Goal: To understand the limitations to the current method of space travel Objectives: To understand how and why rockets work To learn about why rockets have a limitation on the speed they can go To understand how the size of a rocket affects its cost

Newton’s 3rd law For every action there is an equal and opposite reaction. If I push you then you push me back automatically. If you are on the side of a pool and you push the side it pushes back and you move away from the side.

Application to rockets The following will be a bit in depth… In the rocket (depending on which one) you have 2 separated materials. One is either ammonium perchlorate or oxygen. The 2nd is either aluminum or hydrogen

Ignition The two substances are brought together (to make either Aluminum oxide or water) This chemical reaction heats the material making it hotter.

The purpose of heat Heating a fluid increases its PRESSURE Pressure is how much the fluid pushes on its surroundings. This creates a force. Force = difference in pressure * area

The result The rocket exerts a pressure force on the fluid/gas which pushes it downwards out of the end of the rocket. Newton’s 3rd law now comes into effect and the fluid/gas now exerts an equal force but in the opposite direction. If the force exceeds the weight (gravitational force) of the rocket then the rocket lifts off!

What is the catch? Well the catch is that you have to limit how quickly you can lift off. If you push the rocket too fast it will either blow up (you are on a guided bomb after all) or you will kill the passengers.

And so You don’t throw out all the gas at once. So, the initial gas has to move the entire rocket. The next part has to move the rocket minus the gas it has lost so far. And so forth.

The result Most of a spacecraft is FUEL, and not payload. Also, since the gas can only be shot out at a certain speed….

Momentum conservation The momentum (mass * velocity) the spacecraft gets Cannot exceed the momentum given to the fuel in the opposite direction. Since a lot of the fuel is moved with the craft and the craft at the end will move 10 times faster than the fuel can be pushed a lot of the fuel ends up with a velocity in the same direction as the craft, but a bit slower. Ooops…

Speed limit This creates an upper limit to the speed of a rocket. You want to go 2 times faster it takes about 10 times more fuel from a math stand point (so you put a rocket on a much bigger rocket) This gets COSTLY

Space Shuttle Goes into low Earth orbit (8 km/s) Cost: $450 million (from NASA) To go to the moon, simply, would take a about 30% faster velocity. Fuel mass = Spacecraft mass * e(Vcraft/Vfuel) So, if for Vcraft to be 30% more the fuel mass needs to be about 3 times greater. Cost: $1.3 billion.

Mars To get to mars we need about double the velocity than to get into orbit around the earth. That means 15 times more fuel at a cost of $7 billion And if you want to get back, well you need to make it even bigger (another 10 times bigger, so now we are up to $70 billion for just the rocket).

And so… The current method of space travel is sadly limited to our inner solar system. If we want to go anywhere other than low earth orbit or to the moon then we need to either: 1) spend a LOT of money 2) use a new form of propulsion

Quick question: If this is so bad for outer solar system exploration then how were we able to send probes to the outer solar system (such as Voyager, New Horizons, ect)?

Solution Robotic probes don’t need to be as big. They don’t need to keep air or food or water They don’t have humans on them They don’t need to come back So, they can be a lot smaller and a lot lighter. Space shuttle fully loaded: 120 tons New Horizons probe: 0.5 tons

Gravity assist If a spacecraft flies behind a planet (in the planets direction of orbit around the sun) then the gravity of the planet will give the craft a boost Example, New Horizons got a 4 km/s boost from Jupiter.

Conclusion The current form of rocketry is useful for inner solar system However, it is limited by speed and by cost This makes human space travel using this method limited at best and therefore we need other designs if we want to go to the outer solar system and beyond.