Despite Their Ubiquity And Importance, We Know Surprisingly Little About How Clouds Form. The Broad Strokes

The Space Shuttle had a famous double sonic boom when passing overhead during re-entry. This schlieren flow visualization of a model shuttle at Mach 3 reveals the source of the sound: the fore and aft shock waves on the vehicle. The nose of the shuttle generates the strongest shock wave since it is the first part of the vehicle the flow interacts with. This initial shock wave turns the flow outward and around the shuttle. The second boom comes from the back of the shuttle and serves to turn the flow back in to fill the wake behind the shuttle. (The actual shock wave would look a little different than this one because there’s no sting holding the shuttle like there is with the model.) The other major shock wave comes from the shuttle’s wings, but, at least for this Mach number, the wing shock wave merges with the bow shock, making the two indistinguishable. (Image credit: G. Settles, source)

assuming a starship never has to travel through an atmosphere, and never has to worry about collisions with eg. asteroids or attackers' weapons (maybe some sort of forcefield, or a perfect interceptor weapon system), are there any compelling reasons for starships to be any particular shape? (let's also assume the starship can travel at relativistic speeds)

Aha! Excellent question! I love talking about spaceships.

So most of what I know comes off Atomic Rockets, which has far too much love of old whitedude science fiction novels, but also a lot of useful information, and more recently, lots of conversations with arborine, who has thought/knows a great deal about space habitats and is generally wonderful.

There are a few design considerations that constrain what shape your spaceship should have…

it needs to bear the stresses of acceleration without breaking

it needs to dissipate the heat generated by the crew, onboard machinery and direct sunlight into the highly insulating vacuum of space

it needs to be able to manouvre correctly and, for example, not start spinning uncontrollably when the engine does a burn

its rotation needs to be long-term stable in flight

it needs to be able to withstand impacts with interplanetary/interstellar debris, which although consisting only of rarified gas and small particles, may (depending on the mission) may be approaching at extremely high, even relativistic speeds

it needs to protect equipment and crew from space radiation (from stars or cosmic rays)

it needs to protect the crew from any radiation produced by the ship’s engines, or its power source (a big issue if you use something scary like nuclear power or antimatter annihilation!)

if humans are on board it needs to have pressurised compartments for them to live in, and enough space that they will be comfortable for the duration of the voyage

it needs to minimise the extent of the damage if something goes wrong, e.g. part of the ship is depressurised

it needs to survive for the entire length of the voyage, which probably means it needs to be self-sustaining

it needs to carry enough propellant reaction mass to give it the \(\Delta v\) for whatever it’s out there to do

it needs to be as light as possible, because every unit of mass of spaceship also adds a lot of units of reaction mass, depending on the mission \(\Delta v\)

if you want the crew to experience artificial ‘gravity’, it either needs to be capable of accelerating near \(g\) for nearly the entire mission, or have sections of the spaceship capable of rotation to create a centrifugal force near \(g\)

I’m sure there are a lot more of them.

So while there’s no reason for your ship to be streamlined, there are a bunch of constraints!

1. make it strong, and maybe put the engine at the front to save mass

The first one depends a lot on where you position your rockets. Most designs place the propulsion at the back of the ship, pushing the ship forwards.

If you put the propulsion system at the back of the ship, you need your ship to be sturdy against compressive strength, which adds to the mass of the mission - compressive strength usually requires massive beams and so on, and it’s hard to get around that. When you’re accelerating, it will feel as if gravity is pushing down on everything on board towards the back of the ship, and like a building, your ship has to be strong enough to stand up. Otherwise, your engine will punch straight through your ship, or crush it, or tear off, or something similarly bad.

A tiny number of designs, like the Valkyrie, and in fact some of the earliest rockets ever built, have the propulsion at the front of the ship, towing the rest of the ship behind it.

If you do that, your ship needs tensile strength to not snap against the propulsion. You generally don’t need as much mass to have tensile strength as compressive strength - but putting the engine at the front does mean your crew are directly in the path of the engine exhaust! (The radiation problem is a general one for nuclear and antimatter engines, but it’s particularly severe when you’re right in the engine beam…)

The Valkyrie gets around this by having a shadow shield between the engine and the crew compartment, which absorbs radiation from the engine and casts a shadow over the crew compartment.

Another option is to have your engines not point directly backwards, but have two or more engines angled slightly out to the sides in opposite directions - this is apparently what the ship in the film Avatar does (ship’s presumably a still from the film, annotations from Atomic Rockets).

Putting the engines at the front on a flexible tether does make it much harder to change direction, though.

2. make sure it has large flat surfaces, or sprays of recoverable droplets, to act as radiators

Although space is sometimes described as ‘cold’, especially in movies, that’s kind of misleading. Certainly, if not in direct sunlight, something in space without a heat source will eventually cool down until it’s incredibly cold… but for the same reason that we use vacuum flasks to keep tea or whatever warm, it takes a very long time.

If you’ve got something like a spaceship, which is generating a lot of heat, you need to do something to dissipate that heat. The only mechanism that heat can be lost in space, with no atmosphere to carry it away, is electromagnetic radiation. To lose heat through radiation, you need a large surface area (i.e. big flat surfaces called ‘radiators’), and to carry the heat from the rest of your ship to the radiators. You heat up the radiators and they radiate away your heat into the vacuum.

The Space Shuttle would apparently open its cargo bay doors in space to help it radiate away heat (labels once again from Atomic Rockets).

Your radiators should not have their surfaces facing each other, or else they will heat each other up, and won’t radiate heat as effectively.

Another trick to cool your ship down is to use a ‘Liquid Droplet Radiator’ (picture source - apparently the ‘ICAN-II’ design from Penn State University, no idea what that stands for). This involves using your waste heat to heat up a spray of hot liquid droplets, which travel through space, steadily cooling down. As your ship is accelerating, the droplets ‘fall’ towards the back/front of the ship (depending which way you go), and can be collected, heated up and sprayed back in the opposite direction, maintaining continuous circulation.

Atomic Rockets has some really weird-looking designs, including discs, triangles, and even one that looks like a spiral (using magnetic fields).

Have a look at Atomic Rockets - there are lots of radiator designs I haven’t covered, involving bubbles, nanotube filaments, all sorts of stuff.

3. make sure the thrust vector goes through the centre of mass

If it doesn’t - if your engine is off-centre - when you do an engine burn, it will also have a nonzero moment (torque). This means your ship will start to rotate around its centre of mass when you do an engine burn. Sometimes, this is what you want, but usually only by a small amount! And typically a large engine burn would set you spinning wildly. So you will probably have separate small thrusters for turning.

(This gets even more complicated if your ship has flexible elements!)

4. make sure that, if your ship is spinning, it is spinning around its major principal axis

Every rigid body has a thing called its moment of inertia tensor that can be calculated from its mass distribution. Its eigenvalues are called the principal moments of inertia, and its eigenvectors are called the principal axes of the body.  When the engine is not burning, the ship is undergoing free precession.

I remember a series of arguments about free precession in our classical mechanics lectures that say, since a not-quite-rigid body slowly loses energy to internal stresses but the angular momentum is conserved, it precesses (the angular velocity vector moves with respect to the body coordinates, if that means anything to you) so as to end up spinning around the principal axis with the largest moment of inertia.

If your plan is that your spaceship should spin around a different axis, you should be worried, because it will slowly precess to spin around the major axis.

At the time the USA launched their first ever satelite, Explorer 1, in 1958, this was not known. The pen-shaped (shh) satellite was set spinning around its minor axis, the long axis through the middle of the spaceship. To the scientists’ surprise, the axis of rotation changed in flight, until the satellite was flipping end over end. This led to the first development of Euler rotation (the kind of thing we’re talking about) in over 200 years. But we’ve done that now, so, make sure your spaceship is spinning around its major axis if it spins!

5. if you’re going too fast, the ship needs to be safe from relativistic dust

A tiny grain of dust, when raised to relativistic speeds, can have the kinetic energy of a bullet or bomb. And, if your ship is travelling through space at relativistic speeds, every grain of dust in space will be approaching your ship at those speeds! This is bad.

One solution is to put a shield at the front of your spaceship, which will be punched up by relativistic dust, but will disperse most of its energy before it smashes anything important.

I’m going to talk about the Valkyrie again. I am maybe a bit too into this design, but their solution is really cool.

The Valkyrie sprays fluid droplets ahead of the ship. Any incoming particles can detonate harmlessly among the droplets, without touching the engine. Because the ship is constantly accelerating, the fluid droplets will ‘fall’ back down and land relatively slowly on the front of the ship, where they can be recycled. This is also used for cooling - another ‘liquid drop radiator’.

During the deceleration phase of the mission, this doesn’t work - the droplets fly off ahead of the ship instead of being recaptured. The Valkyrie’s solution is to grind its spent fuel tanks into fine dust and release them ahead of the ship as it decelerates. Travelling at a constant relativistic speed, the ground up fuel tanks will punch their way through any interstellar dust particles ahead of the ship, clearing a path.

Some dust will still enter the path ahead of the ship, so in addition to that trick, the Valkyrie extends many layers of ‘blankets’ of extremely thin material (’similar to Mylar’) ahead of the ship. Because the ship is accelerating in the opposite direction, the blankets are stretched out in front of the ship.

Obviously, nothing remotely close to this has ever been tried! So who knows if it would actually work. I haven’t seen discussion of relativistic dust in other designs on Atomic Rockets, though.

6. you also need to be safe from the ordinary kind of space radiation

Even if you’re not travelling at relativistic speeds, you will (without the protection of an atmosphere) constantly be irradiated by cosmic rays and other ionising radiation from any nearby star. This is a problem even for current astronauts, who apparently hide behind water tanks during periods particularly intense radiation. And it’s often discussed as a major danger of a voyage to Mars.

To an extent, you are protected from this by the hull of your spaceship, which absorbs most kinds of radiation even if it’s quite thin. You apparently need two layers to deal with different kinds of radiation, since charged particles entering the e.g. lead you would use for gamma ray shields can create deadly X-rays. Everywhere where people will be living, long-term, needs to be protected this way.

During periods of major radiation, such as solar flares, this may not be enough. You need a particularly well-shielded part of your spaceship to hide in at these times. One method is to use the water (that you’re carrying anyway, for the crew to drink) to protect the crew at these times. You need a way for the crew to access this area, and take anything particularly vulnerable to radiation - such as food - in there with them.

Apparently people are also considering other methods, involving things like powerful electric or magnetic fields, or bubbles of plasma. I don’t know a lot about this.

7. you also need to be safe from the radiation from your ship’s propulsion and power plant

To travel long distances in space, chemical rockets tend not really to be enough - you need something with a very high \(\Delta v\). Many such propulsion systems have been designed, but they tend to have the side effect of producing lots of ionising radiation.

This isn’t necessarily a huge problem. You don’t necessarily need to shield your entire reactor (which would be very heavy), but can instead use a shadow shield, like we discussed on the Valkyrie, to keep just your ship safe from the radiation, and let the rest of the radiation radiate away into space. But it does imply that the rest of the ship, including your thermal radiators, needs to keep itself inside the cone of safety created your shadow shield. And your shadow shield is usually still quite massive, because the main way we attenuate radiation is to make a lump of very dense materials.

Here’s an example of a NASA design which uses a shadow shield, and has to keep its radiatiors inside the shadow:

8. it needs to have pressurised areas for humans to breathe in

The pressure outside the spaceship is nothing. The pressure inside the spaceship is probably about 1 atmosphere. Your ship needs to be strong enough to contain that.

You definitely don’t want to have too many parts of your ship that would be particularly vulnerable to rupturing, so the walls of your pressurised areas are probably going to be nice and curved with minimal sharp corners.

One way to build a pressure vessel, the way we currently do, is to use lots of strong metal. But that adds a lot to your payload mass. So another possibility is to have an inflatable spaceship.

NASA has one such design called the TransHab. Which I assume means it can’t be boarded by cis people. It looks like this (source)…

(all of those weird CGI people are trans. no arguing)

arborine has generally talked about much larger, long-term designs: a big sphere or torus, containing a much more comfortable landscape built for many people (I will have to ask her to help me recreate the details though!). Spheres and toruses are good for inflatable shapes, because the tension is nice and even across the surface (well, this is complicated by rotation, but basically).

The need to have a pressurised hull and radiation shielding does introduce some limitations. Windows in a pressurised compartment generally need to be quite small and rounded, like aeroplane cabin windows, but when you’re far from a planet, there’s not really anything to see anyway. (It seems that, with sufficient engineering, you can make bigger windows, such as in the ISS’s observation bubble thingy. Even then, the bubble’s pretty small, and there’s plenty of structural stuff keeping that together.)

9. you need to be able to deal with problems with mimimal damage

Despite all your best efforts, it seems inevitable that a pressurised compartment travelling through the vacuum of space might have something go terribly wrong. For example, something punches a hole in the hull. You need to have that not end the mission, and cause as little harm to the crew as possible.

This probably means, internally, that your ship needs to be divided into separate compartments, so that if one gets depressurised, the rest of the ship is still safe. If the walls of your ship can somehow be self-sealing, even better.

10. you need self-repair and a self-renewing ecosystem for a long voyage

If you’re travelling on a very long voyage, carrying supplies for the entire journey just takes too much mass. You need to carry with you a functional, lasting ecosystem of plants etc. that won’t die off during the voyage.

Making a closed ecosystem is very hard - as far as I know, we haven’t succeeded yet. I will defer to arborine on the details of one, as I can’t remember what she said or what was in her extremely practical space habitats book off the top of my head.

In addition, parts of your ship will slowly fail and go wrong. Atomic Rockets claims the best NASA probes can last 40 years in space, and that may well not be enough for your purposes (even taking as much advantage as possible of relativistic time dilation). For a very long voyage like a generation ship or something, everything can probably be assumed to go wrong your spaceship to an extent needs to have the capabilities on board to build itself, and apply its repairs outside the safety of crew compartments.

Needing to recreate them en route might constrain the materials you can use, and hence the shapes you can make.

( breathofzephyr made an interesting point that the total computing power, data storage etc. available to the crew of a generation ship will, unless they can build more computers, go down over the course of the mission. which could have interesting social effects…)

11. you will probably need to dedicate a lot of your spaceship to storing reaction mass in suitable containers

A consequence of the rocket equation is that, for every gram (or whatever mass unit) of payload - structural elements, people, food, computers, robots, whatever - you have, you will need a certain number of grams of reaction mass to throw out the back of your rocket.

(The exception is if you are using a propulsion system such as a solar sail or a laser sail where you are propelled by collisions with material that you’re not carrying with you. Or if you’ve figured out a way to get the Bussard Ramjet, or its variants, to be practical!)

The shape of your propellant tanks depends a great deal on what kind of engine you’d use, but basically, you need a lot of pressure vessels somewhere on your ship, likely held separate from the crew modules.

12. it needs to be really really light, as far as possible

The rocket equation is a very demanding thing, and unless you somehow have vast amounts of leftover \(\Delta v\), you want to take every possible opportunity to save on mass. (’loads of leftover \(\Delta v\) is like, you’re using Project Orion to travel through the solar system or something.)

This is why lots of spaceship designs are modules held together by a lightweight framework - you want to minimise heavy structural elements as much as possible.

We’ve talked quite a bit about this already, though!

13. if you provide gravity, you either need a lot of \(\Delta v\) or something has to spin

For a long voyage, your crew’s bones and muscle will atrophy unless they’re given an environment with a source of ‘gravity’ similar to Earth’s.

There are two ways you can do this, which amount to accelerating some or all of your spaceship. One way is to have your engine burning so as to provide \(1 g\) acceleration throughout your voyage. By contrast, current space missions usually involve very short rocket burns, with the spaceship flying freely for most of its mission. To burn throughout the mission, your ship will need vastly more \(\Delta v\), and hence vastly more propellant mass per unit payload mass.

Since that’s pretty demanding, the other option is to have part of your spaceship spinning. This could be for example a toroidal ring, or a cylinder, or a sphere, or a couple of rooms on the end of long arms.

Depending on how large your rotating section is, it will have to spin more or less fast in order to maintain the amount of gravity you need. The centrifugal acceleration in a rotating reference frame is given by \(r\omega^2\), where \(r\) is the distance from the centre, and \(\omega\) is the angular speed.

Note this means the artificial ‘gravity’ in a rotating section with multiple floors, or in a spherical habitat, will vary as you get closer or further away from the centre! The further out you are, the stronger gravity will be.

A rotating habitat section has some implications about the design of the inside. ‘Level’ surfaces will be curved instead of flat, and the Coriolis effect will apply to deflect trajectories from a straight line - particularly strongly in a small, fast-spinning section.

There are lots of difficulties to overcome if only part of your spaceship is spinning. A major one is the problem of the bearing between the rotating section and the rest of the ship - it needs to be near frictionless, or your spin section will slowly spin down (and your ship will slowly spin up.

Another problem comes from spinning up your spin section without setting the rest of your spaceship rotating in the opposite direction (with possible consequences of precession, and the like). One solution is to have a flywheel inside your ship, but this implies a large mass that’s generally useless. Another is to have multiple counter-rotating spin sections, such that the net angular momentum change can be 0 without spinning the entire ship.

Or, you can just let the entire spaceship spin at once, though this makes manouevring the ship somewhat complicated because of the way gyroscopes act (it becomes forced precession, and you need to push at \(90^\circ\) from the way you actually want to turn…)

A further problem comes from the fact that, when your spaceship is accelerating, the direction of ‘down’ will change. This can be accomodated by some very complicated-looking designs involving arms that swing in and out (source), or just ignored because most of the time your ship is not accelerating.

and more stuff, probably

So, yes, there are a lot of physical demands on the shape of a spaceship, even one that never visits an atmosphere!

Building a spaceship is a lot different from building on Earth, and of course nobody actually knows how to build a spaceship for an interstellar voyage. But this is what we think, based on the physical constraints we’ve imagined so far…

Designing new aerodynamic vehicles typically requires a combination of multiple experimental and numerical techniques. The photo above shows a model for an unmanned flying wing-type vehicle. Here it’s tested in a water tunnel with dye introduced to the flow to highlight different areas. The model is at a high angle of attack (18 degrees) relative to the oncoming flow. This puts it in danger of flow separation and stall, the point where a wing experiences a drastic loss in lift. The smooth flow over the front of the model indicates it hasn’t reached this point yet, but notice how both the green and red dyes are separating from the model and becoming very turbulent over the back of the wing. If the model were pushed to an even higher angle of attack, that separation point would move further forward, bringing stall that much closer. (Image credit: L. Erm and J. Drobik; research credit: R. Cummings and A. Schütte)

Reinventing the Wheel

Planning a trip to the Moon? Mars? You’re going to need good tires…

Exploration requires mobility. And whether you’re on Earth or as far away as the Moon or Mars, you need good tires to get your vehicle from one place to another. Our decades-long work developing tires for space exploration has led to new game-changing designs and materials. Yes, we’re reinventing the wheel—here’s why.

Wheels on the Moon

Early tire designs were focused on moving hardware and astronauts across the lunar surface. The last NASA vehicle to visit the Moon was the Lunar Roving Vehicle during our Apollo missions. The vehicle used four large flexible wire mesh wheels with stiff inner frames. We used these Apollo era tires as the inspiration for new designs using newer materials and technology to better function on a lunar surface.

Up springs a new idea

During the mid-2000s, we worked with industry partner Goodyear to develop the Spring Tire, an airless compliant tire that consists of several hundred coiled steel wires woven into a flexible mesh, giving the tires the ability to support high loads while also conforming to the terrain. The Spring Tire has been proven to generate very good traction and durability in soft sand and on rocks.

Spring Tires for Mars

A little over a year after the Mars Curiosity Rover landed on Mars, engineers began to notice significant wheel damage in 2013 due to the unexpectedly harsh terrain. That’s when engineers began developing new Spring Tire prototypes to determine if they would be a new and better solution for exploration rovers on Mars.

In order for Spring Tires to go the distance on Martian terrain, new materials were required. Enter nickel titanium, a shape memory alloy with amazing capabilities that allow the tire to deform down to the axle and return to its original shape.

These tires can take a lickin’

After building the shape memory alloy tire, Glenn engineers sent it to the Jet Propulsion Laboratory’s Mars Life Test Facility. It performed impressively on the punishing track.

Why reinvent the wheel? It’s worth it.

New, high performing tires would allow lunar and Mars rovers to explore greater regions of the surface than currently possible. They conform to the terrain and do not sink as much as rigid wheels, allowing them to carry heavier payloads for the same given mass and volume. Also, because they absorb energy from impacts at moderate to high speeds, there is potential for use on crewed exploration vehicles which are expected to move at speeds significantly higher than the current Mars rovers.

Airless tires on Earth

Maybe. Recently, engineers and materials scientists have been testing a spinoff tire version that would work on cars and trucks on Earth. Stay tuned as we continue to push the boundaries on traditional concepts for exploring our world and beyond.  

Make sure to follow us on Tumblr for your regular dose of space: http://nasa.tumblr.com.  

me too

For humans, swimming is relatively easy. Kick your legs, wheel your arms, and you’ll move forward. But for microswimmers, swimming can be more complicated. For them, the world is a viscous place, and the rules that we swim by can’t help them get around. In a highly viscous world, flows are reversible. Kick one limb down and you might move forward, but when you pull the limb up, you’ll be sucked right back to where you started. So microswimmers must use asymmetry in their swimming. In other words, their recovery stroke cannot be the mirror-image of their power stroke. A new study suggests that simple elastic spheres could make good microswimmers through cyclic inflation and deflation. When the sphere deflates, it buckles, making a shape unlike its inflating one. This difference in shape change is enough to propel the sphere a little with each cycle. Right now the test system is a macroscale one, but the researchers hope to continue miniaturizing. (Image and research credit: A. Djellouli et al.; via APS Physics; submitted by Kam-Yung Soh)

Things Programmers Shout #654

Shout I overheard at work here at NASA. “It’s not like it’s rocket science!” “I will compile your code to a flash drive and send it to the damn sun if you don’t stop it with the null errors.” // submitted by  @space-husband

Recurring slope lineae (RSL) are seasonal features on Mars that leave behind gullies similar to those left by running water on Earth. Their discovery a few years ago has prompted many experiments at Martian conditions to determine how these features form. At Martian surface pressures and temperatures, it’s not unusual for water to boil. And that boiling, as some experiments have shown, introduces opportunities for new transport mechanisms. 

Researchers found that water in “warm” (T = 288 K) sand boils vigorously, ejecting sand particles and creating larger pellets of saturated sand. Water continues boiling out of the pellets once they form, creating a layer of vapor that helps levitate them as they flow downslope. The effect is similar to the Leidenfrost effect with drops of water sliding on a hot skillet; there’s little friction between the pellet and the surface, allowing it to travel farther. 

The mechanism is quite efficient in experiments under Earth gravity and would be even more so under Mars’ lower gravity. It also requires less water than alternative explanations. The pellets that form are too small to be seen by the satellites we have imaging Mars, but the tracks they leave behind are similar to the RSL seen above. (Image credit: NASA; research credit: J. Raack et al., 1, 2; via R. Anderson; submitted by jpshoer)

I don’t doubt it