Sunday, December 21, 2014

New NASA Propulsion Concept

NASA's Idea to Nearly Replace Rockets
RealClearScience Newton’s Blog,
Posted by Tom Hartsfield December 17, 2014

NASA is facing a problem: chemical rocket engines are about as good as they will ever get by the laws of chemistry and physics. It's becoming increasingly difficult to make them any cheaper or safer, and private companies are now doing much of that work. Embarrassing, physically impossible microwave engine pipe dreams aside, what can NASA do?

When pressed for answers by the Obama administration, NASA engineers proposed something interesting: taking some of the load off of liquid-fueled rockets. The slack is taken up by two other propulsion technologies. The first stage of the takeoff is achieved by the use of a railgun. The second is accomplished via an engine called a scramjet. First, the rail gun launches the craft up to speed. Then, the scramjet takes over and pushes the ship to one third or more of escape velocity. Finally, the traditional rocket engine takes over for the final push to orbit.

The railgun stage is a simple idea. Railguns are powered by electromagnetic physics. Two thick metal rails are connected to each end of a capacitor. The capacitor is an enormous storage cell for electric charge, the "fuel" for this system. Electrical energy is stored in the electrical field of the capacitor by holding positive and negative charges close together but separated. So long as the two areas of the device containing positive and negative charge have no connection to one another the device is ready to fire.

One rail of the gun is hooked to the positive charge area of the capacitor and the other to the negative charge area. When a metal object is placed across the rails, the positive and negative areas are connected by this conductive bridge. A massive bolt of charge is immediately driven through the system, flowing from down one rail, across the bridging projectile and back down the other; the pull of the positively charged capacitor area driving an enormous current of electrons.

Flowing electrical currents produce magnetic fields. The magnetic field produced in each rail is proportional to the amount of current flowing through it. For a huge current, the magnetic field can become incredibly strong. Circling each rail in opposite directions, the two fields add together constructively in the center to produce a strong upward field. A law of nature called the Lorentz force says that a current and magnetic field flowing perpendicularly produce a force in the direction perpendicular to both of them. This Lorentz force pushes the projectile down the track at tremendous speed.

The advantage of a rail gun is that it requires no chemical propellant for its energy. The entire system is powered solely by an electrical generator that produces electrons and stores them in the capacitor. This means the first stage of the rocket will not need to load the craft down with any propellant. The second stage of the system also reduces the need for rocket fuel; the fuel is supplemented by air.

The scramjet stage takes over power at roughly Mach 1.5 . The scramjet is a type of jet engine which operates at much higher velocities. A traditional ramjet engine works by compressing air and creating combustion within it as the air flows through. While the ramjet flows this air at velocities less than the speed of sound, the scramjet produces combustion in a supersonic air flow through the combustion chamber, which is far more efficient.

The simple reason that this technology is needed is that as airspeed increases, the air being forced into the engine is moving at higher and higher velocities. This requires more and more and slowdown to drop back below the speed of sound for combustion. This in turn creates shockwaves. Above speeds near Mach 5, the shockwaves become so strong that they disrupt the airflow into the combustion area and restrict any greater air intake, limiting speed.
Scramjets can easily surpass this limit. Supersonic combustion engines have been tested by NASA in such ships as the X-15, X-43 and X-51. These rocket planes have reached speeds as high as Mach 10, roughly one-third of Earth's escape velocity. The scramjet design is theoretically capable of reaching speeds near 100% of escape velocity. Much more research and experimental testing will need to be performed before the feasibility of those speeds is known.

The challenges of this plan are very clear. First, no railgun vaguely approaching this size has ever been constructed. The Navy has built railguns capable of launching 23-pound projectiles; NASA is talking about launching projectiles of 1000 times that mass, with humans inside! Further, the rails will need to be nearly two miles long, and filling the capacitor will require a 180 megawatt power plant. On the bright side, this is mostly achievable with current technology plus research. However, it would require lots of money, initiative and a significant change in course at NASA.

The scramjet is also challenging. There are no declassified tests of a scramjet engine at speeds of Mach 10 for more than 10 seconds. Flights of even Mach 7 have never exceeded four minutes. How difficult it will be to design an engine that can run faster, longer is not at all clear.

Give NASA some credit for thinking big with this proposal. Now, let's see if they are provided the resources and can muster the gumption to really work on it, or if it's just another pie-in-the-sky dream.

Tom Hartsfield is a physics PhD Candidate at the University of Texas


 

No comments:

Post a Comment