spaceship size and fuel tanks

[Lunar Pilot]

Does anyone here know a equation for figuring out tank size of a spacecraft based on thrust and fuel density? I'm trying to design a freighter ship and am trying to build it around the fuel tanks and engines. Please explain what each variable is.
Thank you.

 

[Quick_Nick]

It depends on how long you want the engines to burn.(how much fuel) I'll assume by thrust you really mean ISP(that is more important here). What kind of flights(as in distance and type) are you planning for this craft? What type of fuel, or if not real, what kind of power, ability, etc.?
Basically the volume will be based on the mass and density, the mass will be based on the amount of burn-time and ISP, the density will be based on the type of fuel, the desired burn-time will be based on the types of flights and ISP/thrust, and the ISP will also be based on the type of fuel and the amount of realism. I think I explained this without loops. I could connect things together more but this explaination should basically explain things.
EDIT: I made myself a little diagram. It doesn't make complete sense and no matter what at least one thing may seem wrong depending on how you look at it. But basically everything depends on the type of flights you want.

 

[Zatnikitelman]

The best way to figure this out is to determine the engine type and ISP and how much Dv or acceleration you want from it. Then you'll need to use Tsiolkovsky's equation in reverse. Normally in Tsiolkovsky's equation, deltav=specific impulse*(natural log of full mass/dry mass)
To solve for total needed fuel mass. Use (euler's number ^ (deltav divided by specific impulse))times dry mass.
Then, use the computed mass divided by the density of your fuel (LOX=1141 Kg/m^3 LH2=70.8 Kg/m^3 for example) equals the approximate volume needed. Then use any volume calculation to figure out how big the tank must be.
If you need a specific burntime, use the burntime equation: (T=((MI)/F)) where M=fuel mass, I=specific impulse and F= thrust force in Newtons to compute how much fuel mass you need for a given burn time.
Now remember, the ISP must be expressed in terms of exhaust velocity, this is the same units Orbiter uses, but Wikipedia, Astronautix usually give ISP in terms of seconds. Just take this number and multiply by gravity or 9.80665. So the ISP of an RS-68 engine has an ISP of 304s and an exhaust velocity of 2981.2216

 

[Lunar Pilot]

One question

What exactly is ''dry mass''? Is it the weight of a spaceship without fuel and cargo in it?

 

[ijuin]

Yes, the "dry mass" of a spacecraft is the mass of the craft itself, without any consumables or crew or cargo.

 

[n0mad23]

Lunar Pilot,

You might find this useful: http://orbiter-forum.com/showpost.php?p=5525&postcount=15

 

[Scarecrow]

Yes, the "dry mass" of a spacecraft is the mass of the craft itself, without any consumables or crew or cargo.

While technically that's true, that's not a useful number in the Rocket Equation. If you used the mass of the ship without food, water, oxygen, passengers, or cargo as the dry mass, then you would find the Delta-V for the ship if it flew without all these things. What "dry mass" really means in the context of the Rocket Equation is the mass that the ship would be if it ran out of fuel. This includes everything you would find in the rocket on the launchpad one second before ignition except for the fuel.

 

[Zatnikitelman]

Dry mass is the weight of everything that doesn't get consumed as the fuel burns. Cargo and Crew are part of the dry mass unless an engine works by throwing crew out the airlock? (with spacesuits on guys, come on!)

 

[Lunar Pilot]

launch pad

Well, I get the ''dry mass'' concept, except for a tiny problem. This spacecraft is not intended for atmospheric flight.

 

[Zatnikitelman]

How is that a problem? If anything, it simplifies things because you don't have to compensate for atmospheric drag (if burning engine through the atmosphere i.e. Launch). Earth orbital velocity is about 7.8 km/s, however the required Dv for a launch into Earth orbit is about 9.8-10km/s due to drag.

 

[n0mad23]

Zatnikitelman's right - "dry mass" ultimately is what you want to move. There's really no need for fuel otherwise. Fuel weight and consumption is the trickiest part of the calculations, and definitely made trickier if you're calculating atmospheric drag.

So no problem that it's not intended for atmospheric flight.

 

[jedidia]

unless an engine works by throwing crew out the airlock?

you'll need a darn high exhaust velocity to make this at least slightly efficient! :rofl:

 

[Lunar Pilot]

mass

Doesn't the mass of the spacecraft come in around here?

 

[Lunar Pilot]

volume

I can't find a volume calculator that doesn't need the dimmensions of the tank itself, which I'm trying to find. With the equation that was given earlier, what would 26.232 be?

 

[Urwumpe]

For each kilogram mass at burnout, you would need 26.232 kg - 1 kg propellant.

This mass is split into fuel and oxidizer usually (at least two tanks). The volume needed for the tanks would be component mass(fuel or oxidizer) divided by density. You should also add 5% to 10% to the volume as ullage space and for anti-slosh/anti-vortex equipment.

Additionally, you would need small pressurant bottles (Helium or Nitrogen) for keeping the tanks and the fuel under pressure. More complex rocket engines heat the propellant and turn a part of it into gas for pressurizing the tanks, but that does for example not work with kerosine.

 

[Lunar Pilot]

Tanks

So, would about 1542000000 liters of fuel, in four 500x200 foot tanks, be enough to push a spacecraft 2000x450 feet to Neptune maybe?

 

[Piper]

So, would about 1542000000 liters of fuel, in four 500x200 foot tanks, be enough to push a spacecraft 2000x450 feet to Neptune maybe?

Depends. How much does this 2000x450ft spacecraft weigh? Also, how do you want to get there? Are you going to do a direct Hohmann transfer (which would take a very long time to do to Neptune), or a slingshot past Jupiter, or a high energy transfer (something with an eccentricity greater then 1)? Also, what are you going to do when you get there? Are you just going to do a fly-by, or are you going into orbit? Do you need to return after? Before you can figure out if you have enough fuel, you need to know what your total deltaV is going to be. If you are planning to slingshot past Jupiter, an brake into Neptune orbit, expect a total deltaV somewhere between 11,000m/s and 15,000m/s, double if you want to get back.

 

[Lunar Pilot]

weight

I'm not very sure exactly how heavy it is, considering that the framework is made of a steel/aluminum/carbon nanotube mixture. And since that hasn't been made yet, I don't know how much it weighs. I'm pretty sure that it would weigh over 200 tons, at least.

 

[tl8]

On a side note, It would be nice to have a list of common use items and list their specs for Addon Devs (eg LOX-200kg per 10L, Carbon Fibre 1kg/10m2 etc.)

 

[n0mad23]

On a side note, It would be nice to have a list of common use items and list their specs for Addon Devs (eg LOX-200kg per 10L, Carbon Fibre 1kg/10m2 etc.)

I've thought the same thing. Unfortunately, it's already packed up for my impending move, but in one of my notebooks I've got several pages dedicated to just this. But I've also included the weight of other resources as well. These include things as diverse as basalt sand to coffee beans.

I'll type it up when I can get my hands on it again, and put it up for public consumption.