Main Page

N-Prize reflections

This web site aims to gather to gather my researches in the field of astronautics, rocketry and other launch technologies that can be used for the N-Prize competition. It is not an official web site for the N-Prize. The official web site is here: http://www.n-prize.com/ . The goal of the competition is roughly to reproduce the great achievement of the Sputnik in 1957, but for a 20g satellite and less than £1000. I'm not part of a team, nor did I register a team, because I don't really have the expertise to actually build it in time before the deadline of the project in september 2011.

How to escape from Earth?

Rockets have been used for 50 years to escape the gravity of earth. They are good for three things: create an important thrust, go fast, and burn a lot of ergols. Indeed, the efficiency of a propulsion engine is measured with a specific impulse (I_sp), and for rocket engines, it is quite low. However, they are the only engines that provide the sufficient thrust to climb up with large speeds and to tear of Earth's gravity.

Besides altitude, speed is the most important factor when trying to put an object into orbit. Without it, satellites would fall back down on Earth, even if you climb up at 200 miles. Once again, rocket engines, with their high thrust power can achieve sufficient speed before falling back on Earth.

Rocket trajectories generally roughly form a square angle, with the beginning of the flight is orthogonal to Earth and the final direction is parallel to Earth's surface. The reason is that since they achieve ultra-sonic speeds very quickly, the air pressure on their body (mainly the fairing) is quite important. It is more efficient to first escape the atmosphere and then change trajectory to gain the horizontal speed needed for orbital injection without being slowed down by atmospheric friction.

Rocket trajectory: initial vector is vertical, final is tangent

That particular point of the cost of escaping the atmosphere made me thought about using an aircraft to launch a rocket from the upper atmosphere, reducing considerably the air pressure, the drag, and improving trajectory and efficiency. Moreover, the specific impulse of a turbofan is around ten times greater than the Isp of a rocket engine, since it uses oxygen from the atmosphere to burn its fuel, and not some embedded oxidizer. The fact that it uses a turbo design also has a great impact on the improvement of efficiency. For the N-Prize, the cost of the aircraft could be deducted from the overall price since it would be reused.

I started searching and I found out that Orbital already has developped an air-to-orbit launch vehicle, called the Pegasus. It is able to push onto Low Earth Orbit a payload up to 1,000 lbs (450 kg), and it is launched from a full-sized airplane. My goal is thus to study the feasability of something similar, at low price, even for the aircraft. A rocket would still be used for air-to-orbit link because nothing else is able to achieve a speed around 9 km/s before falling back on Earth. Some specific technologies can be used to improve efficiency, we'll see them below in the rocket section.

Several teams are working on using Helium or Hydrogen balloons (rockoons) to get to the high atmosphere, around 35km and then launch a rocket. It is a nice solution too, and maybe less expensive in the overall, but balloons are not reusable, suffer from imprecise trajectory due to winds, and provide no initial speed.

Single stage to orbit (SSTO) are also a promising research field for low cost orbiting. This one, here captured at SpaceUP, doesn't even allow attitude control out of atmosphere to avoid expensive guidance actuators. The main idea of SSTO is that the launch system (rocket) is the payload. It does not aim to insert a smaller satellite into orbit.

The aircraft

Some aircrafts have been exploring the high atmosphere, around 30km high. Contrary to what one would assume, high flight speeds are not needed. The Helios for example, autonomous solar powered aircraft, flights at this altitude at 20km/h. John Powell [1] is also researching on high altitude propellers and plans to make it to space using a high altitude base for payload transfer to a bigger plane. He describes it well in this video interview. The U-2 is a manned reconnaissance aircraft flying at 21km altitude, but crusing at relatively high speeds (690km/h). Those planes are designed with a very long wingspan, and low weight, similar to gliders. Nevertheless, we would benefit from high speeds of the aircraft, speed that wouldn't be needed by the rocket to reach. It is low speed compared to orbital speed though.

Can electricity energy be considered for that kind of mission ? If not, what fuel should be used, kerozene, alcohol, E85?

Anyway, the main issue with the aircraft is: how to build a £100 turbofan?

Staging and recovery

Separation from the rocket is a big concern. If wings are directly mounted on the rocket body and jettisoned, they would not need some guidance or attitude control electronics, just a basic parachute system. If the wings are able to fly without the rocket, two guidance systems are needed: one for the rocket and one to get back the aircraft in one piece for future launches.

Guidance

GPS can probably be used in the plane for position tracking. Other sensors should be shared with the rocket's onboard computer.

Sun position can be a very good and easy indicator of attitude, and earth curve recognition from 30km altitude can be used for that too.

See the page on the embedded computer.

The rocket

Fuel

Ergols represent the most important part of the weight of what we have to launch. It should thus be chosen carefully regarding to its cost.

Alcohol has been used in the early ages of rocketry, in the german V-2 for examples. It has the advantages to be cheap, and burns quite well. It is not pure, generally used between 75 an 90 percent of volume ratio with water for the rest. The loss of weight due to that water is often a good thing because it burns producing so much heat that the water can keep the engine cool enough to survive. Rocket-grade kerozene (RP-1) has been introduced later to replace alcohol, providing a beter volume efficiency.

To my eyes, alcohol seems to be a very good low cost solution. RP-1 is still used nowadays, and is only 20% more efficient than alcohol with a liquid oxygen (LOX) oxidizer. The next question is thus: should we use some pure alcohol, alcohol/water blend or alcohol/something else blend?

I believe that E85, a 85 percent alcohol and 15 percent gasoline fuel recently used in automotive, is promising. I think that it's efficiency will be slightly better than alcohol, still being very cheap, around £0.5 a liter.

Oxidizer

Liquid Oxygen (LOX) is the obvious/best choice for high Isp. However, it has tons of drawbacks because of the need for cryogenics storage, manipulation, and engine design, that make it quite expensive and too much complicated.

Other leads should then be explored, like Nitrous oxide.

Hydrogen peroxide would even be better, since it's more dense, but it seems complicated and expensive to have it manufactured at a high concentration.

Engine

Aerospike engines should be seriously considered, although they are more efficient than bell shaped nozzles at low altitudes and that we want to launch from high altitude. Webpage on nozzle design.

For the cooling, I only see regenerative cooling as an option.

Trajectory

The trajectory has to be precise enough to get a launch authorization for a specific orbit. I believe that simple cameras can be used on the rocket to determine position of the sun and the Earth's horizon. Accelerometers, digital gyroscopes and a compass are really cheap nowadays and can be used too.

Anyway, if sensors are available, actuators are different story. I see only two possibilities, as fins won't have any impact in the vacuum of space: the rocket engine has to be directionally controllable or control jets must be used to control the attitude of the rocket. Both case imply lot of complications of the rocket's hardware.

The satellite

Lots of strategies for the tracking of the satellite have been exposed: flashing device, radioactive, narrowband EM emitting, mirrors...