Thursday, September 22, 2011

How To Build A Spaceship - Part 4

MPD Thruster
In Part 3, we plugged our numbers into the equations and did the calculations. So let's look at what we came up with:

Mythik Magnetoplasmadynamic Mars Expedition Ship
Mission - Fly to Mars, look around, and return to Earth.
Distance - .5 AU or 74,799,000,000 meters.
Ship - Orbit to orbit Mars Expedition vessel. "Fire Baton" class generating 1G artificial gravity by spinning around the central core. The artificial gravity is good for the health of our crew and also helps simplify things like plumbing and all the other things that act surprisingly weird in micro gravity. It's powered by two 15MW nuclear reactors, generating 6MW of electrical power.
Length - 125 Meters. The reactors are at one end, the Habitat Module at the other end, connected by trusses from the central core. The distance of our crew from the reactors, combined with the radiation shield, keeps the amount of radiation exposure to safe levels for the duration of the mission. There is a "storm cellar" in the center of the Habitat with extra shielding in case of solar flares, etc.
Thrusters - Six Magnetoplasmadymaic (MPD) Thrusters, which have 25 Newtons of thrust each. Three thrusters are on each side of the central core. Our design is what's known as Nuclear Electric Propulsion. The nukes make the electricity which powers the thrusters. Another design is Nuclear Thermal Propulsion, where the nukes are the thrusters.
Wet Mass - 145,400kg
Dry Mass - 75,400kg
Propellant Mass - 70,000kg
Propellant Tanks Mass - 4,000kg
Drive - 32,200kg - Includes all power systems, radiation shield, radiators, etc.
Hull - 5,000kg - Includes central core, thrusters, truss, etc.
Payload - 34,200kg - Consists of TransHab inflatable crew module, including all life support systems, navigation, control systems, etc.
Thrust - 150 Newtons
Specific Impulse - 5,000 seconds
Mass Ratio - 1.92
Exhaust Velocity - 49,050m/s
Ship Delta V - 32,210m/s
Acceleration - .001 m/s
Travel Time - 197 Days (6.6 Months)
Mission Delta V - 17,568m/s

Okay! There are a couple of issues:

1. We're using all of our electrical power for thrust, with nothing left to power all the other systems on the ship. We could solve this by using lesser thrust or taking away two thrusters. But then our travel time would be extended. Or, we could find a way to make our reactors more efficient. 2. We don't have quite enough Ship Delta V to make a round trip. We could solve that by adding more propellant or squeezing out more Specific Impulse from our thrusters. 3. If we solve #1 & #2, we still can't do much once we get to Mars. We have no way of landing on Mars. We also didn't take into account the amount of Delta-V it would take to make the course corrections to match orbits with Mars (and Earth on the return trip.) We could solve that by sending another ship ahead of time that contains the payload of Mars Lander, Mars Habitat, extra propellant, etc.

Also note, that this ship is an orbit-to-orbit vehicle. It needs other rockets to launch its parts into space, then it needs to be constructed in space. It cannot land or take off from a planet. If you want an all-in-one ship, it gets a lot more complicated.

But this does give you a basic idea of what it takes to get to another planet. You can see that Thrust and Specific Impulse are very important if you have a far destination and want a reasonably quick travel time. The fun part is trying out different numbers from various near-future possibly realistic drives and seeing what you come up with. What if there some kind of technological breakthrough and a power source was developed that generated 30MW of electrical power and only weighed 10,000kg? Well then we could string together 25 of our 25N thrusters and get 625N of thrust! Then we could save the weight from the reactors, add some more propellant and cut our travel time less than 4 months! Of course, that's kind of like building a race car out of dozens of lawn mower engines, but you get the idea.

What I learned from all this is that we really need lighter more efficient ways to produce electrical power, or lighter and more efficient (and safer) nuclear engines. Maybe something along the lines of a Nuclear Thorium Laser.

I plan to use a design similar to the Mythik Magnetoplasmadynamic Mars Expedition Ship in an upcoming story. It's about a trip to Mars that doesn't quite go as planned. Think Poltergeist in Space. That's right, a hard SF horror story.

Hopefully, I haven't completely turned off my few readers with all the equations. You had at least a little bit of fun, right?

For your spaceship designing enjoyment, I have put together a little Excel spreadsheet that does all the evil math for you. Feel free to download the Mythik Spaceship Calculator. I based it on a cool spreadsheet from Rocketpunk-Manifesto called the Rocketpunk Manifesto Travel Planner.

Many thanks, also, to Atomic Rockets, a website filled with infinite spaceship goodness. If you go to the Atomic Rockets Engine section, you can see the equations we used (and awesome pictures and examples) in more detail. And check out the Atomic Rockets Drive Table to see what kind of drives you can plop onto your spaceship to see how it affects travel time etc.

Other parts of this series:
How To Build A Spaceship


  1. OK, I have not done this level of analysis when building my ships. I don't usually use metal for the structure, however, and I did study how to make gravity. I stole a ship design from NASA. :)For the one story set with Earthlings. Most of my other stories aren't.

  2. Hi M Pax! That's ok, I don't think it's required ;) Actually I'm not sure why I became inexplicably obsessed with amateur rocket science, but I think it may be out of my system now...