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