So far we've discussed purely theoretical means of traversing the interstellar divide. Wormholes and Warp drives sound great mathematically, but they aren't real world solutions.
Today we are going to discuss an engine that is still theoretical in its scale, but which at least involves technologies that, to some degree or another, have each been shown to physically exist in the universe. Compared to wormholes and warp drives, that’s a big step forward!
First proposed by the physicist Robert Bussard, the ramjet's design relies on an elegant premise which works in a five step process. Refer back to this numbered diagram to follow along.
1. Use A Giant Laser To Ionize Hydrogen Floating In Space Ahead Of The Ship
Bussard's original plan involved a ship with a front end "scoop" shaped like a satellite dish. Except, in order to capture enough hydrogen to undergo fusion and accelerate the ship to high speeds, a purely physical "scoop" would need to cover an area of 10,000 square kilometers while weighing 1,000 tons or less in order to accelerate. New York City is 788 Square Kilometers, so you can see why that's crazy.
The alternative to such an impossibly large physical scoop was to use high powered electromagnetic ("EM") fields to magnetically attract the hydrogen towards the physical scoop from ahead of the ship in space.
The problem is before hydrogen can be attracted to the ship's scoop by the ship's EM field, the hydrogen first has to be ionized, or stripped of its sole electron. Once the hydrogen's sole electron is stripped from it, all that's left is a hydrogen proton, which has a positive charge and is now called ionized hydrogen or a "cation".
All atoms have an ionization energy - an amount of energy needed to remove the most weakly attached electron orbiting the atom. The ionization energy for hydrogen is very high. In order to strip hydrogen of its electron, you need a very powerful ultraviolet laser mounted inside the scoop to shoot into space and ionize the hydrogen atoms. In theory this laser would be swept methodically across space for hundreds or thousands of miles ahead of the ship, ionizing the hydrogen atoms it touches.
2. Use A Giant EM Field To Attract Ionized Hydrogen To The Ship
Once most of the hydrogen in front of the scoop is ionized, you're ready to turn on your EM field generator. The idea is to have the EM field project out from the ship's scoop, covering a gigantic area of space in front of the ship and attracting all the ionized hydrogen in that area toward the ship.
EM fields are created in a variety of ways, and come in a wide range of strengths roughly laid out by the EM spectrum photo above. Radio waves, for instance, are a low energy form of EM field, whereas x-rays are much higher energy.
The exact location on the EM spectrum needed for the Ramjet isn't 100% clear. But in order to attract enough ionized hydrogen out of interstellar space to allow the ship to accelerate, the EM field would have to be many times larger and stronger than any EM field produced by mankind up to this point in human history. It would have to affect an area of interstellar space 1,000s to 10,000s of square kilometers in a cone in front of the ship. Anything less than this and the ship doesn't get enough hydrogen to achieve fusion and accelerate.
3. Physically Capture The Attracted Hydrogen Cations In An Intake Valve In The Center Of The Ship's Scoop
Once the EM field has dragged the nearby hydrogen cations floating around in space inwards toward the front of the ship, those cations are physically captured by the scoop and pulled into an intake valve by a combination of the EM field and the forward motion of the ship. The ship's acceleration pushes the hydrogen cations into a series of spiral tubes which get progressively smaller. As more hydrogen is crammed into a smaller space, the hydrogen speeds up and gains energy until it reaches a critical mass.
4. The Hydrogen Fuses Together In The Fusion Chamber, Creating High Energy Helium
All of that Hydrogen being crammed together in those small spiral tubes eventually forces the Hydrogen atoms to fuse together. Nuclear fusion occurs! Bussard specifically mentions a proton-proton chain reaction should occur, which results in hydrogen being fused together into Helium. Some of the energy from this reaction is siphoned off to run the ship systems.
5. Shoot The Super High Energy Helium Out The Rear Of The Ship, Propelling It Forward Like A Rocket.
The remaining newly fused, high energy helium is spit out the back of the ship, which propels the ship forward like a rocket. As the ship is propelled forward, it collects more Hydrogen which in turn feeds its continued acceleration. Rinse and repeat.
On its face the idea sounds great - harness a functionally unlimited resource to power and accelerate the ship to crazy speeds.
Unfortunately Mr. Bussard probably made some miscalculations
First, there isn't nearly as much free hydrogen just floating around in space as Bussard thought. The result is the need for an even bigger EM field than originally hypothesized. Unfortunately, the original design already called for the largest, strongest EM field in human history by an order of magnitude, and we don't know how to make THAT, let alone an even bigger one.
Second, there are some differences of opinion as to whether the ship would work at all. At least two physicists, Robert Zubrin and Dana Andrews, believe the ship wouldn’t be able to go over 100km per second because at some point the drag created by moving through, collecting, and accelerating the hydrogen fuel becomes greater than the propulsion created by the fusion reaction.
100 km/sec is fast, but would still take over 10,000 years at that speed to get to Alpha Centauri!
Of course, all of this is theoretical. Zubrin and Andrews could be wrong, as could Bussard. Some of the problems with the ship might be solved with the addition of an on board nuclear reactor, or a more conventional initial propulsion system to jump start hydrogen collection. However, no matter what:
- The ship itself would have to be immensely large – one of the largest things ever built by human beings - to collect enough hydrogen to accelerate at all.
- The ship would need a super laser capable of shooting at very high energies for extended periods of time to ionize interstellar hydrogen.
- And the ship would need the largest, strongest EM field ever conceived.
Which is why an actual Bussard Ramjet is still science fiction. Compared to worm holes and warp drives, the ramjet is an order of magnitude closer to real life and most of the principles it relies on have been shown to be physically possible as opposed to just mathematically possible. However, at the same time no one has ever built anything remotely like a Bussard Ramjet before, the cost would be astounding and, in the end, no one is sure it would work.
But hell, at least we’re getting a little closer to real life.
Next Time We Sail To The Stars
Picture Sources:
[1]By NASA [Public domain], via Wikimedia Commons
Information Sources:
https://en.wikipedia.org/wiki/Hydrogen_ion#Anion_.28negatively_charged.29
https://en.wikipedia.org/wiki/Bussard_ramjet
https://en.wikipedia.org/wiki/Electrostatics#Electric_field
https://en.wikipedia.org/wiki/Solar_sail
Hypothetical Spacecraft and Interstellar Travel, By Ezekiel Nygren
http://www.dangermouse.net/gurps/science/ramjet.html
https://en.wikipedia.org/wiki/Ionization_energy
https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction
https://i4is.org/interstellar-ramjets/
https://en.wikipedia.org/wiki/Electromagnetic_f
https://forum.cosmoquest.org/showthread.php?107248-Bussard-ram-ionizing-the-hydrogen
https://www.britannica.com/science/ionization-energy
https://www.angelo.edu/faculty/kboudrea/periodic/trends_ionization_energy.htm
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