From: Johnson Space Center
Posted: Saturday, August 2, 2003
This week I decided I'd write about some exciting new developments that could really change the whole field of space flight. The first has to do with getting off the planet cheaper, which is the necessary first step that could truly make space flight commonplace. The second has to do with getting from low Earth orbit (where we are today) to further destinations like Mars, the moon, and asteroids.
The place to go find out about new and less expensive ways to get into space is Oshkosh, Wisconsin, where the annual Experimental Aircraft Association fly-in is being held this week. It is a great place to see the latest in aviation - thousands of aircraft and hundreds of thousands of people are at this moment converging on Oshkosh. The theme of the show this year is the Centennial of Flight - to mark the 100th anniversary of the first powered airplane flight by the Wright Brothers at Kitty Hawk.
Every year, our friends Pete and Tom set up a campsite on the field. We have a big group of aviation enthusiasts who fly in from all over the place to camp out at what we refer to as Pete-n-Tom's Flights End CafÈ. This will be the first time in seven years that I have missed going to Oshkosh, but I do have a good excuse. My goal is to take a photograph of the thousands of aircraft that will be parked on the airfield during the airshow. I hope the weather cooperates.
Like many of the astronauts, I got my start flying small general aviation airplanes. One of my favorite things to do back home is fly around in my little two-seat single-engine airplane. My airplane is a kit-built airplane, as are many airplanes nowadays. The fact that someone can go out and build their own airplane, or go down to their local airport and take flying lessons is what drives a lot of the innovation in aviation today. Of course you can always just buy a ticket and fly on an airline, too.
I hope that someday flying in space becomes as commonplace as getting on an airplane. Everybody should get a chance to see this view!
Before that can happen though, the cost and complexity of launching things into space will have to come down. A lot of groups and small companies are working on just that, and I think that is a very good thing. None of these groups has actually made an attempt yet at launching into space, so we'll have to see how this plays out.
One thing for certain though is that there is a tremendous amount of creativity and innovation out there, as anyone who has been to Oshkosh can testify.
What most of these groups are trying to do is build a ship that can reach a speed of about 3,000 MPH, which is enough for that ship to coast up out of the atmosphere to an altitude of around 60 miles. Remember that to get into orbit around the Earth, besides just getting out of the atmosphere, you have to attain a speed of around 18,000 MPH. That means none of these privately built ships is going to reach orbit at first, but rather will coast up into space, and then gravity will pull them back down to Earth - hopefully with a soft landing. But at least they will be able to say they have been to space, and they'll get a quick taste of our view up here. It's a start. When private individuals and small companies can reach space on their own, we will really start to see space flight change our lives in the way that aviation has.
Once you have gotten off the surface of the Earth, the next step is getting out of orbit to further destinations like Mars, the moon, and asteroids.
Here is where the Space Station can really help us, as a test platform for the two key technologies we need to really get out and explore the solar system, power and propulsion. Propulsion because everything depends on how fast you can go, and power because something needs to run the engines you are using for propulsion. Huge amounts of power will be required to conduct science and research once you get to your destination.
The number that tells you how fast a spacecraft can go is called the "delta V" capability, which means the amount you can change velocity using your own on-board fuel. The delta V capability is all important because the bigger it is, the more places you can reach and the faster you can get there. To calculate the delta V capability of a spacecraft all you have to know is what percentage of the mass of the ship is fuel, and how fast the rocket engines spit the fuel out the back.
For example, on board the Space Station we have almost 4 tons of fuel, which is used when we periodically fire small rocket engines to boost our orbit higher as our orbit slowly decays (due to the slight atmospheric drag). The Station has a mass of about 200 tons, so that means a little bit less than 2 percent of our mass is comprised of fuel. When we burn the fuel in our rocket engines, the fuel becomes a gas that is ejected out the back end of the engine at a speed of about -3.5 kilometers per second, which provides the thrust to accelerate the Space Station in the opposite direction (Newton's laws again).
The faster I eject the fuel and the more fuel I have, obviously the faster I can go. I can demonstrate this law (known as conservation of momentum) by floating in the middle of the module and throwing some object (say a bag). I end up flying in the opposite direction - but not at the same speed as the bag. If the bag is say one-tenth my mass, then I will only react backwards at one-tenth the speed of the bag. If the bag weighs as much as I do (we have bags of water that are almost that big), then if I throw that bag our resultant speeds are pretty closeÖ in fact they would be identical if we had exactly the same mass. Think of the bags as fuel, which is just something you throw backwards to make yourself go forwards. If we were to burn all our fuel on the Space Station, we would only be ejecting about 2 percent of our total mass (the 4 tons of fuel), and therefore the Space Station will only be accelerated by about 2 percent of the speed of which we exhausted the gas, or in other words about 70 meters per second (or about 150 MPH).
The 150 MPH we would gain would be a fairly small change in our total speed (compared to our orbital speed of nearly 18,000 MPH) and would boost our orbit only about 60 miles higher. What this means is that the delta V capability of the Space Station is sufficient to raise our orbit from 240 miles above the surface of the Earth to about 300 miles. That means the Space Station is pretty much going to stay in low Earth orbit, which is where it was meant to be anyhow.
The delta V capabilities of the Space Shuttle and the Soyuz are somewhat larger, both being about 600 MPH. Of course, I am talking here about how much additional speed they can attain after they have already reached orbital speed and the main booster rockets have burned out. The reason that these very different looking ships have such similar capabilities is that both were designed to be able to reach the altitude of an orbiting Space Station and then return back to the ground. A 600 MPH delta V capability is sufficient for that, but not much more.
So how fast do we want to go?
In order to fly to the moon, asteroids, or any of the other planets in our solar system requires escaping the Earth's gravitational field. The speed it takes to do that depends on how far away from the Earth you are, but very close to the Earth (as we are) you need to be going about 25,000 MPH, which is about 7,000 MPH more than our orbital speed. So once you have reached low Earth orbit, you need to be able to boost yourself by at least another 7,000 MPH just to leave the vicinity of the Earth.
That means the Soyuz or the Space Shuttle, with a delta V capability of about 600 MPH is over a factor of 10 less than the 7000 MPH needed.
The escape velocity of 25,000 MPH is the bare minimum speed necessary to really explore our solar system. Anything less than that means you will remain in Earth orbit. And obviously, the faster you can go the less time it takes to get to your destination (which is important since the solar system is huge - our interplanetary probes take months or years to get to their destinations).
If you want to have people on these ships, it starts to get impractical if the voyages take more than a few months in transit. Even more delta V capability is necessary if you plan to actually stop at your destination and return back to Earth.
Just like flying across the module here - you first push off on one side, coast across the module, and stop yourself on the far side. If you want to return, you have to push off again and fly back, and stop yourself again at your original point. Every time a spacecraft has to use its rockets to speed up or slow down, it uses up precious fuel. What this all means is that a spacecraft that has a delta V capability, after reaching orbit, of about 15,000 MPH is what is needed to really start some exploration of our Solar System. So we are talking about huge increases in performance since this is about a factor of 25 more than our current capabilities.
We do have small, unmanned spacecraft that have delta V capability enough to get them a bit above escape velocity so we can send them to other planets. These probes pick up almost all their speed in the first few minutes after launch, then they coast for the remaining months or years to get to their destinations. That means we are currently able to send spacecraft to other planets, but just barely. We have to wait until certain times when the planets are properly aligned and resort to techniques like "slingshotting" our probes from one planet to the next to pick up additional speed.
This situation is a bit like aviation 150 years ago, which at the time consisted of ballooning. While ballooning was technically flying, it really wasn't useful as a mode of transportation since you were at the mercy of the winds, and you couldn't carry much. If the wind was not blowing the way you wanted to take your balloon, then you had to wait for another day.
The advent of powered flight, i.e., airplanes, made aviation truly useful. From that point on you could fly to your location regardless of the winds, you could carry cargo, and fly more or less whenever you wanted.
What we must do if we want to really open up the solar system is to find a way to get our spacecraft to go very much faster, so we can fly where we want, when we want, and carry lots of stuff. You can see that there are two ways to get more delta V capability: either carry more fuel (it's like carrying more bags to throw); or find a way to exhaust your fuel at a higher speed (like throwing the bags faster). The first solution works up to a point since you can only carry so much fuel. The second solution works provided you can find a way to expel the fuel at a higher speed.
How much fuel can you carry?
If you keep putting larger and larger fuel tanks inside a spacecraft, you have to remove something else of equal weight to make room, and pretty soon the spaceship is almost all fuel. In fact that is the situation with all current spacecraft, if you count the booster rockets used to get them into space. For example, the Space Shuttle and its boosters on the launch pad weighs about 5 million pounds, of which only about 5 percent is the actual Shuttle - most of the remaining 95 percent of the liftoff weight is just fuel. The same is true of the Soyuz. In fact most of the fuel is used just pushing around the rest of the fuel we are carrying!
Think of it this way, suppose I am throwing bags to "rocket-propel" myself. If I am carrying one bag, which weighs as much as me, and I throw it, I will gain speed equal to the speed at which I threw the bag. What if I want to go twice as fast? I need to throw another bag. But to do that I first need 2 additional bags to accelerate me together with the final bag I wish to throw. How about 3 times as fast? Then I need 8 bags, first I throw 4 to accelerate myself and 3 bags, then I throw 2 to accelerate myself and one bag, then I throw the final bag. The number of bags, and thus the total mass of me plus my fuel, is exponentially growing with how fast I wish to fly.
So the delta V capability of a rocket is pretty much limited to a few times the speed at which you expel fuel since otherwise the rocket would have to be enormous (and almost entirely full of fuel), with a small fraction being actual useful cargo. For those of you who have mortgages, it is exactly the same formula as compound interest, and the situation is the same as paying off a loan stretched out over too many years. At first almost all of your payments are just covering the interest, with very little going to capital. By the time you are finished paying off the loan, the original amount you borrowed can be a small percentage of what you paid back in total.
So we are left with spitting out the rocket exhaust at higher velocity. That turns out to be just a question of chemistry - how much energy can be contained in the fuel. As long as your rocket burns some kind of fuel, you won't get much more than about 12,000 MPH for an exhaust velocity, with more typical values around 10,000 MPH. If you want to get 15,000 MPH out of a ship, you will be a flying gas tank, and the size of the actual useful part of the ship will be tiny (a few tons at most). That's one of the major challenges in sending a manned spacecraft to other planets - they have to be large enough to accommodate humans and all the equipment needed to keep them alive.
Luckily, there is another way - instead of burning the fuel, we can use electrical energy to accelerate it out the back of the rocket.
These types of engines are known as plasma or ion engines, and have exhaust velocities as much as 20 times greater than any chemically-powered rocket. Of course that comes with a price, they use a lot of electricity. To date, these engines are quite small, and don't have a lot of thrust, but they get great gas mileage. In fact, none of currently tested engines has thrust greater than a few ounces! While that isn't much, they use fuel at such a low rate that they can run continuously for many months. Since these engines have such tiny thrust, they are of no use for lifting something off the ground into orbit. That's why conventional rockets are still important.
Once in orbit the situation is different. The ship is now weightless and you can take your time and gradually accelerate the spacecraft to high speed. It's the classic case of the tortoise beating the hare. Up here when moving very heavy objects by hand we use the same principle - rather than pushing very hard, you just apply a very light touch for a longer time and you can move objects weighing hundreds of pounds.
We have already flown a spacecraft called Deep Space 1 that used an ion engine to rendezvous with a comet. There are a number of research groups working on making these engines much larger so they will be practical for much larger spacecraft.
So once you have a big plasma engine, how do you power it? Up here on the Space Station we use solar cells for our power, but they are big and bulky (when finished, our truss-work supporting the arrays will be the size of a football field). And, of course, the further away from the sun you travel, the less light you have and the less power you get from the solar cells. The amount of power that would be needed is hundreds of kilowatts for smaller unmanned spacecraft, and many megawatts for larger manned ships. Even the huge solar arrays of the Space Station couldn't come close to providing that much power.
So that means you need a nuclear reactor to provide electricity for your rocket engine. These reactors would be very small by comparison with power plant reactors, and quite a bit simpler. The nice part is that the reactor fuel can be launched "cold" - meaning essentially non-radioactive. The fuel can then be activated in the reactor once off the planet. This way if there is a launch accident there is no risk of spilling radioactive material, since the fuel has not yet been activated. As an added bonus, by having a reactor on board you also have plenty of power to run all the instruments and systems on the spacecraft. Again there are several groups working to perfect the design for small nuclear reactors for use on spacecraft.
The combination of a nuclear reactor plus a plasma engine could provide a delta V capability of 30 to 100 thousand MPH. This is plenty for exploring the solar system.
The exciting part is that NASA has decided to start just such a program, known as Project Prometheus. The first proposed test of the system is to fly a probe to Jupiter.
At first these nuclear-electric propulsion systems will be only large enough to power unmanned spacecraft, but if we are successful, then in the future we should be able to scale them up to large enough systems to carry people.
My feeling is that the Space Station is an ideal place to develop and test many of the components before we try them for real on a deep space mission.
Probably the two biggest unknowns are how to convert the heat produced by the reactor into electricity, and how to make ion and plasma engines both larger and run longer. The Space Station could be the experimental laboratory for the development of these crucial technologies. We could use a large solar reflector to concentrate sunlight to test our system of converting heat to electricity. The power generated could power a small plasma engine which continually (but with small thrust) boosts the Space Station to compensate for the small amount of atmospheric drag.
In this way, by incrementally testing each of the components of the final system we will in the end, we will have much greater confidence that it will work the way we expect once we put all the pieces together.
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