How to Science Your Way Off Mars, Ch. 2 Continued (Patreon)

Well, I have a couple of weeks off the convention circuit, I have no urgent health issues, and I'm mostly caught up on business stuff, which means I have time and energy to throw at writing. And I very much want to get How to Science Your Way Off Mars to the printer- or, at least, PAID FOR at the printer- before the end of the year for tax reasons. So I'll be pushing hard on this project for the next twelve days or so.

This segment is explaining why Andy Weir chose VASIMR as the engine to get Project Ares to Mars- and, in the process, it touches on rocket engines in general.

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In classic pulp science fiction, constant acceleration was a major feature. You pointed your rocket at your destination, you kept the engines firing until you were halfway there, and then you flipped the rocket around and fired the engines again the rest of the way to slow down. This created the shortest possible transit time and, as a pleasant side effect, created artificial gravity without any handwavy solutions like grav-plates or more realistic solutions like magnet boots. (Incidentally magnetic shoes were experimented with on Skylab, as was using hooks on shoes to catch grids in the floor. Both proved more inconvenient than useful.)

The problem with constant acceleration is this.

All spaceship engines devised thus far (that work) are reaction engines. They operate on Newton’s laws of motion. Simplifying it way down, it boils down to: if you throw something out the back of your ship, your ship will be pushed forward with the same force you used to throw the whatever-it-was out the back.

If you want to keep your ship constantly accelerating, you need a constant supply of things to throw out the back- in other words, fuel. And fuel has mass, which means it has inertia.

The equation for calculating a force is, “Force equals the mass being moved multiplied by the acceleration it experiences,” or::

f = ma

A quick bit of algebra gets us the equation for acceleration:

f / m = a

Which means the bigger your mass is, the more force you need to shift it.

Which means the more fuel you need to produce the force.

Which means the more mass that needs shifting.

Which means more fuel.

Which means more mass.

The longer you ride this merry-go-round, the worse your rocket will be.

And, more important for the people building the rockets, the longer you go round and round, the more expensive your gigantic, bloated, fuel-stuffed rocket will be. And the people who cut the checks tend not to want to pay for gigantic, bloated, fuel-stuffed rockets. They want to use the least amount of fuel possible, even if that stretches out the flight time.

So, instead of constant thrust, almost all our rockets make a very limited number of burns to build up initial velocity, then coast as far as possible without burning anything else, and then get to their destination with only a little deceleration burn, some aerobraking (using atmospheric friction to slow down, which requires your destination to have an atmosphere), or nothing at all (i. e. a one-shot flyby). And, since there’s no acceleration on the long coast out to wherever, that means no artificial gravity.

But does this satisfy anybody? No.

The auditors want to spend less money on fuel.

The engineers want the rocket to go faster.

The scientists and medical doctors want the astronauts to get there sooner and come back quicker. (The MDs couldn’t care less about robot probes, but the scientists prefer to see their probes arrive at their destinations before they take retirement.)

And so scientists keep working on the question: how can you make a rocket go faster with less fuel?

Here we bring forward a handy little number called specific impulse.

Specific impulse (abbreviated as Isp) is calculated by dividing the exit velocity of a propellant (X meters per second) by g, or the standard acceleration of gravity at Earth sea level (9.8 meters per second per second, or 32.2 feet per second for Americans). The resulting number is given in seconds, as in, say, 200 seconds. And this number is incredibly convenient because not only does it work in either imperial or metric measurements, but it exists independent of scale. It doesn’t matter how big or small the engine is (so long as there’s enough propellant to ignite- there is a floor to that), the number will still work as a measurement of how efficient the engine type is in terms of fuel.

The main caveat here is that air pressure reduces specific impulse. Reaction engines are always more efficient if there isn’t an outside force damping the force of the reaction, like, for example, 1,013 millibars or 13.9 pounds per square inch of sea-level air pressure. And some engines cope with this inhibition better than others. 

The most common bipropellant combinations for manned spaceflight at launch are either methane and liquid oxygen or RP-1 (carefully refined low-residue kerosene) and liquid oxygen. These fuels are chosen for being very cheap and easy to store, but they also have the benefit of doing better at sea level, going from an Isp of about 265 at sea level to a performance in the low 400s in vacuum. 

Liquid hydrogen has the storage problems I mentioned earlier when talking about the MAV, plus as a propellant it doesn’t do quite as well in atmosphere. In vacuum, though, the liquid hydrogen - liquid oxygen combo is superior, producing about 465 seconds of Isp.

A variant of hydrazine (which we’ll talk about more much later on), monomethylhydrazine, is used for long-term space probes or for maneuvering jets, either as a monopropellant or a bipropellant with either liquid oxygen or nitrogen tetraoxide (N204). Its Isp in vacuum isn’t so good- in the low to mid 300s- but it has the benefit of being a hypergolic fuel, i. e. it doesn’t require a spark, as RP-1, methane, or hydrogen do, to ignite the fuel-oxidizer mix. That, plus the fact that it can be stored pretty much indefinitely, makes it the go-to fuel for any application where either the ship has to go for years without firing the engines or where the engine in question absolutely, positively must not fail.

The highest Isp recorded for a test firing of a chemical rocket is 542 seconds, achieved by a spectacularly toxic and dangerous tripropellant combination of lithium, hydrogen and fluorine. Since these are the three most reactive elements known to exist in this universe, that probably means we’re not going to get much better performance than that no matter how we tweak it. In theory, a straight lithium-fluorine engine might produce as much as 700 seconds, but the impracticability, expense, and sheer unmitigated danger involved, combined with the fact that it’s not really that much better than H2/O2, means we have to look elsewhere for faster rockets. 

In the 1960s experiments were done with nuclear rockets, which basically meant blowing mass past a giant exposed reactor, allowing the reactor’s heat to accelerate the molecules of the propellant, and then flinging it out into the void. Experiments have continued, in a fitful and underfunded way, up to the present day, with some producing as much as 2,000 seconds of Isp. However, the fact that almost all the designs involve blowing hydrogen past an open, unshielded, functioning nuclear reactor has, shall we say, made some people just a tad reluctant to implement this idea at scale. The next nuclear rocket to fly will be the first one. (And that won’t be for a while yet- an effort to launch a nuclear rocket for testing was one of the casualties of the budget cuts in Donald Trump’s Big Beautiful Bill of 2025.)

About here we should discuss Project Orion, the concept of using atomic bomb detonations to propel ships. With that in mind, you, the reader, may discuss it all you like. Here is my part of that discussion: NO!

Moving on.

The next alternative is electrical ion propulsion. This basically consists of turning some gas into a plasma, creating a cloud of positively-charged atomic nuclei and free-floating electrons, and then using electromagnets to fling away the nuclei with extreme force. (A separate device allows the electrons to follow them, because otherwise the nuclei would turn around and come right back to the hugely negative-charged spaceship they just left, nullifying all thrust.) 

The idea, to put it mildly, is not new. Tsiolkovsky and Goddard both noted the theoretical possibility in their writings. The first experiments in building ion thrusters began in the 1930s. The Soviet Union did flying tests of ion engines in the 1960s with mixed results, followed immediately by NASA, whose own test flights had better success. The test-bed cometary probe Deep Space 1 provided the first extended use of ion propulsion in a practical environment, and the Dawn probe to Vesta and Ceres- the first interplanetary space probe to orbit two different bodies after leaving Earth- used it exclusively after leaving Earth orbit. Today the technology is becoming common for satellite maneuvering thrusters or for final burns to put geostationary satellites in their parking orbits.

And how do these perform? Well, ion engine efficiency depends on how much electricity you’re willing to put through the system. And if you’ve got the juice to spare, there’s really no upper limit until the exhaust speed starts getting into whole-digit percentages of lightspeed. Dawn had its three ion engines dialed in to produce an Isp of about 3,100 seconds, giving it over 11.4 kilometers per second of delta-V (acceleration) for maneuvering on its mission. For comparison, that’s about the same delta-V as the Space Shuttle used to get from the ground into low earth orbit- all from a total propellant load of less than half a ton of xenon.

And bench-tests of ion engines, with unlimited power available, have gone as high as 18,000 seconds of Isp.

Now we’re cooking!

But.

There are all sorts of problems with ion propulsion as currently implemented, and the single biggest one is this: for all practical purposes, all of that delta-V is coming through the engine one atom at a time. And yes, it’s giving the ship one incredible kick per atom... but it’s still just one atom. 

No matter how fast you fling it, throwing one atom out the back of your multi-ton spacecraft is not going to speed it up very much.

As a result, ion engines are completely, absolutely useless in even the thinnest atmospheres. Air pressure stops them cold. Only in near-perfect vacuum are they the least bit useful. 

I just said that Dawn had about 11.4 km/s of delta-V when it left Earth orbit, on less than half a ton of fuel. What I didn’t mention is that Dawn, fuel and all, had a total mass of only 1,217 kilograms- meaning that xenon fuel was one-third its total mass. And even then, even with such a small ship, even the smallest burn could take hours. And it didn’t get to Earth orbit on its own; it launched on a Delta II rocket powered by bog-standard chemical fueled engines. 

And that isn’t the only problem. Ion propulsion uses noble gases- argon, krypton, and most usually xenon- because they’re the least likely to react with the walls of the reaction chamber and cause damage. (Xenon is preferred because it’s the most massive non-radioactive noble gas, and thus produces the most kick when it’s thrown, but it’s extremely rare on Earth and thus expensive as hell. Argon has only one-third the mass, but it’s common as dirt- about 1% of Earth’s atmosphere is argon.) 

But least likely is not the same as not at all likely- and the heavier a noble gas is, the less it acts like a noble gas and the more it begins acting like a metal. Again, the larger an atom, the weaker the grip it has on the electrons in its outermost valence shell- which with most other elements makes their chemical bonds weaker, but in the case of noble gases it makes chemical bonds possible. Argon will form compounds under some really extreme conditions, but xenon has all sorts of compounds (admittedly, mostly fragile). And, of course, any ionized gas will react, however briefly, with whatever matter it contacts. As a net result, the reaction chambers of ion thrusters tend to erode, requiring a lot of careful engineering to keep their lifespan as long as possible. 

And, finally, what we gain in Isp per gram of propellant, we have to give back in the form of electricity. The next generation of ion engine after the one used on Deep Space 1 and Dawn, appropriately named NEXT (NASA Evolutionary Xenon Thruster, a clear case of backronym if ever I’ve seen one), used 6.9 kilowatts of solar power and generated only 0.237 newtons of thrust when it flew on the DART probe. (A newton is the force required to accelerate one kilogram of mass by one meter per second. If you threw a softball with a force of 0.237 newtons, you’d only need to take one or two steps, at most, to pick it up off the ground afterwards.)

Scaling this up, to put it mildly, is going to be difficult. Adding more engines would require adding more fuel and also more electrical generation capacity, which leads to the same merry-go-round of bloated ships we had with chemical rockets. 

So, although we look with longing at those beautiful four-digit and five-digit Isps, ion engines just aren’t in any way practical for human space flight, right?

Well, maybe they are- because, after all that explanation, it’s finally time for VASIMR to come into the conversation.

VAriable Specific Impulse Magnetic Rocket- VASIMR- is a close cousin to an ion thruster, but not quite the same. The brainchild of Franklin Chang-Diaz (who, after getting his doctorate and coming up with VASIMR as a concept, went on to become an astronaut and fly on no fewer than seven Space Shuttle missions- seriously, my hat is off to him!), VASIMR’s whole concept is to compromise a little bit of ion engine’s efficiency in exchange for scaling up the thrust to something useful on spacecraft larger than a Reliant Robin.

VASIMR’s neat parlor trick is that, unlike other ion engines, it uses multiple rotating magnetic fields to accelerate the ion plasma a lot faster, and to eject it all- nuclei and electrons alike- out the back at once. The exact nature of  the thrust it produces depends on how much power is run through the radio transmitter that’s used to turn the gas into plasma in the first place- either maximizing specific impulse at the expense of raw thrust or vice versa. The engine has three stages in all, and by the final stage the exhaust can be measured in seven-digit numbers of any temperature scale you like- Kelvin, Celsius, Fahrenheit, doesn’t matter, it’s HOT. And since speed and heat are almost the same thing... 

... yeah, that exhaust is GOING places. 

But are you?

If you’re patient enough, yes.

Right now VASIMR is only in experimental stages- stuck between purely theoretical (“I bet you could run a ship on this”) and practical (“I, a sane human being, would trust my life in a ship that runs on this”). There have been multiple occasions when VASIMR came close to a practical test- most recently as an orbital adjustment engine for the International Space Station- but all of those got cancelled just before an actual functioning engine could be handed over to NASA.

But so far, the bench tests look promising. The VX-200- the engine which was proposed to be added to ISS- has an optimal Isp of about 5000 seconds running on argon propellant, Granted, that only produces 5 newtons of actual thrust, or about the same as a hard slap to your back. And it uses more than twice as much electricity for the same Isp as, for example, NEXT. (Maybe not a fair comparison, as NEXT uses xenon instead of argon, which means more force.) You’re trading off electrical demand and optimal Isp to get about fifteen times as much thrust.

But it’s moving in the right direction- and, as mentioned before, if you have more electricity to shove into the system, you get more shove coming out.

So let’s give it more electricity- more electricity than solar panels can provide. 

Let’s go nuclear.

Yes, I hear what you’re saying. Just a bit ago I was critical of nuclear engines for basically being a wonderful way to spray radioactivity in their wake. And I stand by that, which is why, instead of having an open reactor doing nothing but heating up propellant via waste heat, we’re going to have an enclosed, properly shielded reactor that generates electricity, and that electricity will be used to run everything on the ship.

A single VX-200 engine produces 5000 seconds of Isp running on about 200 kilowatts (hence the number). What happens if we change that to, say, 200 MEGAwatts- one thousand times the power?

Well, then you get from Earth to Mars in thirty-nine days. Or so promises Ad Astra, the company developing VASIMR, founded by Franklin Chang-Diaz (seriously, the brass balls on that man, I salute him). Because VASIMR’s specific impulse is high enough that an interplanetary ship can actually carry the reactant required to constantly accelerate the ship halfway out to Mars, then turn it over and decelerate the rest of the way, for the shortest possible transit time.

And, now that we’ve come full circle, it’s time to take this visionary idea and put it into the story, as Andy Weir envisioned it.

It’s finally time to unveil what Weir described as the single most expensive thing ever built by humanity: the NASA reusable interplanetary spaceship Hermes.