Question A question about manned spacecraft add-ons (e.g. Antares, etc.)

[Pipcard]

How are the dry (empty) masses determined?

 

[Pipcard]

For example, the Antares-SR service module has a dry mass of 5,650 kg, and the command module a dry mass of 12,500 kg. How did the developers calculate this?

Calculating propellant mass/volume is easy (you just need to know the densities of 1 or 2 substances), but a spacecraft's body is made up of various materials, not arranged in a uniform way.

For the stages of the rockets that I'm developing, I'm guesstimating the dry mass of each stage based on empty mass/fuel mass ratios of real rocket stages. Maybe the Antares developers are doing the same thing? I don't know.

 

[Hlynkacg]

Empty mass isn't "determined" so much as it is an emergent property of the design.

The designers can make an educated guess based on the components they are going to use and what they weigh but the only way to actually determine empty mass is to build it. It is not uncommon for actual mass to vary as much as 20% from the calculated, in fact you might as well just add 20% to all your mass calculations and feel happy if you come in under the mark.

http://www.projectrho.com/public_html/rocket/basicdesign.php

 

[boogabooga]

I can't speak for the developers, but I know what I would do. (For the level of accuracy of an Orbiter add-on, anyway.)

Since the things you are talking about are mostly hollow when empty, I would assume that the dry mass is proportional to the surface area, not the volume.

If I were you, I would try to determine the dry masses and surface areas of real-life modern capsules (Soyuz, Dragon, Orion, CST100, etc.). Calculate the ratio of dry mass to surface area for each. Average these ratios. Now, if you know the dimensions of your spacecraft, calculate the surface area of your spacecraft. Multiply your surface area by the average ratio of mass/area for real spacecraft...

Here is something to help:
http://en.wikipedia.org/wiki/Cone#Surface_area
http://en.wikipedia.org/wiki/Cylinder_(geometry)
[ame="http://en.wikipedia.org/wiki/Frustum"]Frustum - Wikipedia, the free encyclopedia[/ame]

 

[Urwumpe]

There is also a first order estimate formula based on crew size and mission duration. The masses calculated by that are a bit on the heavy side, but give you pretty useful numbers for the start.

[math]M_{Initial, Dry} = 592 \times (N_{crew} \times T_{mission} \lbrack d \rbrack \times V_{pressurized} \lbrack m^3 \rbrack)^{0.346}[/math]

 

[boogabooga]

There is also a first order estimate formula based on crew size and mission duration. The masses calculated by that are a bit on the heavy side, but give you pretty useful numbers for the start.

[math]M_{Initial, Dry} = 592 \times (N_{crew} \times T_{mission} \lbrack d \rbrack \times V_{pressurized} \lbrack m^3 \rbrack)^{0.346}[/math]

Source?

 

[Hlynkacg]

The magic webpage of rocket design, I imagine...

Project HRO, Atomic Rockets

 

[Urwumpe]

Source?

Lecture "Human Space Mission Design" by Dr.-Ing Kristian Pauly, OHB- System AG, held at the TU Munich in 2003/2004.