • dan Says:

      Well, if you decided to fly a Shuttle with no payload then I imagine it could get rather higher than the six hundred and something kilometres that was the best it ever achieved in practice, but it still wouldn't get anywhere near true "high orbit".

      (When they deployed the Hubble telescope and on mission STS-103 they made it to around 610 kilometres; that was the record.)

      The solid rocket boosters burned 100% of their fuel every time by definition, and I'm pretty sure they weren't throwing away external tanks with 50% of the fuel still in them. So perhaps a super-lightweight Shuttle carrying two dudes and a toothbrush could have made it as high as a thousand kilometres, barely conceivably perhaps two thousand, I'm hardly an expert. But geosynchronous orbit is 35,786 kilometres, and "high orbit" is defined as any orbit above geosynchronous.

      (Note that the original design goals for the Shuttle were only for a low-orbit delivery vehicle. And, famously, the real Shuttle failed to meet those goals.)

      • TwoHedWlf Says:

        Possibly strap another pair of boosters to the fuel tank would give a good increase in altitude if you're willing to use the kerbal method, any problem can be solved with more boosters.

        • dan Says:

          Possibly strap another pair of boosters to the fuel tank

          Or to the shuttle itself, and make an X-71 :-)!

          ("What? The centre of thrust isn't lined up with the centre of mass? No problem - just balance it out with the flaps on the wings!" - Michael Bay)

    • TwoHedWlf Says:

      Hmmm, Let's see, if I'm looking at the right specs the cargo bay was 18.4mX4.5m, Let's call that 18X4m of tank for 72 cubic meters of fuel. The main engines together burn about 3.9 cubic meters per second.

      So, putting a big fuel tank in the cargo bay would give in the region of 18 seconds worth of fuel.

      I think the extra fuel would be insignificant compared the decrease in weight by not filling the cargo bay with cargo.

  4. RichVR Says:

    No atmosphere in this one but a great deal on a fun sim.
    Lunar Flight. Update of the old Lunar Lander.

  5. Mighty Says:

    I believe that another issue for the shuttle above LEO was radiation shielding. The amount of shielding necessary to spend any time in the Van Allen belts would have been prohibitive. And besides GPS satellites at around 20,000 km there aren't many orbits often used between LEO and GEO.

  6. koolraap Says:

    I question your 11.2 km/sec figure. What if I was Halfway To The Centre of the Earth? Where, as everyone knows, gravity is stronger than 1 earth gravity. You're going to need a bigger spaceship.

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