Obstacles to Manned Space Exploration: Propulsion
Created | Updated Dec 23, 2012
Essentially, manned space exploration involves four stages:
- Get off the ground.
- Stay off the ground.
- Go somewhere else and look around.
However, there are many obstacles to achieving those stages. Among the most intriguing are:
- How do we leave the Earth to start with?
- How do we make our ship go where we want?
- How do we breathe while we're in our ship?
- Where will our food come from? And equally important, where will it go when we're done with it?
In particular, 'How do we leave the Earth?' is one question not easily answered. Here we discuss problems inherent in designing a propulsion system that is able to reach space and take us further into its sparkly depths.
To Boldly Go
The first milestone in going to space is reaching Low Earth Orbit, or LEO (ranging from about 400-2000km from the surface of the Earth). It's not too hard to get there as long as you have enough energy - of course, the big problem is finding enough energy.
Let's make what's called a 'back of the envelope1' calculation to find the amount of energy required to get the space shuttle into orbit.
- The speed required to get into orbit is about 9km/s.
- The shuttle weighs (see footnote 7) about 100 tonnes when empty.
- So, we need to get about 1/2 x mass x speed2 worth of energy to get that baby up there.
- Thus we require (0.5) x (100,000kg) x (9,000m/s)2 = 4.05*1012 joules of energy.
That is equal to the amount of energy the average Canadian dishwashing machine uses over the course of 1,991 years. Obviously dishwasher-propelled spacecraft won't be in common use any time soon.
Once in orbit, however, a space craft doesn't need much more energy to keep going. The speed required to escape from the Earth's pull is about 11.6km/s, and once the space shuttle reaches orbit, it is travelling a bit less than 8km/s (after energy loss from air resistance, avoiding large alien battle cruisers, etc). So, a (relatively) small push in the right direction and it could be on its way to Venus.
Now we know that the biggest energy change is from the surface to orbit. A brief review of the physics involved helps to describe how we get the energy needed.
A few examples:
First, take a football and kick it; it goes up because energy was transferred from your foot to the ball. It comes back down because the force from gravity is constant and the force from your leg only lasted a split second.
Next, start your friend's aeroplane and fly into the sky2; the plane goes forward because the jet or propeller pushes on the air giving you forward motion. It goes up and stays up because of the interactions between the wings and the air - those forces being greater than the force of gravity.
Finally, take a 100-tonne rocket, strap 1500 tonnes of high-explosive to it, light the non-pointy end3, and run. The rocket goes up because of the force of the burning fuel, and it stays up because it has enough speed to 'keep falling around the planet' once it is in orbit.
The idea is always the same; give something more energy than it needs to stay still and it will move. If you also make sure that the energy you give it is sufficient to keep it going, it will stay up.
The problem lies in the fact that we would require lots and lots of dishwashers to get a spacecraft up there. A part of the solution, in terms of understanding rockets, is boiled down for simplicity (even among rocket scientists) to: how fast the craft is going? For rockets this speed comes from the rocket fuel (which is much more efficient than utlising dishwashing machines) that's put in the blunt end.
The Rocket Equation
The energy required to launch a rocket to a certain speed is calculated from a series of curly symbols and funny letters called 'The Rocket Equation.' This includes such things as air density, fuel mass, velocity4 of the fuel being pumped out, and velocity of the rocket. Patching this equation into a computer with enough power to play the average video game will give an answer.
The Rocket Equation basically tells us that the faster we want a rocket to go, the more fuel we need to get it going. Simple. Obvious, even.
But, once more fuel has been added to make it go faster, the rocket needs more fuel to lift the fuel that has just been added. And then the rocket needs more fuel to lift the fuel that was added to lift the fuel that was added to make it go faster. This is where the problem starts to hurt normal brains.
Luckily however, one of the more famous virgins in world history, Sir Isaac Newton, didn't have a normal brain like the rest of us5 and he helped to develop a way to calculate where this whole thing would stop. Fittingly, it's called calculus6.
Once we know how fast we want to go, calculus enables us to find out how much fuel we need. Normally, at least for spacecraft like the space shuttle, the fuel and fuel tanks can mass7 as much as 19 times more than the spacecraft itself. And that's just to get to 400km altitude. To get higher we need a lot more fuel than that. Using the rocket equation we see that even though we have gained most of our speed once we are in LEO, and even though to go faster we need only a little more fuel, lifting that fuel into space to use it requires a lot of energy. This is why rockets get so heavy when you want to send them to far-off places. The Apollo rocket (called Saturn V) had a mass ratio of almost 150 to 18, and that was with rocket staging which lowered the total mass required9 to go fast enough to get to the moon.
Solutions for Getting it Up
To solve this problem, either we have to make the spacecraft mass a lot less, find a more efficient fuel (or a more efficient way of burning the fuel), or find a more efficient mode of travel.
The problem with losing unnecessary mass is that there isn't much. Unnecessary mass, that is. Fuel, if conventional (read: glorified gasoline), can't be reduced, life support can't be reduced on manned missions, scientific equipment can't be reduced or we lose our only convincing reason for playing about up there, and relegating space travel and exploration to robots modifies the definition of manned space travel. Therefore, we must look elsewhere.
There is really no way to make conventional fuel more efficient without discovering a new exotic substance. Efficiency rates, in terms of how well the fuel burns, are pretty much at a maximum. Making the rocket do more work by designing better nozzles, lighter ships, etc. permits room for improvement, but the general consensus is that improving the efficiency of conventional fuel is a dead end and that someone should figure out anti-gravity instead.
The only obvious way to lower the cost of launches using conventional propulsion is to start launching the things all the time. Everyone knows that the more of something you make, the cheaper it is to make each one. Unfortunately there are currently no motels in space, and there isn't much to do up there except deal with motion sickness and take pictures of floating liquid. So until there exists a reason for the general public to fly to space, there doesn't seem to be much chance of Airbus building a 399 series rocket to take you to Moon Base Zeta. At least in the near future10.
Using Present Technology
To illustrate the points, here we look at a few of the previous designs for achieving a sustainable and cheap launch system.
Project Orion (USA): A really neat idea involving tossing exploding H-bombs behind a ship to take advantage of the resulting wave of energy released. In fact, this ship never flew (nor was it even built), but testing was done on the idea using small conventional bombs formed to create a specific blast pattern - and it worked. Unfortunately, until either the International Space Treaty is modified to allow for the use of nuclear weapons in space, or people stop caring about simple things like fallout drifting from the launch site into their breakfast cereal, this option will remain on the drawing board.
The Space Shuttle (USA): The idea behind the Space Shuttle was for a relatively cheap, reliable replacement to building intercontinental missiles and stacking people on top of them. It didn't work because in the original plan the shuttle was supposed to have a flight turnaround time of 3-7 days. That means that the six11 shuttle fleet would have a total of about 300 launches a year. With an efficient enough infrastructure, enough demand for launches, and a user-friendly interface the shuttle could have lived up to expectations. Then they realised that half of the shuttle tiles, used to insulate the Orbiter during re-entry, fell off after every flight; that 'gassing up' a space shuttle with liquid hydrogen and oxygen isn't as easy as the guy with the brochure said it would be; and that the infrastructure was a heck of a lot more complicated than was originally planned. To top it all off, one of them blew up at launch near the beginning of the program, and one broke up during re-entry 17 years later. So, instead of scrapping the program and trying to come up with a new idea, they decided to push on and waste billions of dollars on the project.
Buran (Soviet Union): Buran launched once on a sub-orbital flight in 1988 and then the project was cancelled - for almost exactly the same reasons the Space Shuttle should have been cancelled.
Ariane 5: Probably the only successful attempt at building a cheap heavy-lift launcher, though it has had its share of launch failure (the very first one blew up because someone coded the words 'allez a la gauche maintenant' instead of 'go left now'). The first problem is that it is designed for payloads - no monkey flights are planned anymore after the failure of the French Hermes project. The second is that it isn't really much better than the rockets in use before. It is a perfect example of squeezing all the efficiency possible into the tools available. It works great, but still costs a couple of hundred-million US dollars per flight.
Kankoh Maru/Uchu Maru: Designed by Kawasaki Heavy Industries for the Japanese Aerospace Exploration Agency, the Kankoh Maru is supposed to make passenger travel a reality. It can carry 50 passengers, and is meant to be used on sub-orbital and orbital flights for tourists interested in space, and business people interested in going from Tokyo to New York in two hours. It hasn't been built yet, but it is in this category because control system tests have been done on small rigs, and all of the technologies used are off-the-shelf12. It might be built soon, but it will be quite expensive, and the business plan that would make it economical involves thousands of launches a year, and a fleet of 50 ships. Sound like the Space Shuttle to anyone?
Combined Cycle (Hybrid) Propulsion: This idea is interesting because it aims to reduce the energy losses due to air, gravity, etc. by using different forms of propulsion for different stages of launch. For instance, a ship would launch using a jet that would take it to a speed of Mach 3, then, a RAM jet would kick in taking the ship to a speed of Mach 8; then a SCRAM jet would take over raising the speed to Mach 15, and the rocket would finish things off by bringing the speed up to orbital velocity. Benefit #1: we don't have to take any oxygen with us for the first three stages of flight because we can just use the air around us. Problem #1: a rocket with wings loses more energy than a rocket without (because of air resistance) and any vehicle using jets needs wings to get off the ground. This might cancel out the benefit of using jets. Benefit #2: we can make a more leisurely ascent and use the lift generated by the wings to get more height before we have to use the high fuel rate rockets. Problem #2: energy loss to gravity is enormous. The longer we spend using energy to go parallel to the Earth, instead of perpendicular, the more energy we lose unnecessarily. Benefit #3: flying a combined cycle ship would be just like flying a plane. Problem #3: RAM jets and SCRAM jets run at such high temperatures that they melt the craft around them when they are in use. This would have to be fixed before any serious contenders could arise in the field of combined cycle propulsion.
The X-PRIZE: Also worthy of note in this section, the X-Prize was conceived as the 21st Century's answer to the Raymond Orteig prize, that Charles Lindbergh won in 1927 for crossing the Atlantic solo. It's a great idea that may help to spur on public interest in rocketry. However, all of the entries to the competition are incapable of reaching LEO. They are designed specifically to jump up into space, stay there for three or so minutes, and fall back down to Earth. The designs are great for rich tourists who want a couple of minutes of weightlessness, but are useless for anything else... yet.
Proven New Technologies
The last13 possibility is to design a completely new way to get to space that doesn't depend upon conventional rocketry.
Ion Engines: Take a few tiny little pieces of ionized xenon, accelerate them to enormous speeds with a grid of opposite charge, et voila - the spacecraft is propelled forward. The weird thing is that this type of propulsion works great (in space, not for getting off the planet), it just takes a long time to build up speed. The Deep Space One probe had an ion engine as its primary propulsion.
VASIMR: A newer idea that involves high temperature plasmas. This is a great idea that allows for less fuel, lower overall dry mass and good velocity. Like the ion engine, it takes a while to build up speed, but it has a higher maximum velocity. Also, it can adjust the flow rate of the fuel so that when the ship is travelling at higher speeds it can use less fuel at a higher exit velocity to gain more speed itself. Again, like the ion engine, VASIMR can only be used in space, but once up there, it rocks. Problems relating to funding seem to have stalled the development of this engine for now 14.
Solar Sails: Picture the HMS Bounty sailing across the Pacific Ocean in search of gold, spices, and women with no shirts on. Now, in your mind make the sails one million times greater in area, make the ship one tenth the size, lose the prospect of gold and spices, and make the women green things with three heads and tentacles. That is the idea of the Solar Sail. It works by unfurling a gigantic sail made from mylar (a fancy plastic) and by utilising the wind of particles that blows off the sun on a regular basis. It goes slow, costs a lot, and looks absolutely fantastic. Beautiful even. Of course, because of something called the inverse square law15, we can't use it when we get too far from a star. Yes, it is useful, and beautiful, but it has its limitations.
These are a few of the ideas bouncing around the halls of astronautidemia. A clear-headed reader will easily notice the one problem in the three solutions above. Namely, that none of these systems are useful for getting off the planet - only for use once we are already up. We need some better ideas, maybe using some not-so-proven technologies.
The Space Elevator: Physically possible as soon as we work the kinks out of a subject that no one really knows how to work the kinks out of: carbon nanotechnology. Take a really long piece of burnt rope, hang it from space, and stick an elevator on it. Exactly what it sounds like it is, and just as fantastically difficult to realise as it seems, but it does offer an option.
The Rail Gun: If you were to put a high-speed Maglev train on the side of a mountain, have the end of the track curve upwards off the top of the mountain, and then strap a rocket engine to the back of the train, you would essentially have the general idea behind the rail gun. The idea is that we can reduce the amount of fuel necessary to get a rocket off the ground by giving it a lot of speed before it even leaves the ground. Of course, the energy requirements are still there, but instead of putting the fuel on the rocket, we can just tap into the local hydro utility. Oh, and spend billions of dollars building the tracks up the mountain.
Warp Drive: Impossible16? Roddenberry isn't back from the future yet.
The most difficult obstacles to creating a propulsion system for space exploration by humans are here laid out: lack of money, lack of technology, and not enough desire for it by the public. Barring the complete destruction of humanity, manned spaceflight will one day become near commonplace. The only questions are: 'how long will it take to get to that point', 'how much money will it cost', and 'how many physicists and engineers have to go insane in figuring it out?'