I am a leaf on the wind…watch how I soar.1

—Hoban “Wash” Washburne, pilot of Serenity, a space vessel from the TV series Firefly and the film Serenity

Up, up and away!

—Superman

The word spaceship first appeared in an 1894 novel called A Journey in Other Worlds by Jacob Astor IV.² We waited until 1934 for the word starship to make its debut in Frank K. Kelly's story Star Ship Invincible.³ After that, science fiction was not turning back. It was the moon, Mars, Titan, Proxima b, or bust. This is where the science of rocketry comes in.

Science and science fiction have both come a long way since H. G. Wells used a space gun to shoot a bullet ship to the moon in his story Things to Come. At least his use of a space gun is more scientific than the spaceship he devised for The First Men in the Moon. The spaceship was made out of fictional cavorite, a material discovered by none other than Mr. Cavor, as a means to negate gravity.⁴ If only cavorite were real. Many problems of space travel would be solved, and this book would be a lot shorter.

KEEPING IT LOCAL: ATMOSPHERIC TRAVEL

For planetary travel, our aircrafts rely on the air within the atmosphere. I guess that's why they are called aircrafts. Of course, this assumes that the planet has an atmosphere. For atmospheric travel, three modern types of engines zip across the globe: turbojet, ramjet, and scramjet. All have made appearances in real life and in science fiction.

The more streamlined a jet or car or animal is, the faster it will move given a fixed amount of energy. You should also know that the faster you travel it isn't the friction that will get you (although it does exist), but it will be the air compression that will give you a figurative headache. No matter what you use (bike, boat, or jet plane) or who you are (superhero Flash), movement forward forces air to curve to get out of the way. When the movement is too fast, air molecules can't jump aside fast enough. So they bang into each other, heating up and pushing back.

To go faster through the air (or land or water), the key has always been aerodynamics. At a point near the sound barrier, aerodynamics is no longer enough. The air begins to assert more and more drag, causing extreme turbulence. Traditional turbine engines suck in air and use spinning rotors called turbines to pump the air backward, producing thrust. They will not work at such high speeds because the red-hot blades liquefy.

Here, things get interesting. Unlike turbojets, both ramjets and scramjets (supersonic combustion ramjets) can use that piled-up air without a turbine. No moving parts. They blast air through a specially designed tube and fire it from a nozzle at supersonic speed, producing (in the vernacular of Battlestar Galactica) a fraking lot of thrust. Ram pressure is how ramjets and scramjets got their inspired names.

Ramjet combustion only works at subsonic speed (the combustion chamber can't handle supersonic velocities). They can only fly through an atmosphere at high speeds and rely on booster rockets in the first portion of their flight. The scramjet burns conventional fuel to reach an initial flight speed before the system starts up. After that it can fly at supersonic speeds because the combustion chamber is designed for supersonic airflow.

THE LONG JOURNEY: SPACE TRAVEL

The dinosaurs became extinct because they didn't have a space program. And if we become extinct because we don't have a space program, it'll serve us right!

—Larry Niven

It is now time to leave Earth and go into space. Travel between planets takes a whole different type of technology than atmospheric flights. Just to get off the planet, you need a lot of thrust. Weight matters. A lot. In good science fiction, weight is always a concern.

To escape Earth's gravity, you need to accelerate from the launchpad to 11 km per second (24,606 miles per hour).5 So to get off Earth these days, we fire rockets. Another way we could, if we had the full technology, is to use a cable space elevator. The idea for such a system can be traced to 1960 when Russian scientist Yuri N. Artsutanov, in an interview for Komsomolskaya Pravda, suggested a cable tethered to an orbiting geostationary satellite (a satellite that remains stationary to a fixed point on the earth) with a counterweight extending out into space above the satellite and the other end of the cable lowered and anchored to the earth.⁶ The competing forces of gravity, in theory, result in a taut stationary cable.

If such a system could be made to work, elevator vehicles could carry equipment up into orbit where you build your spacecraft, eliminating the need for large chemical fuel tanks. Lower weight and less fuel? It's a rocket engineer's dream.

Although Arthur C. Clarke didn't think up the idea, he popularized the space elevator to the general public in his novel The Fountains of Paradise.⁷ Today it is a commonly used tool in science fiction. Someday, I hope it will be common on our nonfiction earth.

Without further ado, here are ways for us to do some space travel.

Fig. 17.1. Space elevator diagram. (Wikimedia Creative Commons, author: Skyway, user: Booyabazooka. Licensed under CC BY-SA 1.0.)

CHEMICAL ROCKETS

All types of chemical engines have a combustion chamber where fuel and an oxidizer are combined. Fuel and oxidizers are nothing more than something that burns and something to start the burning. Three types of chemical rockets are in use: solid, liquid, and hybrid. Each has its own advantages and disadvantages.

Solid propellant engine. The fuel and oxidizer are pre-combined into a solid. Once the burn is activated, the engine won't stop until the fuel has been consumed. The amount of thrust is constant, at least until the fuel is expended. The advantages of this over a liquid propellant are that it offers more thrust and is more stable.

Liquid propellant engine. Both the fuel and the oxidizer are held in liquid form in separate tanks. This type of engine has the advantage of allowing control over the amount of thrust. The pilot (human or AI) can control the mix ratio. The disadvantage is that the oxidizer must be kept extremely cold.

Hybrid propellant engine. Solid fuel is in the chamber while liquid oxidizer is stored in a separate tank. This provides the best of both propellants. It is a very stable fuel with controllable thrust. But it also has some of their disadvantages. It isn't easy to replace the solid fuel (as a liquid would be), and the oxidizer-to-fuel ratio must be constantly rebalanced to maintain efficiency.

SOLAR SAILS

Solar sails use the sun for propulsion. Photons shooting from the sun carry energy and momentum. A solar sail captures the momentum and reflects it off, resulting in continuous acceleration.8 While a chemical rocket provides a short burst of thrust to speed up quickly, the solar sail starts off with little acceleration that builds up over time. You can only use a solar sail off planet, so it would be cool to have a space elevator.

A groovy electric version has a sail lined with a spiderweb of wires that can be electrified with a positive electric charge—the same charge as the photons shooting from the solar wind. You guessed it: like charges repel and push the ship forward.⁹

Fig. 17.2. Illustration of a solar sail. (Wikimedia Creative Commons, author: Kevin Gill from Nashua, NH, United States. Licensed under CC BY-SA 2.0.)

NUCLEAR PROPULSION

Chemical engines don't have enough thrust to travel far, and solar sails are good for long distances but need time to accelerate. A nuclear rocket wouldn't suffer from either problem. It uses liquid hydrogen that is heated, rather than ignited, in a nuclear reactor. Boom! (The sound effect is to help you imagine a ship powered by nuclear explosions.)

The advantage is the higher amount of energy relative to the fuel for chemical engines. Nuclear rocket technology is popular in near-future science fiction. The Discovery One ship from Arthur C. Clarke's original story 2001: A Space Odyssey used one. The movie version did not.

In Dan Simmons's novels Illium and Olympos, inhabitants of the far future build nuclear ships to replicate twenty-first-century technology.10 (As to why, go read the books.) In the television miniseries Ascension, the colony ship is Orion-class. (Project Orion, active between 1958 and 1963, studied the use of this type of engine.)

Nuclear rockets do have a problem. They are illegal. The Partial Test Ban Treaty of 1963 (which ended Project Orion) and the Comprehensive Nuclear-Test Ban Treaty of 1996 prohibit aboveground nuclear detonations. Even when launching a rocket from the ground or a low Earth orbit for a peaceful space project, an electromagnetic pulse (EMP) could damage satellites. So this type of rocket would need to be launched away from Earth.

ION ENGINE

Ion propulsion is a technology that involves ionizing a gas to propel a craft. Instead of using standard propulsion chemicals, xenon gas (which is like neon or helium only heavier) is given an electrical charge to ionize it. The gas is then electrically accelerated to a speed of about 30 km/second. When xenon ions are emitted at such high speed as exhaust, they push the spacecraft in the opposite direction.

ANTIMATTER ENGINE

As described in the first interlude, when a particle and its antiparticle meet, their masses are converted into a pair of gamma ray photons with the amount of energy given by Einstein's famous equation E=mc². Put simply: when they meet, they annihilate each other. This fact gives science fiction plausibility for matter/antimatter engines.

With antimatter, a lot of power comes from only a little bit. A study in 2003 showed that with seventeen grams of antimatter, a spacecraft could cross one light-year of space in just a decade.¹¹ There are four problems I can think of that spoil this plan. And all of them have to do with one of my least favorite words—practicality.

For starters, it takes a lot more energy to create antimatter than you can ever get out of it. Then there is the cost. Today, we spend about 62.5 trillion dollars per ounce, which feeds into the next problem—availability.12 Less than twenty nanograms of antimatter have ever been made by humans.

All the antimatter ever made and annihilated at the Conseil Européen pour la Recherche Nucléaire (the European Organization for Nuclear Research, better known as CERN) is only enough to light a single light bulb…for a few minutes.¹³ The antimatter at CERN is mainly used to study the laws of nature. Reasonable fictional solutions include one I used in a short story. The characters used a particle accelerator that circled an entire planet to easily produce antimatter.

The final problem is storage. The property that makes antimatter a great source of energy is the same property that makes it tricky to contain. If you try storing it in anything made up of matter, boom. Instead of attempting to store your antimatter in materials made of normal matter, it might be possible to contain it inside magnetic fields. The science is solid, but the application is difficult.

These hurdles are mountains for scientists but are mere molehills in science fiction. That said, for some reason, I'm still willing to suspend my disbelief on matter/antimatter drives. I'm looking at you, Star Trek universe.

ELECTROMAGNETIC DRIVE

This type of engine converts electrical energy into thrust. For launches from a planet, the huge advantage is the lighter weight because all that heavy fuel is unnecessary. This type of engine uses a magnetron to push microwaves into a resonating cavity (a fancy name for a closed cone). The microwaves bounce back and forth, pushing on the cone's wall. More shoving takes place at the narrow end, which pushes the spacecraft forward.

Not all scientists are convinced that this is possible. One group believes this is as effective as moving your car by pushing the steering wheel, or lifting yourself off the ground with a self-inflicted super wedgie. Other scientists think it might be possible if the microwave field pushes against virtual particles popping in and out of the quantum vacuum.

WARP DRIVE

The inspiration for a warp drive comes from the geometry of general relativity. Did you forget about this? You can find the basics in chapter 1. Spacetime can be warped in a way that will propel a spaceship faster than the speed of light.

Don't worry, we can do this theoretically without breaking any of those pesky laws of physics. As I hope you recall, according to general relativity, the speed limit only applies to objects traveling through space and not space itself. Think back to the metaphoric taut sheet with the dimples caused by objects that lie on it. A warp drive would expand a dimple by adding energy.

Quick reminder (mass/energy equivalency): More energy→ more mass→ more mass pressing into spacetime→ bigger dimple→ more gravity.

The drive would push the sheet behind the starship and create a hill (repulsive antigravity). The starship makes the spacetime in front of it dip downward (creating a gravity well). All that is left is for the ship to slide along. Unfortunately, we can't use this type of drive right now because of the amount of energy necessary to cat scratch spacetime.

There is a speculative solution to the energy problem for the warp drive I described—use negative mass to change the geometry of spacetime. Scientists don't know if negative mass exists, but a lot of math has been done to show its properties. Assuming it does exist and we can manipulate this type of exotic matter, we could expand space behind the starship and contract it in front. The starship would then be able to ride along a wave of flat spacetime called a warp bubble. This type of warp drive was proposed by theoretical physicist Miguel Alcubierre in 1994.¹⁴

GRAVITY AND THE SPACE TRAVELER

Now that you have a taste for space travel, know that astronauts, future space colonists, and people living on the International Space Station need to worry about gravity.

As described in chapter 12, our bodies evolved within Earth's gravitational pull (1g). The prolonged effects of living in a low-gravity environment include muscle damage, bone damage (similar to osteoporosis), loss of red blood cells, and a whole lot more.

A short-term solution for the physical discomfort of space travel is artificial gravity. The long-term answer is adaptation through transhumanism, but for this chapter I'll stick with humans staying as we are today. Isn't this true for most of the characters in science fiction? Here is a list of scientific ways to produce artificial gravity:

1. Take advantage of one of Einstein's greatest discoveries: the gravity-acceleration equivalency. Your starships could constantly accelerate. This is obviously not practical because you would have to constantly accelerate at 1g.

   Acceleration (increasing velocity), not constant velocity, makes you feel gravity. For this to work in science fiction, a starship would need to be orientated to ninety degrees of the direction of thrust. Imagine this orientation for the USS Enterprise.

2. Store a lot of mass on your starship. According to general relativity, more mass means more gravity. If you want to maintain 1g, you need to store something that has the same mass as the earth. Science fiction usually has some kind of gravity plate or generator that converts miscellaneous fuels into synthetic gravity.

   Samuel R. Delany came up with an interesting explanation for gravity plates in the novel Triton. Einstein's special relativity shows that, as an object is accelerated, its mass increases. At our earthly speeds the increase is negligible, but near the speed of light the effects are (literally) massive. Delany had the atomic nuclei of the gravity plate spin in place at near light speed. Even if we forget about all the power necessary for such a device to work, the plate would have the mass of the earth. Good luck pushing that ship through space.

3. By spinning various sections of your starship, your crew could take advantage of centrifugal force. This worked well enough for USS Discovery One in 2001: A Space Odyssey and the Hermes spacecraft in The Martian.
4. Your spacecraft could use a gravitomagnetic field generator. I'm including this even though it's more speculative than science. The idea for the generator was first suggested by Russian engineer Eugene Podkletnov in the 1990s.¹⁵ This device hypothetically produces a gravitational field from the angular accelerations of a spinning superconductor.

Fig. 17.3. Illustration of a spinning wheel and artificial gravity.

IF SCIENCE FICTION (ADVENTURE) USES ARTIFICIAL GRAVITY, THEN WHY NOT…

In science fiction, starships zip all over the place. Sometimes I don't see (or read) about these ships rotating, or continually accelerating, or having a gravity plate, yet somehow they simulate gravity. Come on! Give me something.

Anyway, no matter how it happens in science fiction, it is done. What I can't excuse are authors who sometimes forget the technology they've created for their world. So, if a space station or starship has artificial gravity, why can't it be increased or turned off? If I'm ever a captain of a starship being boarded by a hoard of space pirates, I would turn up the artificial gravity in certain cabins just as the pirates walked into them. End of story.

A HOPEFULLY TRUE STORY: A TALE OF APPLICATION

First the bad news: given the vast time scale of space travel, we humans most likely will not make it out of our solar system, at least anytime soon. The good news is that this isn't the end of short-term space projects. They just won't involve human bodies. I'm talking about sending technology.

In 2016, Yuri Milner and Stephen Hawking announced an initiative called Breakthrough Starshot.16 They intend to develop high-speed nanocrafts (sometimes called nano-spacecrafts or nanoprobes) and send them to the Alpha Centauri star system. Once there, the nanocrafts will relay all types of information including photographs of Proxima b, our nearest earthlike neighbor.

If these tiny crafts can be limited to around a gram in weight then, given the physics of fuel to weight travel, these little guys could be accelerated to a significant proportion of the speed of light. The fuel of choice would be light itself.

A nanocraft could be a protective graphite shell equipped with a microprocessor, radio transceiver, and navigation gyroscope. The tiny craft would then be tethered to lightweight, highly reflective solar sails measuring only a few meters in area. Of course, the sails would be designed to absorb just enough light to get the job done. We wouldn't want them to burn up the craft that they are carrying.

Now, before these nanocrafts can make their interstellar journey, we need to somehow get them outside of our atmosphere (I'm assuming they are built here on Earth). We could launch them with conventional rockets and deploy them in space. Boring. I'd love to see them sent up in a space elevator and launched from a space station, but maybe that's just me.

However they achieve orbit, a ground-based laser array would target the sails and accelerate them to speeds about 20 percent of the speed of light. Once at speed, the solar sails would fold up to become antennas. During the approximate twenty-five-year voyage to Proxima b, the nano-fleet could perform additional scientific tasks. Some could coast past Jupiter's moon Enceladus and sample the water plumes. Others could zip to Pluto (the flight would only take three days).

Only four years after they arrive at Proxima b, we would start receiving messages from our intrepid space explorers. All in all, not too shabby a cosmic project…and one that can be completed within a human lifespan.

PARTING COMMENTS

The sky (space) is the limit. Actually, the limit is the speed of light, but I don't want to ruin the moment, so continue looking at stars.

It might not be geology, but aerospace science rocks. We can travel inside our atmosphere in jets (turbo/ram/scram) or, if you like the retro thing, a prop plane. If your intent is to travel between planets, chemical rockets, solar sails, nuclear propulsion (but not launched from the earth), and ion engines are your ticket out of here. If we move from the practical to the theoretical, you could imagine flying starships equipped with antimatter engines, or electromagnetic drives, or warp drives (each less feasible than the last).