Art is Art and Water is Water

March 10, 2019

Enterprising Spacecraft

Filed under: Twitter Threads — foone @ 7:45 pm

Original Twitter thread March 9, 2019

So mentioned the SpaceX Dragon 2 capsule being docked on the “front” of the International Space Station and that made me think about something I’ve always vaguely wondered and never really put into words:

Is there a “front” to the ISS, with regards to how it orbits?

And it turns out the answer is: Yes! Other than occasional rotations to help with docking (or re-boosting), it maintains a consistent orientation relative to the direction it orbits in. So in the above picture, it’s always orbiting with the part on the top of the picture as forward. It maintains this orientation in 3 ways (aside from simple rotational inertia: it’s rotating, so it’ll rotate at that space without external input):

1. Thrusters on Zvezda. This module has 28 small attitude control thrusters (and one main engine)

2. Control moment gyroscopes. These are devices similar to reaction wheels which spin up a rotor and then tilt it with a motorized gimbal, creating torque on the station to rotate it, using only electrical power (and no propellant)

3. Attached Spacecraft! Along with reboosting the ISS back up, spacecraft docked to it can be used to rotate the station to a different orientation, when needed.

Image result for gif of craft docked to iss

Keeping one part of the space station facing forward at all times makes sense for a couple reasons, like making it easier to communicate with earth, and simplifying micrometeorite shielding: it’s far more likely to get hit in the “front” than the “back.”

For the record, since the ISS is divided into US and Russian sections, it means one of them is in front. It probably won’t surprise anyone to learn it’s the US. (But that does mean the US modules are more likely to get punctured by impacts!)

Random tangent I thought of while looking at that top picture: You see these big white things on the ISS? Those aren’t solar panels! The solar panels are the pivoting (to track the sun) black panels. So what are they?

Those are the “External Active Thermal Control System“, or EATCS. They’re actually radiators! Ammonia is pumped through the space station and along the radiators, where it’s cooled back down and recycled along the heat pipes. They’re needed because despite space being “cold,” you can’t actually cool down that fast in it, because it’s also a great thermal insulator. It’s like being inside a giant pile of blankets in the middle of the antarctic. And all the people and machinery on the ISS do generate heat, so it has to be radiated away or the station will just get hotter and hotter. And keep in mind it’s in direct high-noon no-atmosphere-to-block-it sunlight half the time, too!

So heat is a problem in space. With the EATCS it can remove up to 70 kW of heat from the station. It’s an upgrade from an earlier system that could only do 14 kW, installed back in 2001. There’s a neat design thing here you might not notice: See how they’re all in the same plane? It’s a flat surface, rather than looking like a computer heatsink, with lots of little parallel pieces of metal.

This is because they’re radiators, in the strict sense: they remove heat through thermal radiation, and not convection or conduction. So you get lower and lower efficiencies the more each surface can “see” each other. Basically the idea is that each surface is going to emit thermal radiation in a random direction from its surface, and if they are placed in such a way there’s a direct path from one surface to another, some portion of that thermal radiation will end up hitting the other radiator. So you have more surface for your radiators, but at the cost of lowering the efficiency. and in an environment like the space station where every kilogram you launch costs something like $50,000? You want maximum efficiency!

The awesome Atomic Rockets page by has a great section on Heat Radiators as part of the Basic Spacecraft Design page:

Warning: Don’t click that link if you have anything else you were planning to do today. That site is (amusingly for a site about space) a black hole of time. Way too much interesting content on there.

He points out this is how the Shuttle bay doors worked: They had integrated freon cooling loops, so while it was in orbit it’d keep the doors fully open to better radiate away heat.

BTW, if you’re not familiar with why it has to be radiative cooling, there’s four basic mechanisms for heat transfer:

  1. Advection, which involves heat moving within a fluid. The space station isn’t a liquid unless something very bad happens, so that doesn’t help.
  2. Conduction (diffusion). This requires contact. Since the space station is in orbit, it’s not touching anything, so it can’t transfer any heat to it using conduction. So this doesn’t help.
  3. Convection. This transfers heat between an object and its environment, using the motion of fluids/gasses. Since there’s pretty much no atmosphere as high up as the ISS is, this doesn’t help either.
  4. So we’re left with only: Radiation. (Not the uranium sort) This is where a surface emits electromagnetic waves, lowering the thermal energy left in the surface. So it works in space! Unfortunately it’s the least efficient of the four, but that’s the price you pay for being in space.

BTW, keep in mind that the ISS needs radiators this big for a space station that maxes out at only 120 killowatts of solar power, with a usual crew of 3-6 people, maxing out at 13 during a handover.

So your cool spaceship in your favorite sci-fi universe? it should have HUGE FUCKING RADIATORS!

And the bigger your cool reactor is at the heart of it, whether it’s Fission, Fusion, Antimatter, or something even more exotic … the bigger your radiators. Otherwise the question is always going to be: where’s all that heat going?

It has to go somewhere. Thermodynamics doesn’t play around.

It’s something you have to worry about even without the reactor, to be honest. Let’s take everyone’s favorite fictional starship, the USS Enterprise D. It supposedly has a complement of about a thousand people.

Which is actually quite low but let’s pretend. So ignoring that some of them are gonna be non-humans, a human expends about 8 million joules of heat a day. Given there are 86,400 seconds in a day and a watt is a joule a second, that means each person puts out about 93 watts. So the Enterprise D would have to dissipate at least 93 kilowatts EVEN WITH THE REACTOR TURNED OFF, just from all the people on board!

Recall that the ISS has nearly half a square kilometer of radiators just to keep our dinky little 6-people-and-no-reactor space station cooled. Sure, by 2361 they’ve probably built more efficient radiators so they might be smaller, but it still needs to dump heat somehow.

BTW, if you’re writing a sci-fi story and want to easily fix this problem in your universe without resorting to sticking giant fragile “wings” on all your space ships? One alternative is to have exhaustible heat sinks in your ships. The idea is that you have some material you can dump a lot of heat into without it melting (or with it melting, optionally!). Tungsten for example, or if you just want to go the cheap route, plain ol’ rock from asteroids. You take that heat sink and cool it way down, using radiators or convective/conducting cooling at a planet or asteroid. Now as you fly around, you’re slowly warming it up. It’s like a battery, but for heat. So eventually you have to either stop off at a planet/asteroid to cool it back down, or deploy the radiators. But it’d be real handy for things like combat, where otherwise your radiators make big tempting targets to shoot holes in.

In any case, to get off this tangent of a tangent of a tangent and back onto an earlier tangent I didn’t expand on: 1000 people on the Enterprise D is silly, and you know why? Here’s the other Enterprise. No, not the Kirk one, the first nuclear powered aircraft carrier.

It’s about half the length of the Enterprise D, right? 342 meters vs 642 meters. But in terms of volume, dear god, it’s so much tinier! It’s basically a long stick, the Ent-D is a big fat thing. It has lots of internal volume. The aircraft carrier has a volume (well, displacement, but it’s close enough) of 84,626 metric tons. That’s pretty fucking huge! You see these 4 little specs on it? Those are grown men. This thing isn’t a boat, it’s a small town that can move.

The Enterprise D has a volume of 4.5 MILLION metric tons. It’s not a spacecraft, it’s a CITY that can fly through space.

So, given that the Ent-D has a thousand people on it, and it’s got 4.5 million metric tons to the 86,000 of the aircraft carrier, you’d expect the crew of the aircraft carrier to be what, 100 people? And even that’s stretching it. By the math, if it scales linearly, the carrier would only have 18 people on it. Does that number seem at all right to you?

Wanna know what the actual crew of the USS Enterprise aircraft carrier was? 4,600.

By that scaling, the Enterprise D could have the same population as Boise, Idaho.

And that’s silly, of course. The USS Enterprise aircraft carrier was a military vessel so it had lots of cramped quarters and lots of excess crew to be backups and second backups and third backups and fourth backups. Because otherwise you could get hit with a torpedo and it kills all 3 of the guys who know how to fix a coolant leak in your nuclear reactor, and guess what else the torpedo broke? All 4,597 of you left better evacuate fast before you glow in the dark.

And the Enterprise-D is not a military vessel. I mean, the people running it have military ranks and it’s very heavily armed. But it’s not a military vessel, still. There’s civilians, it’s not packed that densely, it’s not as heavily redundant as a warship. But the thing has (according to the technical manual) at least 250 Photon Torpedoes and they have a minimum yield of at least 64 megatons so it’s a “non-warship” that could level every city over 150,000 people in the US without having to reload.

I’m just saying, that sounds pretty fucking Warship to me.

In any case, I’m always amused by fan discussions about the “1000 people on the enterprise” number. They always seem to think “WOW THAT’S A LOT OF PEOPLE!” When really it’s “how the hell do they run that ship with only 1000 people who aren’t all active staff?”

Cause for the Ent-D that 1000 people includes lots of scientists (who aren’t gonna be able to re-route warp plasma around the hole the Klingons just shot in Deck 13) and families. You’ve got kids running around, and kids means you got teachers & babysitters and pediatricians and now you’re pretty much a small city in space to be able to handle all that and that’s okay, the spaceship is giant and can fit them all, but still: 1000 people!?

Automation will help with that, yes (the aircraft carriers the US is replacing the Enterprise with will have fewer people on them, but only by about a hundred), but you still need people there to fix the automation when it breaks. And if your automation is so good and complete that you don’t need all the people on hand to fix it, why are you even sending humans into space anyway? Leave them on the ground and let the robots explore. They’re far more disposable. (and I don’t mean Data. We might cry if Data dies, but no one is shedding a tear when the damage control system for Deck 5 is blasted to bits by a Romulan Warbird).

Anyway. Tangents, won’t you? I’m gonna stop here before I rant about star trek forever.

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