Question How do you estimate the weight of a vessel?

[ar81]

If I want to create a vessel, how do you estimate its empty weight?
For example, if it has 2 engines or 4, of it has a solar array, if it has several tanks, or how about the weight of internal systems...

I may try to see if I can make a spacecraft.dll vessel capable of dropping cargo into the sun (the right dV) from LEO. Unfortunately, working with parameters is not my field of expertise.

dV = F / m * t

I could get the dV from IMFD.
I have a guesstimate of thrust (F) and I might like to see how much time does it take to accelerate, but I need the mass.

I also may need to see if the ship gets out from Earth SOI or not.

This seems a math exercise but it is not. I want to test my idea in Orbiter, making a spacecraft.dll ship, so it belongs to dev forum.

How do I need to estimate vessel mass to make this addon?

 

[mc_]

"Формула Циолковского" (sorry, dont know how it will be in english)

M1 - empty mass, M2 - full (with fuel), I - engine Isp, V - max dV.

 

[jedidia]

The above mentioned Tsiolkovsky equation is the single most usefull formula to figure out dV. However, you'll also be interested in fuel ratio and stuff. I'd recommend, as allways, project rho:

http://www.projectrho.com/rocket/rocket3b.html
http://www.projectrho.com/rocket/rocket3c.html

these two subsequent pages deal with the subject very nicely.

As for how far your systems will weight, what the hull will weight etc, you'll have to do a bit research and educated guessing for that. You just can't say "a cubic of spaceship is about this heavy". The weight of your systems will depend on what you need, how big you need them and the techlevel you're designing your vessel in (like, now, or a bit in the future where we might have some lighter alloys, or in ten thousand years, where you can make everything up as you'd like it)

 

[EtherDragon]

I think what ar81 is asking is, how do you estimate the mass of a vessel that is, say 100m in length by 20m wide.

How do you estimate the mass of the life-support, navigation, control, and automation systems?

How do you estimate the mass of the frame, empty fuel tanks (for a given fuel capacity), etc...

 

[jedidia]

as I said above... tons of research and educated guessing.

 

[dbeachy1]

One way to get a ballpark estimate is to come up with an average density for your entire ship and then get the volume of your entire mesh using Orbiter's shipedit.exe utility: then your ship's mass is simply "average density" times volume. Of course, the tricky part is coming up with a reasonable average density.

 

[Urwumpe]

I have a first order estimate formula in a lecture hand-out, maybe I can find it again. It is giving you a good upper estimate of the dry mass for a capsule, all extras have to be calculated based on that.

The formula is set for Mercury/Vostok as reference, Soyuz is still pretty close, but from Apollo on, the dry mass by the formula would be much higher as possible by conservative engineering.

MInitial, Dry = 592 * (#Crew * Mission Duration (d) * pressurized volume (m³))^0.346

 

[EtherDragon]

Personally, I use reference objects for my masses.

If I have an interplanetary battleship that is about the size of the USS Liberty, then I might fudge a number for "space age materials" (say 65% of Liberty's dry mass) and throw in a bunch of fuel capacity.

So, yea. Lots of research.

 

[Istochnikov]

In the case of my commercial satellites, i use a 60% fuel 40% structure + payload mass relationship (knowing that at GEO orbital inserction, it should remain near the 12% of fuel ~200 kg)

For the rockets, it's useful the proportion 90% fuel 10% structure + engines.

 

[tgep]

Here's how I do it.

1) Multiply the X,Y, and Z mesh demensions to find the mesh volume.
2) Multiply that answer by 1000 to get a metric tonnage.
3) Divide you tonnage by trhree to find it's CUBIC base tonnage.

Now, depending on what type of vessel your talking about, you can subtract 85 % of your base cubic mass to find your vessels EMPTY MASS
After that you can fiddle with fuel mass and ISP to get numbers you like for the vessel.

This method has brought me good results since I started using the process.

 

[Hielor]

Here's how I do it.

1) Multiply the X,Y, and Z mesh demensions to find the mesh volume.
2) Multiply that answer by 1000 to get a metric tonnage.
3) Divide you tonnage by trhree to find it's CUBIC base tonnage.

Now, depending on what type of vessel your talking about, you can subtract 85 % of your base cubic mass to find your vessels EMPTY MASS
After that you can fiddle with fuel mass and ISP to get numbers you like for the vessel.

This method has brought me good results since I started using the process.

Except that only works if your ship is exactly rectangular. Some modelling programs (Wings3D i know can) can give you a much better approximation of the volume of your ship.

 

[Notebook]

If you have got as far as converting your mesh into .msh format, you can use Shipedit (found in SDK/utils directory) to find volume.

N.

 

[ar81]

I loaded ISS to MIR mission and estimates a tangential transfer to the sun. It tells me 29900k . Does it mean it takes a dV = 29,000,000 m/s? Is the magnitude that big? Or am I reading it wrong?

 

[ar81]

"Формула Циолковского" (sorry, dont know how it will be in english)

M1 - empty mass, M2 - full (with fuel), I - engine Isp, V - max dV.

Are you sure?

If I have:
V = 30,000 m/s
I = 4,000s to 40,000s
ln(M1/M2) = 7.5 to 0.75

then
M1/M2 = 1808 to 2.11

So it means M1 is bigger than M2

 

[Urwumpe]

ISP in proper units: Ns/kg

M1/M0 = e^(-dv/isp) = e^(-30000/40000)

M1 = 0.472366 * M0

for electrical propulsion.

For chemical hydrolox:

M1 = 0.0012726 * M0

 

[ar81]

ISP in proper units: Ns/kg

M1/M0 = e^(-dv/isp) = e^(-30000/40000)

M1 = 0.472366 * M0

for electrical propulsion.

For chemical hydrolox:

M1 = 0.0012726 * M0

Isp units are seconds.
I estimate 4,000 to 40,000 seconds for the VASIMR addon I want to make.
This doc says 3,000 to 20,000 but I want to imagine an improved VASIMR, which is in my opinion feasible for the feature.

If I am correct, the limits would be if normal operation allows higher Isp and also the power of the solar array.
How do you convert s to Ns/Kg?

For low thrust navigation I imagine a spiral trajectory as described here.

 

[Urwumpe]

Isp units are seconds.

No, the SI unit of specific impulse (impulse per kg propellant) is Ns/kg. Seconds is the result if you use improper math. That many people do just that, is not really making it better.

If you don't want to calculate with SI units, you need to change the formula. The correction factor is simple:

Specific impulse in Ns/kg = 9.81 m/s² * Specific Impulse in (kg (force)/kg(mass) * ) seconds.

 

[ar81]

So...

Non SI:
Isp from 4,000 to 40,000 seconds

SI:
Isp from (4,000 * 9.81) to (40,000 * 9.81) Ns/kg

Right?

 

[Urwumpe]

Right.

 

[ar81]

I still have a problem

dV = Isp * ln (Me/Mt)

dV = Delta V (30,000 m/s)
Isp = Specific impulse (39,240 to 392,400 Ns/kg)
Me = Mass (empty)
Mt = Mass (total)

then

ln(Me/Mt) = dV / Isp =
0.76452599388379204892966360856269 to 0.076452599388379204892966360856269

using inverse ln with Windows scientific calculator

Me/MT = 2.15 to 1.08

which means empty mass is bigger than total mass...