Alternative Propulsion Systems - Pushing For Lightspeed!!

---

Getting Moving.

Something that has always interested me is moving through space, and if we're going on a journey of interstellar distances, well, we better be doing it at pace unless you want to spend tens of thousands of years getting there. Nothing in this universe (as far as we know so far) goes faster than light, so let's look at how we can get there. Or even bend space around us to push past those limitations.

It seems rather silly to go for high speed with no method of slowing down again, so let's cover that for a moment before diving into hitting the 'Go Baby Go' button.

There are a number of methods to slow down a space craft, and here are a couple for you.

• Aerobraking - using interstellar dust and debris or atmospheric interaction to create friction against the craft, reducing its speed. You could even use solar wind to brake your craft with a reversed solar sail. Or within an atmosphere by deploying parachutes or balloons to slow you down. Even a heat-shield can be considered an aerobrake as it uses friction from the air to slow you down and keep your craft safe from the elevated temperatures.

• Lithobraking - essentially running into the ground, this is generally used as a last method of braking as you meet the ground, ensuring you have a strong enough craft and bodies able to resist the impact is required.

• Retrothrusters - either by turning your craft around and using your main thrusters to apply reverse thrust, or by carrying thrusters mounted in the opposing direction of your craft to slow you down. This is a rather traditional method of scrubbing off speed.

Now that we have discussed a few methods of slowing down, let's look at going fast! (My favorite bit).

Nuclear Power!!

In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature by a nuclear reactor, and then expands through the rocket nozzle to create thrust.

In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant in a traditional rocket.

The thermal heater / inert propellant compared to the reactive propellants of a traditional rocket produces a superior exhaust velocity, and therefore has a higher efficiency, with specific impulses up to twice that of chemical engines.

The overall lift-off mass of a nuclear rocket is about half that of a chemical rocket, and when used as an upper stage it roughly doubles or triples the payload carried to orbit.

This means less propellant and engine mass required and more payload. Good if we are going on a long journey. A engine such as is envisaged above could reach speeds of up to 6,700 m/s given current technology.

Ion Drives and Plasma Thrust.

VASIMR, which is an Electro-thermal Magneto-plasma Rocket, uses radio waves to excite and heat the propellant, which is then accelerated with magnetic fields to generate thrust.

It has been described as an electrodeless version of an arcjet rocket that can reach higher propellant temperature and therefore efficiency and velocity by limiting the heat transfer from the plasma to the structure of the engine.

Both types of engine have no electrodes. This eliminates the erosion that shortens the life of other ion thruster designs. Since every part of a VASIMR engine is magnetically shielded and does not directly contact plasma. So the durability of this particular engine will be greater than many other ion/plasma engines.

Why have the ability to go fast if you cannot sustain it for a decent duration? We have a need to spread across the stars, and journeys take a very long time. So better have an engine that lasts the trip!

Pushing Light?

A laser-pushed lightsail is a thin reflective sail similar to a solar sail, where the sail is being pushed by a laser, rather than the sun. This could be achieved by ground based or orbital platforms.

The advantage of lightsail propulsion is that the vehicle does not carry either the energy source or the reaction mass for propulsion, and so the limitations of the Tsiolkovsky rocket equation to achieving high velocities are avoided.

Use of a laser-pushed lightsail was proposed as a method of Interstellar travel that would avoid extremely high mass ratios by not carrying fuel. Allowing massive payloads for those long distance hops.

Light On board.

A photonic laser thruster is a concept for space propulsion that works on the principle of a photon-pushed sail, generating thrust directly from the momentum of a photon by use of a mirror reflected laser.

The concept, differs from other solar sail and laser propulsion concepts in that an amplification process is used, in which the beam is re-used by being reflected from a stationary mirror, with an amplification at each reflection.

Because of the recycling of energy, the photonic laser thruster is claimed to be more efficient than other laser-pushed sail concepts.

The practical usage of the photonic laser thruster for space propulsion would require high laser powers and overcoming technological challenges in achieving the laser power and fabricating the required optics for adjusting the focus and aiming of the beam over the duration of the trip.

Photonic laser thrusters have a very high specific impulse, and can permit spacecraft to reach much higher speeds that approach a high fraction of the speed of light than with conventional rockets which are limited in comparison.

Stuffing The Ram.

The Bussard ramjet is a theoretical method of spacecraft propulsion proposed by the physicist Robert W. Bussard.

He proposed a ramjet variant of a fusion rocket capable of reasonable interstellar travel, using enormous electromagnetic fields as a ram scoop to collect and compress hydrogen from the interstellar medium.

High speeds force the reactive mass into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy as rocket exhaust in the intended direction of travel, accelerating the vessel.

One could even theoretically do a low level flyby of a solar body or a atmosphere that is fusible and scrape a portion of the fuel from the surface. By collecting your fuel as you travel, this allows for very long duration's of rocket burn, allowing for very high payloads and long distances traveled.

Capturing Black Holes?

A black hole starship is a theoretical idea for enabling interstellar travel by propelling a starship using a black hole as the energy source.

The concept was first discussed in science fiction, notably in the book Imperial Earth by Arthur C. Clarke, and in the work of Charles Sheffield, in which energy extracted from a black hole is described as powering the rockets engines.

In a more detailed analysis, a proposal to create an artificial black hole and using a parabolic reflector to reflect its Hawking radiation has been discussed in 2009 by Louis Crane and Shawn Westmoreland.

Their conclusion was that it was on the edge of possibility, but that quantum gravity effects that are presently unknown will either make it easier, or make it impossible.

Anti-matter - Big Booms?!

An antimatter rocket is a proposed class of rockets that use antimatter as their power source. There are several designs that attempt to accomplish this.

The advantage to this class of rocket is that a large fraction of the rest mass of a matter/antimatter mixture may be converted to energy, allowing antimatter rockets to have a far higher energy density and specific impulse than any other proposed class of rocket.

Antimatter rockets can be divided into three types of application: those that directly use the products of antimatter annihilation for propulsion. Those that heat a working fluid or an intermediate material which is then used for propulsion. And those that heat a working fluid to generate electricity for electric spacecraft propulsion. Similar to water in traditional nuclear reactors or molten salt/steam in some high output solar farms.

The alternatives to direct antimatter annihilation propulsion offer the possibility of vehicles with vastly smaller amounts of antimatter but require a lot more matter propellant. There are also hybrid solutions using antimatter to catalyze fission/fusion reactions for propulsion.

Due to the matter/antimatter reaction utilizing close to 100% of the energy stored in the propellant for output, it allows for speeds previously unheard of and efficiencies that are absolutely mind blowing. Perfect for traveling between the stars.

Prepare for Warpspeed!!

The Alcubierre drive or Alcubierre warp drive is a speculative idea based on a solution of Einstein's field equations in general relativity as proposed by theoretical physicist Miguel Alcubierre.

Alcubierre theorized that a spacecraft could achieve apparent faster-than-light travel if a energy-density field lower than that of vacuum (that is, negative mass) could be created.

Rather than exceeding the speed of light within space itself, a spacecraft would traverse distances by contracting space in front of it and expanding space behind it, resulting in effective faster-than-light travel.

Objects cannot accelerate to the speed of light within normal spacetime. So instead, the Alcubierre drive shifts space around a craft so that it would arrive at its destination faster than light would in normal space without breaking any physical laws.

The proposed mechanism of the Alcubierre drive implies a negative energy density and therefore requires exotic matter. So if exotic matter with the correct properties can not exist, then the drive could not be constructed.

However, at the close of his original article Alcubierre argued that the Casimir vacuum between parallel plates could fulfill the negative-energy requirement for the Alcubierre drive vastly reducing the energy requirements and the need for exotic materials.

---

All of the above methods of propulsion will accelerate a craft close to or beyond the speed of light, far faster than conventional rocketry, I hope you enjoyed the topic at hand today and I thank you for your time spent dearest readers. If you would like to hear more perhaps we can dip into wormholes at a later date, let me know in the comments below. Much love. <3

Sources for this article include, Nuclear thermal rocket here, VASIMR concept rocket here, Alcubierre drive here, Laser propulsion here, Photonic laser thruster here, Black hole power here, Antimatter rocket here, Black hole power here and the Bussard ramjet here.

---