One could characterise the whole environmental debate, or the debate about issues like peak oil or overpopulation as all boiling down to one key issue – resources, or lack thereof. We live on a single planet, which has only a limited supply of materials and a limited area in which to live and grow food. And regardless of how much fossil fuels there are, the atmosphere has a limited capacity to absorb carbon dioxide and the natural world can only deal with so much environmental damage and habitat destruction before a collapse is a risk (with a knock on effect on food supplies).
Now we can debate the exact scale of these resources, if you believe the more optimistic voices on fossil fuels we could stretch supplies out a good deal longer than many assume. But they will start to run out sooner or later, almost certainly within this century. And keep in mind there’s some who say we’ve already hit the peak. And a growing population means a growing demand for resources which sooner or later we will not be able to meet. There’s a wide range of arguments on what is the sustainable population of earth, with estimates ranging from under 2 billion (if we all lived like Americans) to 24 billion (if we all lived like hippies!). But my point is that regardless of the number, there is a number, there is a limit. Its just a matter of finding it….hopefully without causing catastrophic environmental damage to the ecosystem in the process!
However, there is perhaps a loophole we are over looking – Space. If we could gain access to space that would change everything. Suddenly we’d have access to vast amounts of resources. For example a joint NASA and University of Arizona study took a seemingly average asteroid 3554 Amun and attempted to estimate how much resources we could extract from it. They concluded that it contained 30 times as much metal as the human race had mined throughout history, including iron, nickel, cobalt and platinum. Suffice to say one or two space mining colonies and any shortages of platinum or rare earth metals, in support of, say, fuel cell or solar panel production, would be a thing of the past.
Moving earth’s more polluting industries into space would eliminate the threat they pose to the environment. And space could also provide power directly to the earth beamed down from giant orbiting solar power plants. So given the potential game changing nature of space colonisation, I think its important for us to assess how likely it is that we could colonise space.
In order to properly assess the possibility of space colonisation, we need to break the question down a bit. Firstly we are talking about “space colonisation” (going to Mars, for example, not to collect rock samples, but to stay) as opposed to “space exploration” (e.g. temporary crewed expeditions, robotic probes). Obviously exploration is a lot easier, indeed we’re already doing it. Colonisation is likely to be a lot harder and opens a whole can of worms in terms of problems.
Also we need to break the problem down into two broad areas – getting into space and an assessment of the technical problems and possible solutions therein. And secondly, the problems associated with living in space, or on other planets, on a long term basis. I’ll be tackling the first of these points below, looking at the habitation issue in a later post.
The cold equations of space flight
To achieve orbit around the earth you must accelerate a spacecraft to the seemingly unimaginable speed of about 25 times the speed of sound, about 8 km’s per second. If there’s one thing you need to know to understand the difficulties of achieving orbit its Tsiolkovsky rocket equation.
The rocket equation states that to achieve a delta-v (i.e. a change in velocity) sufficient for orbit, typically about 9 to 10.2 km/s (varies depending on where you launch from, note this accounts for air resistance) will equal the exhaust velocity of the rocket times the natural log of the mass ratio (i.e. the fully loaded and fuelled mass on the launch pad divided by the dry empty mass of the rocket plus its payload). Do the maths and you’ll see that even using the best fuel available, with the highest specific impulse, which would be liquid hydrogen (LH2) and liquid oxygen (Lox) producing an exhaust velocity of 4.4 km/s, means that about 90% of the initial take off mass will be fuel.
This should make it obvious why rockets are built the way they are, in other words like giant gas tanks. Keep in mind that the remaining 10% of mass that isn’t fuel has to include everything else, the engines, the fuel tanks, the payload, recovery system (heat shield, parachutes), etc. Every gram of extra empty weight basically means several more grams of fuel must be carried. So the empty weight of the rocket has to be pared down to the bone. The truth is that rocket science isn’t that complicated (I pretty much just explained it in a few sentences above!). Rocket engineering on the other hand is the tricky bit. If rocket engineers had a motto it would be “yes it has to be exact”. Over-designing something means excess weight and no useful payload, under designing means it likely fails and gets blown to bits during launch. Naturally this explains why rocket parts are so expensive, we’re talking about applying a level of precision to parts exceeding that of the aviation industry. Then throwing away nearly the entire vehicle in the process of launch.
The rocket equation also explains the weight saving trick of staging. By splitting a rocket up part way through the launch, we can discard some of the useless dead weight of empty fuel tanks and unwanted engine capacity (since the rocket’s getting lighter, you don’t need as much thrust to keep going) as we climb, meaning we can deliver more payload. Staging also allows the rocket to switch between different propellants as it climbs.
Different rocket fuels have their own particular “sweet spots”. Solid boosters are useful as a first stage as they can provide a very high trust and are relatively cheap, so they can get the launch vehicle up off the pad. However, the Isp for solid rockets isn’t as good as with liquid bipropellants (like LH2/LOx), so normally we’d now switch to one of those. Historically no large LH2 burning engines existed (not the case anymore) and LH2 is somewhat expensive, so some rockets use Kerosene/LOx for the next stage (or for the first stage altogether), often followed by LH2/LOx for the intermediate stage that follows (as noted this supplies the best Isp possible). Once in orbit a space craft must manoeuvre and fire its engines on a periodic basis to achieve the desired orbit. Given that cryogenic fuels like LH2 or LOx would simply boil off over time, we now need some sort of storable propellant instead. Hydrazine is a common choice, either as a monopropellant, or in combination with an oxidiser of some sort. So by splitting up the rocket into stages allows us to use a range of different propellants in the same launch vehicle.
As the above should indicate, the end result is that any launch vehicle quickly becomes very large, complex and expensive. Which, as noted, is largely thrown away in the process of launch. As a result it currently costs tens of thousands of dollars to launch a single kilo of payload into orbit. Getting an astronaut up to the ISS costs tens of millions of dollars….and that doesn’t include the costs of supplying him when he’s up there (or building the ISS, the most expensive object in history). Its widely assumed that to allow space colonisation, we’d have to pull those launch costs to less than a thousand per kg, or possibly a few hundred per kg. Elon Musk recently suggested a figure of $200,000 per person. That’s a big ask, so what can be done to lower these costs?
Single Stage to Orbit
One proposal is to switch from expendable rockets to reusable launch vehicles, most notably an SSTO (single stage to orbit). The logic is that this would allow airliner like operations, no need to assemble stages first (which is also incidentally a risky process, NASA purposely keeps the number of staff in the giant VAB to a minimum in case of a catastrophic fire during launch preparations) and no throwing stages away either. How long do you think BA would stay in business if they had to throw away a 747 after every flight to America? This could allow more frequent operations with the same hardware and a smaller launch crew, with much of the hardware reused for hundreds of launches rather than just one, thus substantially lowering costs.
However, developing a working SSTO is no easy task. Recall that roughly 90% of its take off mass will be fuel. The rest will consist of everything else, the fuel tanks, engines, guidance, crew (if any), life support, recovery system (e.g. extra fuel for a powered landing, parachutes or wings) and of course the heat shield. And that heat shield doesn’t just need to protect a small capsule but the entire launch vehicle. Keep in mind it was largely the cost of maintaining the shuttle’s heat shield that pushed its launch costs to double that of a conventional rocket.
In the 90’s and 00’s NASA attempted to develop two SSTO’s the Delta Clipper (from the DC-X prototype) and the Venture Star (X-33 prototype). As both vehicles developed and more and more systems were added, they either ended up seeing drastic reductions in payload, or the size (and cost) of the vehicle started to balloon in size. The last iteration of the Venturestar would have weighted in at 1,000 tons. Think about that for a minute, a vehicle 3 times the take off weight of a jumbo jet, just to get half a dozen astronauts up to the ISS. I’m sceptical if the servicing bill for such a behemoth would actually have worked out cheaper than current launch methods.
On a wing and prayer
I’ve heard some argue that winged SSTO’s would get around the rocket equation because it doesn’t apply to aircraft. Well that’s not entirely true. Certainly wings generate lift, but they also generate drag. Now for most aircraft, cruising along at subsonic or low supersonic speeds, the benefits of lift outweight the problems caused by drag. However for a launch vehicle cruising at hypersonic speeds, its a little bit different. The drag would actually increase fuel consumption (potentially up to three fold more). This is why the aforementioned Venturestar went for a lifting body approach and vertical take off (minimising time in the atmosphere during launch), while the X-15 (which was air dropped from a carrier plane) went for short stubby wings. In both case the intention is to allow the vehicle to perform a glider landing but not aid with the boost and climb phase, by minimising drag during these phases.
And speaking of which glider lands, as proposed in some SSTO studies, are not necessarily as safe as proponents claim. Landing such a large (and often quite delicate) aircraft at the sort of high speeds an SSTO would typically be travelling at, is a tricky manoeuvre. One or two X-15’s were written off in similar high speed landings. NASA actually fitted ejector seats to the shuttle for the first phase of its operations, for these very reasons. They only removed them because it was seen as unethical that the pilots would have a route of escape but the rest of the crew would not. Even so they developed an alternative bailout process for crew to ensure they could abandon ship if the mission commander decided a landing was too big a risk.
Taking the heat
Also while generating drag those wings (and indeed the entire spacecraft) will heat up…..alot! So the entire vehicle will have to be build to withstand such heat. You’ll also have to find a way of insulating the crew and electronics from the heat as well. On which point there are three approaches to heat shields. The ablative kind (used on expendable vehicles as it amounts to a form of sacrificial protection), the insulator/thermal soak variety (ceramic tiles that insulate the vehicle from heat, radiating it away by glowing red hot, come with all the disadvantages of ceramics, i.e. brittle, difficult to machine, expensive), or simply building the spacecraft out of metal or composite materials with a very high melting point.
Given the problems with traditional heat shielding, a number of SSTO proposals have cited the latter of these approaches as their intended solution to the problem of heat shields. The trouble is that while the launch vehicle will survive re-entry, given that metals (and composites) are very poor insulators, everything inside it will be cooked to a well done crisp.
The only serious proposal for a space vehicle using a metal heat shield that ever came close to flying was the Dynasoar. And it needed an elaborate 3 stage cooling system to protect its single person crew and electronics from the heat of re-entry. It also needed to land on skids rather than wheels as there was no way any tire could survive re-entry heat intact (even inside the plane!). So there are some serious questions to be asked as to whether a number of these proposals are actually viable.
Another proposal is to use air breathing engines for part of the journey, cutting down on oxidiser. For any spacecraft a vast proportion (typically 70-80% in fact) of that “fuel” at launch will be oxidser. So using the atmosphere as an oxidiser can produce a significant weight saving.
However air breathing engines tend to have a much worse thrust to weight ratio‘s. While the Saturn V’s F-1’s have a thrust to weight ratio of 100:1, a typical jet engine comes in at closer to 8:1, 5.4:1 for concorde’s engines,5.2:1 for the Mach 3 capable blackbird’s engines and about 2:1 for a scramjet. You’ll notice how the thrust to weight seems to get worse the faster an air breathing engine goes. This is not surprising because it needs to work at higher temperatures and endure higher working pressures, meaning it ends up getting heavier.
The general assumption is that the weight saving benefits of carrying less oxidiser will be cancelled out by having to use heavier engines (and burn more fuel overcoming all that air resistance), quite apart from the fact that at some point you must switch to conventional rocket power (to get into space where there is no air) hauling a set of now useless air breathing engines all the way to orbit and back again.
The British Skylon project proposes to get around a number of these issues by using an air breathing hybrid rocket engine called Sabre. Initial tests have been promising, they’ve demonstrated that they can potentially cool down the incoming airstream. However there are still a number of obstacles to overcome. By their own admission, the Sabre engine will need to generate a thrust to weight ratio of at least 14:1 (i.e. twice that of a normal subsonic engine!) to be effective. That’s a big ask and its far from proven that this is technically possible.
And recall that with past SSTO’s its not been the engines that have been the problem but the development of everything else that makes up the vehicle. Recall that the X-33 was cancelled because of the failure of a fuel tank during testing (well officially anyway, I think in truth NASA has by now given up on it and this was more of an excuse to kill off a project they could no longer afford to fund).
Two stages to orbit?
Indeed my main criticism of Skylon is why an SSTO? It would make sense to me to split Skylon in two. An air breathing lower stage that accelerates and raises the 2nd stage to a suitable speed and altitude, then separates and returns for a glider landing. The rocket powered 2nd stage then flies on to orbit. Yes this would mean facing the problem of mating these two stages back together again sometime. But I’m of the view that all the technical problems (and in-efficiencies) an SSTO presents us with do not outweigh the benefits of avoiding the job of bolting the two halves of the craft together again.
Keep in mind that the vast majority of space missions involve launching space probes and satellites, often to either higher orbits or beyond earth orbit. In such cases we have no desire to recover the 2nd stage (nor the payload). It seems to me to be rather foolish to risk a multi-billion dollar launch vehicle, flying all the way into orbit and back again, for a satellite worth a fraction of that.
Of course one stage or two stage, any reusable launcher still comes with the fundamental problem that my making the vehicle reusable you are increasing your initial capital costs building and developing it, adding in servicing costs between launches and decreasing the payload you could otherwise deliver, if it was non-reusable. The logic is that reusing the spacecraft with then cancel out these extra costs and the cost implications of the lower payload. However to date, the evidence doesn’t suggests this is feasible.
The big dumb booster approach
It is for all of these reasons that some argue that a reusable space launch vehicle, or an SSTO is a waste of time and would (like the shuttle) likely end up being more expensive to maintain than it is to just keep building conventional rocket stages. Instead they argue in favour of taking a cheap and nasty “Ryanair” style approach. Rockets are instead mass produced in large volumes on the cheap. A good example is the OTRAG concept which would have used large numbers of mass produced smaller self contained rocket stages bundled together to make larger launch vehicle stages.
The main down side of this approach is that launch vehicles like this would probably have a much higher rate of failure. If a rocket engine has, say a 1 in 200 chance of blowing up every time you start it up, the more engines on our rocket, the more dice we’re rolling, the more likely we’ll get a money shot. So probably not a good idea for crewed missions. Also the environmentalist in me has to shudder at the thought of 20 spent rocket stages ending up in the bottom of the ocean every time we launch a payload. Surely we’d need to send someone out to fish those spent stages out of the sea and recycle them? How much is that going to cost?
The not-so dumb booster
SpaceX have gone for a sort of hybrid approach somewhere between reusable launch vehicles and the BDB. On the one hand they are putting their faith in conventional rockets, which are mass produced using as many common components as possible. On the other hand they are also experimenting with reusable lower stages and upper crew modules. So far they’ve pulled down their launch costs to about half that of their nearest competitor, with further cuts expected once further refinements are added and larger launch vehicles are brought into service.
However, its likely that there’s a floor to how low SpaceX can make space flight. And my suspicion is that this floor price will still be too high to allow space colonisation. Certainly it will make a number of the many things we’d like to do in space cheaper and thus more likely to go ahead. These include new space telescopes, space probes (notably to the moons of Jupiter) perhaps even crewed missions to Mars and the Moon. But colonisation (going to Mars or the Moon to stay) will require much lower costs to be viable (again Musk himself admits a figure of $200,000 would be necessary, about 1/20th the current cost).
Also rockets (in general, not just SpaceX) aren’t noted for their reliability. Most have a failure rate of about 1:12. So one has to question whether it would be feasible to take such odds with the lives of the many thousands you’d need to move into space for colonisation to happen. And its worth keeping in mind that those accidents will have legal consequences. Keep in mind that the space shuttle failure rate made it about as dangerous to fly into space on board it, as it was to fly combat missions over Europe in the 1940’s.
Up until now we’ve only discussed the difficulties of getting into orbit. However going further afield, to the moon or Mars for example, requires further engine burns. In terms of Delta-v’s while launching off the earth will be the largest single segment, going all the way to the lunar or Martian surface and back can add up an extra 50 – 100% onto that “delta-v budget”. And recall the “payload” for the earlier stages into orbit will be the fuel to perform these manoeuvres. Of the Saturn V’s take off weight of 2,300 tons the LEM, Apollo Crew modules and life support systems represented a mere 18 tons combined. Everything else (99.2%) was just fuel and engines.
Alternative propulsion options
Naturally this has led some to question whether it is practical to rely on “kick and coast” chemical rockets when moving beyond earth orbit. One idea would be to have a space station in orbit, where astronauts transfer from a launch vehicle to some sort of dedicated space vehicle. This would use engines that provided a much higher specific Impulse per kg of fuel. A solar thermal rocket (which uses large mirrors to superheat hydrogen gas) or a nuclear rocket engine (same idea but with a nuclear reactor doing the heating) can provide an exhaust velocity at least double that of a chemical rocket (9 km/s v’s a maximum of 4.4 km/s for chemical rockets). A VASIMR thruster could potentially provide an incredible 30-120 km/s.
Another interesting idea is that of solar sails or magnetic sails. One such proposal is the M2P2 system. This uses a plasma generated by the spacecraft to inflate a magnetic bubble around the ship. This bubble then reacts with charged particles released by the sun and gets blown along like a tumbleweed bush in the wind. Such systems could offer 50 times the Isp of a conventional engine. A ship equipped with such a system could reach Jupiter in about a year (v’s the 5-7 years it currently takes). As an additional benefit, the magnetic field would serve to protect the crew from cosmic radiation.
You may enquire why I’ve not mentioned Ion thrusters (of Tie fighter fame) or Arcjets. Well because they are low thrust engines. They have a high Isp and a high exhaust velocity, but very low thrust (i.e. pushing power). It would take an Ion engined spacecraft about a year to reach the Moon. One assumes that any astronauts inside would need a good Netflix subscription and that any fuel savings would be cancelled out by the amount of Twinkie’s and Cheeto’s they’d go through over that year. More importantly you’d be increasing the time exposed to danger of the astronauts. The longer they spend in transit, the more likely it is that they’ll get fried by a solar flare.
Even putting such issues of safety aside, there are problems with a number of these alternative propulsion methods. Firstly, most of these ideas have never been flight tested. We have no idea how reliable they would be or what costs would be involved, as the technology is too immature to tell us. How long would it take to develop such propulsion methods? Your guess is as good as mine!
Also a lot of these alternative propulsion methods nearly all rely on some sort of additional power source on board the ship. Solar power (either thermal or PV) is one possibility, but the quantities involved here means you’d need a very large array. So its possible some sort of nuclear reactor would be needed. Regular readers of this blog will know I’m somewhat skeptical of nuclear power, as I think its too expensive, comes with too much excess baggage and there are other far simpler alternatives available (renewables, or energy efficiency). In space however, I’d argue that it might be very difficult, if not impossible, to colonise space without the use of nuclear energy in some way shape or form. And powering spacecraft is just the start, I’m thinking about how to keep astronauts warm during those long lunar nights.
However this is not to suggest all of the problems with nuclear energy magically disappears just because we are in space. Actually, they get worse. Our reactor will need to be very lightweight, which basically means a much more expensive design that the types preferred for power generation on earth. And a design that simple and easy to maintain (the astronauts may not be able to access it during flight). We’d need something that’s inherently safe (yet high performance) as there will be no mass budget to take along triple redundancy backups or multiple layers of containment. Keep in mind that if a nuclear power plant goes into meltdown on earth, we can just evacuate everyone (for 100,000 years!). The astronauts will essentially be strapped onto the top of a melting down reactor, they’ll have nowhere to go in an emergency. Stephen Baxter, in his excellent “hard” sci-fi book “Voyage” considers such a scenario…and the PR disaster that follows!
Space agencies have used RTG’s (essentially a lump of plutonium with a heatsink and a Seeback effect power generator attached) to power deep space probes or Martian rovers, but they’ve never been used as the primary means of power/propulsion on a manned space mission. Nor have we ever launched a nuclear reactor into space. Back in the 1970’s NASA did attempt to develop just such a nuclear engine, in a programme called NERVA. While many technical aspects of the project were proven to be sound, it was ultimately cancelled during the testing phase. It would take further design and development to confirm the viability of such a design.
And then there’s the problem of launching our nuke into orbit. Again this brings us back to our choice of launch vehicle. Those with beach front property down range from the launch site might not be terribly keen on a 1 in 12 gamble every time NASA wants to launch a Mars mission. And what happens if our reactor accidentally falls back to earth after its been used? That’s happened with RTG’s before, but not a large nuclear reactor. Space cynic Jeffrey Bell discusses the likely consequences of such an accident…and the PR disaster that follows!
Hence there’s a lot of political implications (as if the technical problems we’re enough) for using nuclear power in space and they would need to be addressed first. Simply telling people there, there, we know best, we work for NASA and smoke pipes ain’t going to get you very far.
All in all we can see that the obstacles to space colonisation are indeed numerous. There are possible solutions and research is ongoing on some of those as we speak. However I find it doubtful that space colonisation is going to happen any time soon. We are probably going to face some sort of a crisis (energy or environmental) within the next 30-50 years, unless there’s a major change in policy (of course there’s some who’d say we’re already at crunch point now). And its unlikely all of these technical hurdles could be overcome within that time frame.
Not least because we’ve only discussed up until now the technical problems of getting into space. Even if all these propulsion problems could be solved there are a further range of problems which come with trying to live in space, something I will discuss in the 2nd part of this article.