Hull size and mass

Deryn_Rys's picture
Deryn_Rys
February 7, 2011 - 11:38am
I was reading through this section and had to comment on something that I found was missing, in your description you made no mention about the required space needed aboard the ship for its fuel payload. While the engines might be mounted externally on pylons or struts and thus do not take up space in configuing the mass of the ship, fuel (particularly chemical fuel) takes up a lot of space, and should add to the initial cost of the ship for the amount of space that it displaces if not for the fuel itself.

Space and fuel costs might become a major determining factor in determining the engine types that are purchased, with cheap chemical rockets being not so readilly chosen because of the largue volume of space required for fuel payload, while atomic or ion engines might be more economical. And of course if you add scoops to streamlined ship to enable them to suck up fuel from gas giants, Ion drives might be even more popular than atomic drives.


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Comments:

TerlObar's picture
TerlObar
February 7, 2011 - 11:53am
I agree, this is something we need to work out.  The current system skirts around it by saying the engines hold x amount of fuel but never really define what that is.  This is something that we need to think about and figure out.  However, to do this right, we need to decide exactly how the different types of engines work and their fuel consumption.
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TerlObar's picture
TerlObar
February 10, 2011 - 5:18pm
Here's what I propose.  Numbers can be worked out once the basic idea is decided on and we have average mass values for the ships.

1) We keep our three types of engines (Chemical, Ion and Atomic).  We can either keep the three sizes or add a few more if we want.

2) For all engines, we assume that the fuel storage is part of the engine itself and give each engine type and size a fuel capacity, measured in ADF (total delta-v) or something equivalent (ADF isn't quite right since that depend on other factors as well, so maybe burn time).  So for a Chemical rocket, it will be really small (say 4-10).  A quick look through on-line references, it takes the space shuttle about 2 ADF worth of fuel to get into orbit (actually about 3 g for 5-7 minutes, but it averages out).  The capacity would be more for the other engines (I think an Ion drive can hold enough for 10,000 ADF). 

3) Each engine is rated to provide X amount of thrust (over and above what is required to move itself and its fuel load).  This will be fairly high for chemical and atomic engines but lower for Ion engines.  This value would be in ADF x mass.  So simply dividing this thrust by the mass of your ship would give you ADF.  As an example if a HS 1 ship massed 200 tons we might say a Class A Atomic drive has a thrust rating of 1000, Thus a fighter would have a total ADF of 1000/200 = 5

4) To add engines to your ship, you detemine the HS and thereby the maximum number of engines you can have, pick the number of engines you want and their type.  Once you've done that, you can determine the max ADF of your ship by adding up the thrust of all your engines and dividing that by the mass of your ship.  All fractions round down to the nearest integer.

As another example, Say a frigate masses 10,000 tons and a Class B atomic drive has a thrust of 20,000 ADF-tons.  And in our new system, a HS 5 vessel can have up to two class B engines or 6 Class A engines.  Since the Class A engines only produce 1000 ADF-tons of thrust, six of them would give us a total thrust of only 6000 ADF-tons which, divided by our mass of 10,000 tons only gives us a thrust of 0.6 ADF, not very impressive for a warship.  So we go with the two class B engines.  Each giving 20,000 ADF-tons for a total thrust of 40,000 ADF-tons.  Dividing that by our 10,000 ton mass gives an ADF of 4, much better.

5) We could add in an additional fuel storage option in a couple of ways.  We could either define volume and mass values for the extra fuel storage and require that to be added mass and space on-board the ship.  Or we could just have it as part of the engines and state that the added fuel capacity makes the engines bigger and reduces their thrust value.  Exact numbers could be worked out but it would be something along the lines of every 10% increase in fuel capacity reduces the thrust rating of the engine by 5%. 

So say my class B atomic engine could store 2500 ADF worth of fuel normally.  I'd really like to be able to sqeeze one more jump out of that (say a single jump takes 400) and I'd like to increase my capacity to 3,000.  That's a 20% increase in the capacity and I'd see a related 10% decrease in total thrust available since I need to have a larger mass to hold the extra fuel.  The new thust value for my engines would be 18,000 ADF-tons instead of 20,000 (a 10% reduction).  If I was putting these on my frigate, it would have an ADF of 3.6 (which rounds down to 3) instead of 4.


We would define a number of engines of each size that a ship of a given HS could have.  This would allow for players to pick and choose the engines you have on your ship (and maybe even mix and match, both types and size).  Maybe you have two pair of Ion drives that give your ship the one g of acceleration you need most of the time but also have a pair of smaller atomics which you don't normally use, but when turned on, they boost your ADF up to 2 when in combat.
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iggy's picture
iggy
February 10, 2011 - 7:32pm
@TerlObar  I'm liking how you are doing this.  This leads to a system that we can build a spreadsheet and graphs or look-up tables for.  I'd like to start coming up with some simple equations for what you are proposing above.
-iggy

TerlObar's picture
TerlObar
February 10, 2011 - 8:08pm
If we want to go this way, I think the next step really is to determine mass (or at least volume abd ) for each component that could go on a ship.  Once we have that, we can mock up the various ships and see what falls out.  I suspect that to make it work out rationally, we'll actually need 4 or 5 classes of engines instead of the standard three or allow a lot of engines on some of the hulls.
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jedion357's picture
jedion357
January 20, 2012 - 12:22pm
Just curious, does anyone know how much volume an ocean going freighter has in its cargo hold? EDIT: I've done a Google search and could not turn up a rough ball park number. On a related topic standard shipping containers are 20' x 8' x 8.5' or 40' x 8' x 8.5' or 40' x 8' x 9.5'. Perhaps we should establish a PanGal STD. Shipping container in meters that equally loads on trucks shuttles and star ships. There can be a second common container that was the standard in the Rim that showed ifshnit influence. The ifshnit STD. Container was under consideration for being changed due to contact with the osakar and the humma but contact with the UPF lead to the adoption of the PanGal container. Many a ifshnit container found itself repurposed on outpost worlds as temporary/semipermanent housing or storage etc. But still you see this odd sized container being shipped.
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