Space – Getting To Mars Part 3: Propulsion

We kicked off my series on “Getting to Mars” last time with a look at Orbital Mechanics – showing that the physics of getting from one planet to another can be mostly explained with a stapler, a pen, and Kristen Wiig looking unimpressed. This time we’re looking at the propulsion systems that we’ll use to get to Mars.

Of course because every armchair expert has their own pet propulsion project they think is critical to the future of space exploration, this is probably the article I’ll have to delete the most hate-mail for. That’s right – I don’t even read your unsolicited and poorly-spelled bullshit before deleting it, but thank you for reading all of mine! And if you haven’t already figured it out this is also the article you’re probably going to get me at my snarkiest, because there are three phrases I hear on a fairly regular basis that genuinely get under my skin and strangely all three are connected in some way to spacecraft propulsion…

#1 “Space is hard” – The lame catch-cry of everyone that’s just watched a spacecraft disintegrate in a “rapid unscheduled disassembly“. Don’t whinge that space is “hard” – find the cause of the problem and learn from it. Space isn’t hard, it’s just unforgiving of screw-ups. Screw-ups like when someone puts in a gyroscope upside down on a US$1.3 billion rocket launch, or when someone else loses a Mars probe because it was built by the world’s biggest aerospace contractors in the only country besides Liberia & Myanmar still fighting the Metric system.

#2 “It’s not rocket science” – The sarcastic accusation that something you’re struggling with isn’t really that difficult. You know, instead of helping you, someone will suggest you’re an idiot. Here’s something for all of you unhelpful jerks: Rocket science is not difficult. Rocket science can be explained with literally ONE equation (aptly called the “Rocket Equation”) that’s not even remotely complex. Ready for it?
Where \Delta v\ is the change in the spacecraft’s velocity, v_{\text{e}} is how fast things are being shoved out the back of your spacecraft (eg. the rocket exhaust), and you multiply that by the natural logarithm (\ln ) of your spacecraft’s initial mass (m_{0}) over it’s final mass (m_{f}). You can also express the same equation in terms of specific impulse, but if it’s all feeling too complex just remember you go faster if you throw bits of your spaceship out the back really fast to make it lighter.

Rocket science is not difficult, however rocket engineering is ludicrously complex and exceptionally challenging*. So next time you decide to be an obnoxious and holier-than-thou wanker to someone trying to do something they’re struggling with, how about at least getting the terminology right?

*For why I still refuse to say rocket engineering is “hard”, see point 1 above

#3 “We need to develop better solar electric propulsion to get to Mars” – I’ll get to why you’re what’s wrong with the space industry a little later, but for now lets just say you’re a piece of shit and I can prove it mathematically.

Spacecraft propulsion can be broken down into two big categories: Thermodynamic (using heat to move gas) and Electrodynamic (using electricity/magnetism to move gas).

Thermodynamic

This category is mostly the kind of spacecraft propulsion everyone is familiar with: rockets. Absolutely no one is doubting that rockets look super cool. They’re also dangerous, wasteful, noisy, and prone to going boom because of the most tiny and obscure things… like super-chilled liquid oxygen turning solid on your carbon-fiber wrapped helium tanks.

Rockets are also ridiculously expensive and absurdly inefficient at getting things to space. The Saturn V that launched men to the Moon* weighed nearly 3 million kilos on launch, but only 5,560kg of that was left by the time the Command Module splashed down in the ocean. To put it in context, 0.185% of the original rocket’s mass came back to Earth and the other 2,964,440kg was either burnt as fuel, dumped in the ocean/space, or left on the Moon. Considering each Saturn V launch cost about US$1.16 billion in 2016 figures, that’s a whole lot of specialised and expensive stuff to be just throwing away.
* Don’t even start with me Moon Hoaxers – I will destroy you

I’d talk about how NASA’s “Space Launch System” is supposed to (eventually) be more powerful than Saturn V… buuuuuuuut since SLS & the Orion capsule are basically the worst parts of the Bush-era Constellation program that have already cost US$18 billion and are now projected to reach US$35 billion in 2025, at this point it really looks like it’s just a pork-barreling jobs program for a bundle of US Senators through the old conservative aerospace manufacturers. A jobs program which is also takes funding away from real exploration opportunities (like the underfunded Commercial Crew Program) to build a rocket that’s going anywhere. #NotEvenSorry

I currently have a bet with a fellow space geek about SLS: I’m convinced it will be cancelled before it ever flies, whereas she thinks it’ll fly once before it’s cancelled. The loser has to buy the other a ticket to Mars aboard this…

Did you see that gigantic rocket flying itself back to the launch pad to refuel and launch again? That’s SpaceX’s “Interplantary Transport System”, and once it’s up and running in the 2020’s there will be several of these taking 100 to 200 people to Mars every few years for about US$200,000 each – return trip included. They can afford to talk about sending people to Mars and back for less than the median cost of a house in the US (or 1/4 of a house in Sydney) because they’re not dumping most of their rockets into the ocean every time they launch – they’re landing them, refueling them, and launching them again. Building better rockets and not throwing most of them away after a launch means the cost of getting stuff to orbit has decreased dramatically in recent years.

We’ve never used rockets for their efficiency though – we use them because they produce a huge amount of thrust. If you have to get something from the ground into Low-Earth Orbit, it needs to push through the air with enough raw power and velocity to break free of the atmosphere and start falling around the Earth with enough velocity not to hit it again. Right now the only thing we’ve got that can push hard and fast enough to reach orbit is rockets, and no matter whatever weird propulsion system other folks might be dreaming about this is also the only way we’re going to get to Mars in the next 15-20 years*.

*Bring it on Solar Electric Propulsion people – I’ve got your number at the end of this article.

That’s not to say all rockets are the same though – we’ve got all sorts of different ways of making things go boom to get somewhere fast:

Solid Rockets – Basically really big and complex versions of the little gunpowder rocket engines you can buy at a hobby store. They’re cheap, powerful, and easy to make – perfect for launching things like cargo and probes into space.

It’s probably not a great idea to use solid rocket boosters on anything carrying people though – once you light a solid rocket you can’t stop it burning if something goes wrong… like when one on the space shuttle burned through an o-ring and into a 760,000kg tank fuel of rocket fuel, which then exploded and killed seven astronauts. But NASA is planning to use solid rocket boosters again with the crewed SLS (test fire pictured above). So, you know… YOLO.

Liquid Rockets – Pumping flammable liquids into a chamber and having them explode in a specific direction. While the Chinese were the first to get serious about solid rockets back in the 1200’s, it wasn’t until the 1900’s that a guy called Robert Goddard started to set fire to liquids to push rockets around. Unfortunately the US’s scientific community and the New York Times just made fun of him for suggesting rockets could work in space.

Correction the New York Times published 3 days before Apollo 11 launched (on liquid rockets) to the Moon… and 24 years after Goddard had died.

Fortunately some people payed attention to Goddard’s research into liquid rockets. Unfortunately those people were also the Nazis, who then used that research to bomb Europe with these:

Liquid rocket engines are way more complex than solid rocket engines essentially because the fuel is sloshing around and needs to be pressurised through tanks & fuel lines for them to keep flying. Going back to my earlier “rocket science is easy, but rocket engineering is hard” – the national security restrictions imposed by each country on who can work on their rocket technology often has little to do with the rocket itself, and is almost entirely about protecting the technology behind the turbopumps that push the fuel and oxidiser at high speed & pressure into the engine bell.

Liquid rockets generally get broken down into two further categories depending on their fuel too. Bipropellants are what you see in a usual rocket launch where an oxidiser (usually liquid oxygen) and a fuel (kerosene, liquid hydrogen, methane, ect) burn to produce thrust. Monopropellant is a single liquid that ignites when it touches a catalyst, and is often used once you’re in space to turn your spacecraft around or give it a gentle push. It’s also usually made of hideously toxic, carcinogenic and explosive liquids like Hydrazine, that apparently smells like fruity-ammonia if you live long enough to tell someone.

Hybrid Rockets – A surreal mix of a solid and liquid rocket. The most obvious and well-known example of a hybrid rocket powers this:

Virgin Galactic’s Spaceship Two

Hybrid engines have a liquid/gas oxidiser that runs through channels in the solid fuel to burn it. They avoid the complexity of liquid rocket engines, and unlike a solid rocket you can stop them once they’re lit by cutting off the oxidiser supply. The downsides are they’re not as efficient as solid or liquid rockets, and most of them are filthy polluters. The fuel going into hybrid engine in Spaceship Two has been changed a lot, but it’s usually nitrous oxide burning rubber. So pumping soot directly into the upper atmosphere isn’t exactly fantastic for things like Global Warming…

Nuclear Propulsion – Launching tonnes of hot, radioactive material into space because it’s really good at getting you places fast… provided it doesn’t explode on the way.

Now I’m only including this because it is a form of thermodynamic propulsion, people have talked about for more than 60 years, folks like NASA & the Soviets have designed entire working systems around it… and even at it’s absolute safest it’s still fairly insane.

Nuclear rockets are outrageously powerful – even the most basic designs are twice as powerful as what’s possible with a chemical rocket. There are dozens of different (theoretical) varieties, however only two have ever been developed properly: NASA’s NERVA and the Soviet Union’s RD-0410. NASA actually had the closed-cycle NERVA XE flight ready and deemed suitable for a Mars mission in 1969, right before NASA’s funding was cut because it was clear the US was going to win the race to the Moon. Both the NASA and Soviet systems still involved using a flying nuclear reactor to super-heat hydrogen in space, however they were designed to be comparatively safe “closed cycle” systems.

I say comparatively, because you have to compare it to the other crazy shit other people were suggesting in the 1960’s. Fun things like “open cycles” designs that used weapons-grade radioactive material and deliberately spewed out clouds of radioactive exhaust.

See the bit saying “Uranium 235 T~55,000 K” leading to an open nozzle? Because fuck everyone else on the planet, right?

Then there’s the folks who designed Project Orion, who clearly felt the only thing better than using a nuclear reactor in space would be to use actual nuclear weapons. Project Orion was about literally firing a nuclear weapon behind your spaceship to propel it in the other direction: for anyone who’s ever played Quake or Team Fortress 2 this is basically a rocket-jump but with a nuke.

We’re not talking about just one nuke either: the idea was to have one going off every second, and some of the interstellar designs called for a spacecraft 20km long that carried 300,000,000 1-Megaton nuclear weapons, or “pulse units” as they were so eloquently renamed. Strangely enough Project Orion pretty much ended when most of the world signed the “Treaty Banning Nuclear Weapon Tests in the Atmosphere, in Outer Space and Under Water” (aka the Partial Nuclear Test Ban Treaty) in 1963.

The fever dreams of Dr Strangelove

Chances are we’ll need some sort of nuclear propulsion in the future to take humans beyond Mars though. Jupiter barely gets 4% of the sunlight the Earth does, so the diminishing light from the Sun makes solar power a lot less viable. It’d also be a great way to reduce the nuclear stockpiles we have, and there’s even some semi-reasonable arguments for taking small nuclear power plants to provide electricity to a colony on Mars – the big issues are obviously what do you do with the waste and what if something breaks?

Nuclear propulsion isn’t completely insane… but do we need to take the risk, when we can get to Mars just fine using conventional chemical rockets? No. 

Do you know what else we don’t need to get to Mars? Solar Bullshit Electric Fucking Propulsion.

Electrodynamic

Maybe you’ve heard on the news about some crazy space propulsion system that uses lasers, ions, or something else that sounds really complex and weird. Chances are it’s either a solar sail (which are slow but cool in their own “Star-Surfing with Sagan” kind of way) or you’ve heard about some variant of an ion drive (which are also slow but cool in their own “Star Trekking with William Shatner” kind of way too).

Ion drives are not some far flung science-fiction fantasy though: Harold Kaufmann built the first ion thruster in 1959, the Russians launched their own variant (known as a Hall Effect Thruster) on a satellite in 1971, and almost all modern communication satellites use some form of ion drive for “station-keeping” – correcting for variations in Earth’s gravity to maintain a highly precise “geo-stationary” orbit.

Essentially ion drives use electric fields to accelerate a gas (usually Xenon) out an exhaust at incredibly high velocities to produce a tiny thrust. The high exit velocity (aka “Specific Impulse”) means ion drives are insanely efficient and capable of reaching much higher maximum velocities than any rocket ever could, and there’s been some really exciting improvements… but because ion drives only throw out only a tiny bit of gas (eg. roughly the same amount of force you feel blowing on the back of your hand) they’re also incredibly slow to accelerate up to those high velocities.

How slow? NASA’s Dawn mission has three Xenon ion thrusters capable of 90mN of thrust (about the same force as the weight of a postage stamp) that can accelerate the probe from 0 to 100km/hr over four days.

Ion drives absolutely have their place, but no matter what bullshit spin some of the old aerospace players might try to pull that place is not getting people to Mars. Ion drives are improving, but unless VASIMR unexpectedly gets a demo flight and proves it actually works electrodynamic propulsion simply won’t be powerful enough to shorten the trip to Mars for humans any time in the next few decades. Especially if you’re only using solar power.

Improved ion drives that run on solar power will be really useful however for… getting communication satellites from Low-Earth Orbit into a Geo-stationary orbit.

Here’s a fun fact: the global satellite communication industry generates over US$200 billion in revenue each year, and makes up nearly 2/3’s of the entire space industry. Reaching Low-Earth Orbit (160km to 2000km altitude) with a rocket is relatively simple, however getting to Geo-stationary orbit (~36,000km and where almost all large communication satellites need to be placed) is much harder, requires far greater velocities, and usually needs an additional stage on the rocket. This extra velocity and additional staging brings greater risks of things going wrong, so naturally launching something to such a high orbit is also a lot more expensive.

So if telecommunication companies can launch new satellites to a much cheaper Low-Earth Orbit and then use solar powered ion drives (aka “Solar Electric Propulsion” aka “The bane of my existence”) to slowly shift new satellites up to geo-synchronous orbit over several months, they’ll save literally billions in launch costs alone.

Are you bored by this yet?  

No shit – the satellite communication industry is boring, but it’s also really big money. Do you know what is not boring, but also means risking lives for something that won’t make anywhere near as much money? SENDING PEOPLE TO MARS.

Which is why there’s a huge amount of money and research going into solar electric propulsion at the moment, and why I roll my eyes obnoxiously at everyone who tells me it’ll “help with NASA’s #JourneyToMars”. Because they either don’t understand how weak solar electric propulsion currently is, or they’re trying to bullshit me and others into believing a technology being developed to reduce the cost of deploying communication satellites around Earth will somehow get me to Mars.

I’m happy to be proven wrong on all of this, and I’m certain in the far future we’ll use ion drives to zip between Earth and Mars. I’m even sure some of them will even use solar power. They’ve been trying since 1971, but maybe Ad Astra will finally get somewhere with VASIMR afterall. Maybe the EM Drive will be completely validated and change everything. But don’t tell me we to need to pour billions more into solar electric propulsion research to get to Mars – chemical rockets have been getting things there just fine for decades.

In the meantime, Mars One was founded with the express purpose of permanently colonising Mars, and SpaceX was founded with the express purpose of establishing a sustained human presence on Mars too. Do you see either of them talking about needing further research into solar electric propulsion?
No? Just using conventional liquid rockets you say?

Funny that…

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Space – Getting To Mars [Part 1: Overview]

For the last few years I’ve structured my school visits and public talks primarily around answering questions about the Mars One project, rather than lecturing. For an average 90 minute school visit for example I’ll usually only speak for the first 10-15 minutes – with plenty of images of Mars and no text on the slides – before spending the next 75-80 minutes answering every question under the Sun about life on Mars. School visits in particular are incredibly entertaining, mostly because kids have absolutely no shame and no chill – they will ask absolutely every obscene thing you could ever imagine, while literally bouncing up and down in their chair with excitement, and I have to try to honestly answer their question about how sex, death, shitting, and/or cannibalism will be different on Mars than it is on Earth while their teachers look on in horror.

“Mr Richards, what would you do if there was an ACCIDENTAL fire in your Mars house?” *giggles*

When people hear about Mars One though, their questions almost always focus on what it would be like a) leaving Earth behind, and b) living on Mars without any prospect of coming back. Besides “how long will it take to get there?” though, I don’t usually get a lot of questions about the journey to get there itself. Kids want to know how you shit in space, and they understand the idea of living in a special “house” on Mars… but drifting for months through the inky darkness of interplanetary space to get to your new home is a concept so far removed from their regular lives they don’t even know where to start with questions.

And if kids won’t ask questions about the trip to Mars, you can be damn sure that adults won’t… unless they’re a massive space geek, in which case it’s 50/50 if they’re asking a question because they’re really excited about what you’re doing, or if they’re trying to “correct” you to show off their own knowledge.

So with all of this in mind, I’ve decided to write a series on how we’ll actually get to Mars. I’ll inevitably follow it up with another series on how we’ll live on Mars once we get there, but there’s definitely a huge knowledge gap in comprehending just how difficult (but perfectly achievable) the journey itself is.

Orbital Mechanics & Interplanetary Transfers

Contrary to what most kids (and plenty of adults) might think, you can’t just point your rocket at Mars and hit “GO!” (as awesome as that would be). With Earth and Mars orbiting the Sun at different distances, inclinations and orbital velocities; going from one to the other involves a lot more swinging and looping than people expect, and orbital mechanics has a great way of messing with people’s heads.

The short story is it will take us roughly 7 months to get to Mars, but because of the alignment of Earth, Mars and the Sun we can only launch things to Mars every two years or so. I can already hear the angry space geeks mashing their keyboards at that sentence alone… but if you can hold off for a few weeks from sending me hate-mail filled with delta-V equations and screaming in all-caps about “BALLISTIC CAPTURE”, I’m going to delve deep into orbital mechanics. As always I’ll be writing equally for comedy AND science-communication, so don’t panic if you’re the type who doesn’t break out into an excited sweat at the sight of a Hohmann Transfer equation – I”l be aiming to help you understand why there’s no straight lines when you’re trying to get anywhere in space, but without you needing to become a full-blown pocket-protector-wearing nerd in the process.

Launch Vehicles & Propulsion

There’s no shortage of folks gushing about how you’ll need a “big rocket” to get to Mars (don’t talk to me about SLS, I’m only going to sigh at you) but there’s a lot more to rockets than just “burn lots of fuel really fast to make things go up”. Payload fairing size, solid vs liquid fuels, payload harmonics, staging, crew/cargo separation – it all gets pretty complex pretty quickly. I cringe any time someone sighs and tells me “Space Is Hard”, but using rockets to get places is definitely expensive, risky, and utterly unforgiving if something goes awry.

It’s also not just the “getting out of the atmosphere without being ripped apart” bit you need to worry about either – between ion engines, solar sails, Neumann Drives and nuclear propulsion (if anyone mentions “Solar Electric Propulsion” I will scream at you), there is a mountain of different ways to move between planets without an atmosphere to contend with that are a lot more efficient than just firing up a hypergolic rocket like the US used in the Apollo program to get to the Moon (DO NOT EVEN START WITH ME, MOON HOAX PEOPLE. I’M ALREADY PISSED OFF ABOUT SLS AND SOLAR ELECTRIC PROPULSION – I WILL DESTROY YOU).

Life Support & Psychology

If you’re putting people in an aluminium can and launching them for 7 months to live on a cold, desolate planet for the rest of their lives…. you kind of want them to survive the trip. While there’s still a lot of discussion about the design of Mars One’s transit habitat, we already know it will face unique challenges that nothing rated to carry humans in space has ever had to contend with. Operating somewhere between the space shuttle (which never spent more than 18 days in space) and the International Space Station (which has so far spent more than 18 years in space), the Mars One transit habitat will need to keep four astronauts fit and healthy during the trip to Mars, but once it reaches Mars orbit it also won’t ever need to be used again… so life support systems that are reliable for 7+ months, but also can’t be repaired with critical supplies from Earth.

There’s also that little factor of how do you keep the crew from going bonkers and opening the airlock – preferably by not taking a suicidal British botanist for starters. While I’ve already talked about how to use Ernest Shackleton’s approach to crew selection as a template when selecting a Mars crew, the psychology of space exploration is a particularly fascinating topic generally so get ready to be bombarded with discussions on Breakaway Syndrome, the 3/4 Factor, the Overview Effect, and Facebook use during Antarctic over-winter studies!

Radiation

*sigh* I’m only doing this because there is a ridiculous amount of fear-mongering around it. Yes, we will be exposed to radiation and it will probably increase our risk of heart attack… which is fine, because we’re not coming back and I’d be having a heart attack ON MARS. Which is way more awesome than having a heart attack in an Earth-bound nursing home. NO – it will not make us stupidNO – it does not make a Mars mission impossible. Mars One has written up a great article on what the actual radiation risks are and how they can be mitigated, but I’ll be writing a far more in-depth article on why radiation is NOT the biggest hurdle to sending people to Mars.

Because realistically the biggest hurdle to getting people on Mars has always been…

Entry, Descent & Landing (EDL)

A fractionally elevated risk of cancer and/or heart-attack is nothing in-comparison to the risk of hitting the top of the Martian atmosphere at 9km/sec without bouncing off into deep space, using your spacecraft as a brakepad as it heats up to glow white-hot while ripping through the atmosphere, firing a rocket engine into the hypersonic winds to try and slow down, and then using those rockets and their highly limited fuel to land without becoming an impact crater.

The challenges of Entry, Descent and Landing (EDL) is why the heaviest thing anyone has successfully landed on Mars to date is Curiosity Rover at around 900kg. If NASA wants to send astronauts to Mars and bring them back, they need to be able to land a Mars Return Vehicle that will weigh roughly 30,000 to 40,000 kg. For comparison though Mars One’s Environmental Control and Life Support System is the single heaviest component that needs to reach the surface of Mars safely at 7,434 kg, while SpaceX is talking about being able to deliver 13,600 kg to Mars with Falcon Heavy.

Above all else not being able to land heavy stuff on the surface has been the biggest engineering hurdle faced in the race to Mars, but it looks like the folks at SpaceX are up for the challenge.

So there you have it! I’ve been looking forward to hooking into some serious space engineering and psychology posts to off-set the more personal posts I’ve been working on lately, and I’m really interested to seeing what I can feed from these new posts back into “Becoming Martian” as I continue to edit it.

Onward and upward!