Finding the Volume of Consumables Needed

[Spike Spiegel]

Let's say I wanted to make a fairly realistic spacecraft for Orbiter, and I wanted to know how big my various tanks needed to be. For example, hydrazine for the RCS, some kind of fuel for the main engines (LOX?), and something for my crew to breathe so they don't die.

How would I figure that out?

For example, I took a look at the Dragonfly model and found that 3DS Max calculates the volume of those two pill-shaped vertical tanks on the back as about 15.6 million cubic centimeters. Using the FuelTransferMFD by computerex, I found that there's about 10 grams of fuel in the thing. From the information I've found about hydrazine, 10 grams should probably fit in your hand. Not that you'd want to hold that stuff.

So from my limited understanding, something is not matching up. If someone could help me figure this out, as well as the volumes/masses of the other consumables I mentioned, I'd appreciate it.

 

[Calsir]

So from my limited understanding, something is not matching up. If someone could help me figure this out, as well as the volumes/masses of the other consumables I mentioned, I'd appreciate it.

Maybe the MFD does not recognize the two spheres as fuel tanks. To gather information about the density of your propellants, you can check on www.wolframalpha.com for "liquid oxygen" "liquid hydrogen" etc. It gives you the data you need for density.

As for the quantities, it depends upon the deltaV budget, the engine ISP and the oxidizer to fuel ratio.

I have no clue about human consumption, but I am sure you can figure out both food and oxygen need from the caloric needs.

 

[ar81]

To bring a craft into orbit you need an approx of 9 parts of fuel and 1 part of payload.

 

[Spike Spiegel]

Thanks Calsir, I didn't know about WolframAlpha. I'll give it a try. And I had heard about the 9:1 ratio before, so I'll have to remember to take that into account.

 

[Wolfer]

You might want to check the atomic rocket page, there's a lot of useful information about how to design realistic spacecrafts. http://www.projectrho.com/rocket/index.html

 

[Spike Spiegel]

Wolfer, thanks for the link. Lots of fun stuff there and good info.

Now, I'd love to know if I'm on the right track, because I'm quite the noob when it comes to this stuff. Here's what I did with the crew's air supply.

I'm assuming a person uses 0.83 kg of air per day, and assuming that this is a mixture of 79% nitrogen and 21% oxygen, both in liquid form. WolframAlpha tells me that liquid nitrogen has a density of 0.8116 g/cubic cm and liquid oxygen has a density of 1.146 g/cubic cm. So in the case of oxygen, for example, I took my number of grams (174.3) and divided it by 1.146 to find the number of cubic centimeters. If I'm wrong here, please correct me.

I did this for the nitrogen as well, then added my numbers to get about 960 cubic cm total. Going into 3DS Max, I created a sphere with this approximate volume. The sphere has a radius of about 6.16 cm. This seems awfully small to be correct, but like I said, I'm a noob.

Can anyone tell me if my math is right?

 

[tori]

Your math is right.

It's noteworthy that you don't need to bring a large supply of nitrogen with you, as the human body doesn't modify it in any way (it's just a filler). You can have a cabin with 79/21 atmosphere and just remove CO2 and replenish O2 (and you can use CO2 scrubbers for that, no need for tanks at all).

You'd want to bring just enough N2 to handle events such as loss of atmosphere and airlock cycling.

 

[Calsir]

Wolfer, thanks for the link. Lots of fun stuff there and good info.

I forgot about that site, it is really awesome.

As tori said, your math is correct and you don't need to carry much nitrogen, as it is not consumed by respiration. I understand that your spacecraft barely has a day of oxygen for a single crew member. Where did you find out the .86kg/day consumption? Is it the basal consumption (i.e. at rest) or does it take in consideration the workload of an astronaut?

One other thing: it is impractical to carry around cryogenic stuff, as it requires thermal insulation, both from the atmosphere when the vehicle is being readied for lift-off and from the rest of the vehicle when it is in orbit. The main sources of heat in orbit are the orbiter itself and celestial bodies. The first option would make me think of keeping the reservoir at a certain distance from the main body, via a boom/outrigger. While for the second I'd use a shade. I think that it is easier to just keep the Oxygen in compressed bottles. The whole assembly will likely weigh less, especially for short duration flights.

I just found an article about this issue: http://science.nasa.gov/headlines/y2000/ast13nov_1.htm. It is interesting because oxygen is stored as water (H2O - density: 1g/cm^3) and extracted via electrolysis, a process that uses electricity to separate Hydrogen and Oxygen. Water is made of roughly 88% Oxygen. Hydrogen could be vented, it is highly reactive and corrosive.

Or, you can think that there is a way to extract Oxygen from CO2, since the main effect of human respiration is to extract O2 from air and release CO2. It would cost energy, but it would essentially make an "unlimited" (with all due caution) life support. You could use plants to do so (via photosynthesis).

 

[Spike Spiegel]

I got the kg/day number from that atomic rocket site that Wolfer mentioned, on the life support page somewhere.

I basically wanted to break down the requirements for oxygen (and other things) into person-day units so that I could then easily multiply the amounts by the number of people involved and the number of days that I wanted to keep them alive. As far as being at work or at rest, I'm not sure. I'd probably add a bit to the amounts as a safety margin, just to be sure.

And thanks for replying. It's nice to know that I don't entirely suck at math.

 

[Piper]

There's a calculator I made up a while ago, that not only calculates how much fuel you will need for a given deltaV (or vice-versa), it also tells you how pick your tanks need to be (based on three different shapes). It's called the "Rocket Equation Calculator," and you can find it at:

http://www.ioss.ca/tools.html

There's also a program called "Solar Panel Calculator" that tells you how pick you need your solar panels to be for a given amount of energy, and distance. You might find that useful too if you are trying to be as realistic as possible, plus a few other tools as well.