Obstacles to Space Exploration: The Original Article

Artist's rendering of the Nuclear Pulse Drive Spacecraft courtesy of NASA

I love rocketships!

I am the author of the article below. The original can be found at http://www.bbc.co.uk/dna/h2g2/A3708669. I’m basically rewriting it to make it easier to understand, and to make it suck less. I think the first time I wrote it, I was trying too hard to be funny, and not hard enough to get across the ideas.

If you find any mistakes, please let me know. I will be updating this over the next few days to add pictures that I steal from the web. Thanks to Asuka for pointing out the incorrect bits.

Obstacles to Space Travel Part I: Propulsion.

Manned space exploration has three stages;

1. Get off the ground
2. Stay off the ground
3. Go somewhere else

Simple, right? Absolutely. No matter what anyone tells you, rocket science ain't difficult. Rocket engineering is insanely difficult, and you'll find out why later.

There are many questions we have ask when we are thinking about travelling in space, and here are just a few.

1. How do we get to space?
2. How do we get to our destination once we're in space?
3. How do we breathe while we're on board our spaceship?
4. Where will our food come from?
5. Where does our food go when we're done with it? (Eww.)
   
6. Finally, how do we stop ourselves from killing each other on that really long trip?

Most of these questions have simple answers, and we'll discuss them in future articles.

'How do we get to space?' and "How do we get to our destination once in space?" are the questions not so easily answered. In today’s article, we’ll talk about space propulsion.

To Boldly Go

The first goal for every space traveler is called Low Earth Orbit (LEO). LEO lies between 300km and 2000km above the surface of Earth, it's not too hard to get there as long as you have enough energy, and enough money to buy the energy. Of course, the big problem is finding enough energy.

What does “energy” mean? Energy is the word that scientists use to describe pretty much everything. In simple terms, if something has a lot of energy, it’s either moving really fast (kinetic energy), or it has the potential to move really fast (potential energy).

For example, if you throw a baseball, it has kinetic (moving) energy. If you stand on top of the Empire State Building with a baseball in your hand, the baseball has potential energy because if you drop the ball, it will go faster and faster until it hits the ground. All of the potential energy will change into kinetic energy as the ball speeds up.

It's exactly the same with spaceships. When you have a spaceship sitting on its launch pad, it has zero kinetic energy, because it’s not moving. But it has a lot of rocket fuel in its tanks, so it can be said that it has lots of potential energy. When you light the fuel, it will go fast.

Let’s look at how much energy the Space shuttle needs to get to LEO.

· The energy we need is almost the same as if we wanted the shuttle to go 32,000km/h. Look at the picture below. This cute little picture shows how orbit works. Isaac Newton drew it to show how gravity works. If you shoot a cannonball horizontally away from the Earth (from point V) with a certain speed, it will land at point D. If you shoot it with a slightly higher speed it will land at point E. Even higher and it lands at point F.

If this drawing belongs to anyone it belongs to Newton, but I don't know his email address so I can't confirm it.

He shows that if you shoot a cannonball fast enough, it will fall down to the Earth at the same rate as the Earth curves, and the cannonball will go all the way around the Earth. It will go right back to where it started, probably hurting the cannon shooter in the process. This is what orbit is.

TO stay at a certain height you have to be going a certain speed. In the case of a spacecraft travelind around the Earth at the same height as the Space Shuttle, the speed is about 25,000km/h. But, we lose some speed because of air resistance, so our guess is that we need to be going 32,000km/h.

· The shuttle weighs (masses) about 100 tonnes empty.

Weight does not equal mass. Weight is what you have when you feel gravity, and it's usually measured in Newtons (N). Mass is an absolute number that never changes, except when you're on a diet or drink too much, and it's measured in kilograms (kg).

If you go to the Moon, your weight will be one-sixth what it normally is. But your mass does not change. Scientists always use mass when they talk about science, because saying "weight" is confusing. Non-geeks say weigh, so I'll use "weigh", and "weight" here, too. But I'll use the word "mass" as well just to be clear. Just remember that mass means kilograms, and not pounds (lbs) or Newtons, and it doesn't change in low gravity.

· Now that we have the speed we want to go, and the mass of the space shuttle, we use the kinetic energy equation to find out how much energy we need.

Kinetic Energy = 1/2 x mass (weight) x velocity²

OR

(0.5) x (100,000kg) x (9,000m/s)² = 4,050,000,000,000 Joules of energy

You would have to run a dishwasher for 24 hours a day, 365 days a year, for the next 1,991 years to use up that much energy.

Or eat 1,844,262,295 Big Mac hamburgers.

That is a lot of energy, but the cool thing is that once we get to orbit, we have a lot of kinetic energy, so we don’t need that much more to go to another planet. The problem is that although we only need a bit more speed to go to Mars, we need to carry fuel with us.

The Rocket Equation

Here’s the problem with rockets.

The faster we want a rocket to go, the more fuel we need to get it going. Makes sense?

Okay, but once more fuel has been added to make it go faster, the rocket gets heavier, right? So 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, one of the more famous virgins in history, Sir Isaac Newton, designed more than just the gravity cannon. He helped to develop a new type of math, called calculus, and rocket engineers use it to find out how much fuel they need to go a certain speed. We won't use any calculus here, because it's too hard and only Math professors like it, but here is what the Rocket Equation looks like.

Thank you, NASA!

Heavy, eh?

'The Rocket Equation' is the calculus equation we use to find the answer. The Rocket Equation tells us that the full fuel tanks of Space Shuttle have to weigh 19 times more than the spaceship itself. To get higher we need a more fuel than that, and therefore bigger and heavier fuel tanks.

Look at the Apollo program which sent people to the Moon in the 1960’s and 1970’s. The Apollo rocket (called Saturn V) weighed 3000 tonnes at lift-off and the Apollo capsule weighed only 20 tonnes. So the fuel plus fuel tanks weighed 150 times more than tiny little part where the astronauts sat. See the little red circle in the picture below? That's how big the capsule was.

I love you, NASA

Why does it matter how heavy a ship is? Because fuel costs money. And bigger spaceships cost more than smaller space ships. And a hundred other things become more expensive when a rocket is really heavy. We want cheap rockets, not expensive ones.

To solve this problem, we have to make the spacecraft weigh a lot less, or find a more efficient fuel, or find a more efficient way of burning the fuel, or find a completely new way of space travel. We just can't afford these heavy, expensive rockets.

Can we lose weight?

The problem with losing weight is that there isn't much left to lose. Fuel can't be reduced, life support can't be reduced, scientific equipment can't be reduced or we lose our only convincing reason for playing about up there, and robots suck. I want to go to space, I don't want to send some stupid automatic satellite. We're talking about manned space travel here! We must look elsewhere.

Can we find a better fuel?

There is really no way to make conventional fuel more efficient without discovering a new exotic substance. Efficiency rates - how much energy we get from the fuel - are pretty much at a maximum. Making the rocket do more work by designing better nozzles and stuff helps a little bit, but the general consensus is that conventional fuel is a dead end.

So how do we lower costs?

One way to lower the cost of launches using conventional propulsion is to start launching rockets all the time. The more of something you make the cheaper it is to make each one. This is called “Economy of Scale”, and it’s the reason why McDonald’s is so cheap.

The big problem with this argument is that there are no motels in Space. There’s really no reason for anyone to go up there. And there isn't much to do up there except deal with motion sickness and take pictures of floating liquid.

So, until there is a reason for the general public to fly to space, there doesn't seem to be much chance of companies building more rockets to try to take advantage of lowered costs. Maybe we need to find a new technology?

There must be another way...

If we can't make normal rockets cheap, we need to look at different technology. Here are a couple of ideas, old and new, for you to check out. These are by no means all of the propulsion ideas being researched, these are just the ones I like.

· PROJECT ORION (USA):

This was a really neat idea from the 1950's that involved dropping Hydrogen-bombs behind the ship, and then detonating them. The ship would then kind of surf on the energy from the hydrogen bombs. Of course, as you can see from the picture below, the whole idea is insane.

Stolen from NuclearSpace.com

This ship never flew, nor was it even built, but it's still theoretically the fastest and most efficient ship design anyone has come up with. Successful tests were done on the idea using small conventional bombs, but unfortunately, until people stop caring about unimportant things like nuclear fallout drifting into their Cheerios, this option will remain on the drawing board.

These are non-nuclear Cheerios.

· Ion Engines:

Take a few tiny little pieces of ionized Xenon, accelerate them to enormous speeds with a grid of opposite charge, et voila, an engine that exerts the same force as a piece of paper sitting on your hand.

Sheesh, NASA, can't you think of an easier way to describe this stuff?

It's not exactly powerful, and it can't be used to get off the planet.

There are two problems. First, on Earth, a spacecraft has to fight off gravity and air resistance to get into space. The force of this engine is so small that the spacecraft wouldn't move at all. And the second problem is that the engine wouldn't even start on Earth. It only works in a vacuum. In the atmosphere, particles in the air mix with the fuel and stop the engine from working.

In space though, there is nothing slowing the spacecraft down (like air resistance) so it eventually speeds up.

The Deep Space 1 (DS1) probe had an ion engine as its primary propulsion.

·VASIMR:This idea involves high temperature plasmas. It uses less fuel, has a low overall dry mass, and makes you go REALLY fast. 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. 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, but it also might be that it works really well and the US Government doesn't want anyone else getting their filthy hands on it.

· 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 with three heads and tentacles. That is the idea of the solar sail.

Here's a drawing of a NASA concept sail courtesy of the APOD website.

It works by unfurling a gigantic sail made of mylar (a fancy plastic) and uses the wind of particles that blows off the Sun. It is slow, but it is pretty cheap and looks absolutely fantastic. Beautiful even. Of course, because of something called the inverse square law, we can't use it when we get too far from a star. Yes, it is useful, and beautiful, but it has its limitations.

· The Space Elevator : The space elevator will come as soon as we work the kinks out of carbon nanotechnology. Unfortunately, that looks like it's going to take a while. The elevator basically climbs up a really long piece of burnt rope that hangs from space. It is just as fantastically difficult to realise as it seems, but it does offer an option. It is also fantastically expensive. It could cost hundreds of trillions of dollars. But man, it looks cool.

Courtesy of the Christian Science Monitor, but I couldn't figure out if they drew it or borrowed it.

· 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.

So what's the conclusion?

This article isn't exactly a full treatment of the subject, but I think we covered the basic ideas.

The conclusion is that we don't really have a conclusion. Space research is a constantly changing field. Every year new rockets are developed, and every year new wierd and wonderful ideas are thought up to get us into space. Some are feasible, some are not, but all of them move the field a little bit further forward.

One answer that a lot of rocket engineers like is the hybrid space ship. Basically it is a space ship that has many different types of propulsion on it for the different environments it goes through.

For example, a space ship could launch like the Space Shuttle, use ion engines to get away from Earth, use a solar sail to get to Jupiter, and then use a VASIMR engine to get to Pluto. It would look pretty cool, though this kind of space ship has problems as well, mostly engineering ones. And engineering problems equal money problems, so I'm not sure a hybrid design is really much of an answer at all.

In regards to popularizing space travel, I can say one thing. Barring the complete destruction of humanity, manned spaceflight will one day become more commonplace. How much it will cost to go to the Moon Hotel, I have no idea, but one day, you will be able to stay there.