The 1% Project

[hobbsyoyo]

I want to throw together ideas on how you might go about creating a propulsion system capable of propelling a starship to 1% of the speed of light for lulz. The rules:

1. It has to be achievable with current technology (not even near-future will suffice)
2. The starship that the propulsion system propels must be capable of safely carrying 5 humans in space for at least 1 year
3. The starship has to accelerate to 1% of the speed of light within 6 months
4. I am not examining supporting technologies beyond cursory assumptions

My goal in this is to create a concept for a realistic system that has general utility for manned operations to explore the solar system. I'm not really interested in accelerating a microchip to the speed of light, for example, or really any other hyper-specific goal.

The high-level take of how I'm going to tackle this:

1. Define the problem/outline assumptions
2. Create back-of-the-envelope boundaries to the solution
3. Identify potential solutions and trade them against each other
   
4. Perform detailed technical analysis of winning trades
5. Detailed design

To start, I have to define the goal, which means verifying that the goals I've set forth are realistic with current technology in the first place. I also need to lay out the assumptions I will use and check that those are realistic as well. This is going to be trickier than it seems because even answering the question 'What does it take to keep 5 people alive for 1 year' is an open-ended question with endless permutations. I have to answer that question before I even start on the engine, because it dictates the amount of payload the ship must carry at a minimum to do nothing else but keep the crew alive.

Because I'm intending to only focus on the stardrive itself, I will only loosely bound the problem such that it is generally representative of a manned space vehicle rather than try and invent specific architectures for the space vehicle.

Anyone is free to contribute and I'm especially grateful for criticism.

Mods please be nice about double posts, I doubt I'll hit more than 1 a day but it's not like this is a heavily trafficked section of CFC and I fully expect to mostly be talking to myself.

Oh and I decided I'm going to call the system the Ventura Stardrive.

 

[hobbsyoyo]

I know it's bad form to skip ahead, but I am very keen to examine various nuclear-powered propulsion systems as I think that's the only way this is going to work. I really want to work on a nuclear thermal rocket design (which is basically a reactor that shoots flame out of a traditional nozzle) but I think in the end it will end up being a electrical propulsion system that is powered indirectly by a reactor or some sort of exotic nuclear contraption.

I've seen some concepts where you spin fissile plates around in such a way that they have moments of criticality and the charged particle fragments that are ejected in these moments are magnetically funneled out the back as reaction mass, rather than used to heat up some fluid. It's pretty funky stuff and I'm excited to jump in!

 

[hobbsyoyo]

1. DEFINING THE PROBLEM / OUTLINE THE ASSUMPTIONS

So on the assumptions, I need to work out how much stuff you need to keep 5 people alive for a year. This is important because the mass of this amount of stuff is going to be the absolute minimum mass you need to accelerate with the engine to 1% of lightspeed. After we calculate these basic assumptions, we'll also look at some more assumptions related to the rest of the ship (structure, communications, etc).

One thing that I am confident that we are going to find out by going through this process is that the final mass of the ship will be largely insensitive to considerations for manned flight. What I mean is that inevitably, you have to carry so much fuel to get up to speed that you end up using most of your fuel just to push the rest of the fuel. The rocket equation is a harsh mistress, and it dictates that the final mass of the vehicle will enter an exponential upward climb after a certain point.

This is actually almost nice for us as humans because it means that once you have an engine + fuel system capable of reaching this speed, the extra mass required for an arbitrarily large crew can be tacked on almost for free. Below a certain crew size, I'm guessing that the mass of the crewed part of the ship is just a rounding error compared to the mass of the fuel.

The basics essentials:

• Air
• Food
• Water

Air
Google says people need about 8 L/min (I saw 6 - 7.5 and I am a conservative designer), and to calculate the total volume they'd need, you multiply that by 525,600 (60 min/hr, 24 hr/day, 365 d/yr) to get the total volume. And since the ship has five people, you have to throw on an extra x5 at the end as well!

How much air do we need?
21,024,000 Liters of air for are required for the 1-year trip! But is this really the answer? This assumption is for air at sea-level pressure and it assumes the elemental ratio (~4/1 nitrogen to oxygen) is the same as on Earth as well. We can change those parameters! We're not stuck with Earth-normal conditions, though our bodies do have limits that must be accounted for. And of course the even bigger elephant in the room is whether or not we're including any air recycling.

Well are we doing all that?

I've done some research today and the current international standard for the atmospheric makeup of manned space vessels are basically Earth-normal. This greatly simplifies logistical considerations around docking and is safer. So I'm not going to try and work with the pure-oxygen atmospheres like they had in the Apollo days. So we know what kind of breathing gas we're going to use but we haven't yet decided how much of it we can reuse. That's my next item to tackle.

One other tidbit I picked up is that the ventilation system on the ISS works better and has less breakdowns than it would have it the air pressure was lower since that means it would have to work harder to push the air around. Also, it is easier to provide a stable thermal environment with thicker air as well.

So I did some digging and I found that the ISS recycles about 40% of the oxygen that the astronauts breathe and about 80% of the water they drink. I'll worry about the water later, but for now let's say that we can cut the amount of O2 we need to bring by 40%. Since O2 makes up about 20% of the air our astronauts take, we are only saving 40% of 20%, so not massive savings there.

The good news is that our bodies do not actually use the nitrogen we breathe. It's a neutral buffer gas (to our anatomy), so we only need to take enough to fill the ship, plus enough to replace any lost through leaking. This dramatically reduces the amount of gas we have to bring along! I'll run the numbers tomorrow; I need to start a dedicated spreadsheet for this project anyways.