Steve Jackson Games - Site Navigation
Home General Info Follow Us Search Illuminator Store Forums What's New Other Games Ogre GURPS Munchkin Our Games: Home

Go Back   Steve Jackson Games Forums > Roleplaying > GURPS

Reply
 
Thread Tools Display Modes
Old 07-12-2020, 08:41 AM   #1
the-red-scare
 
Join Date: Sep 2014
Default [Spaceships] Mass reduction system

Suppose rather than a contragravity lifter system you had a mass reduction system. It’s some form of negative mass that reduces the apparent mass of a spaceship, as in Star Trek (per offscreen tech manuals) or in Mass Effect. Reaction mass (which I will call propellant below to avoid having to say mass a million times) has full mass when throwing it out the back. Disregard the potential effects of reduced mass on basic physics and chemistry inside the ship and the crew for now.

Acceleration seems easy. Full propellant flow plus reduced spacecraft mass = increased thrust, the math is simple, right?

How does the rocket equation work?

Option 1: Technically, the mass ratio as literally defined (wet mass divided by dry mass) stays the same since both are reduced and mass reduction wouldn’t change delta-V at all.

Option 2: The propellant being used, when compared with the dry mass, is actually more than the mass ratio shows. It would seem the ratio should be “reduced dry mass plus full propellant mass” divided by “reduced dry mass.” So a spaceship with a mass ratio of 2 (say, 100 tons wet, 50 tons dry) reduced in mass by 10x would have an adjusted mass ratio of 11 (5 tons dry plus 50 tons propellant, divided by 5 tons dry).

Taking the natural logs that would be a delta-v increase of about 3.5. So we could give a “10x Mass Reduction” system a flat 3.5x delta-v increase.

Given this is all made up nonsense, which, if either, seems correct?
the-red-scare is offline   Reply With Quote
Old 07-12-2020, 08:52 AM   #2
Fred Brackin
 
Join Date: Aug 2007
Default Re: [Spaceships] Mass reduction system

Quote:
Originally Posted by the-red-scare View Post
Given this is all made up nonsense, which, if either, seems correct?
Spaceships never uses the term "dry mass". So you're totally on your own if you start trying to do calculations with it.

You're effectively getting more thrust without spending more reaction mass so you've got a reactionless thruster that only works in tandem with a reaction drive.

You could just change the Acceleration and Delta-V numbers for your reaction drive by the "mass reduction factor" you decide upon. This is a lot like inventing a new reaction drive that takes 2 systems to install, one of which is a High Energy user.

You could just invent a new "Reaction Drive" with very good stats of your choice and assume that it's about half old-fashioned reaction drive and half mass reducer.

The last might be the simplest thing to do.
__________________
Fred Brackin
Fred Brackin is offline   Reply With Quote
Old 07-12-2020, 09:08 AM   #3
the-red-scare
 
Join Date: Sep 2014
Default Re: [Spaceships] Mass reduction system

For the final system I will probably do as you suggest.

But in the planning stage I wanted to try to do the math because there may be the option of, say, a Super Mass Reduction system that doubles the reduction using 2 Power Points or something, or maybe you install more than one. So there is variety and that might have different impacts rather than just adding them together and saying it’s twice.
the-red-scare is offline   Reply With Quote
Old 07-12-2020, 09:12 AM   #4
AlexanderHowl
 
Join Date: Feb 2016
Default Re: [Spaceships] Mass reduction system

Negative mass propulsion (NMP) does not require reaction mass and, when you look into the math behind negative mass theory, it would be completely useless anyway. Since negative mass is repulsed by positive mass, the system effectively 'borrows' acceleration for the positive mass from the negative mass by creating a mass imbalance in the opposite of the desired vector and 'returns' acceleration to the negative mass by creating a mass imbalance in the desired vector. Since inertia remains '0' throughout the system though, any NMP system should result in an inertialless drive, which may allow for FTL, depending on which mathematics you are looking at.
AlexanderHowl is offline   Reply With Quote
Old 07-12-2020, 09:15 AM   #5
the-red-scare
 
Join Date: Sep 2014
Default Re: [Spaceships] Mass reduction system

I am not interested in negative mass propulsion, luckily.
the-red-scare is offline   Reply With Quote
Old 07-12-2020, 09:22 AM   #6
Fred Brackin
 
Join Date: Aug 2007
Default Re: [Spaceships] Mass reduction system

Quote:
Originally Posted by the-red-scare View Post
For the final system I will probably do as you suggest.

But in the planning stage I wanted to try to do the math because there may be the option of, say, a Super Mass Reduction system that doubles the reduction using 2 Power Points or something, or maybe you install more than one. So there is variety and that might have different impacts rather than just adding them together and saying it’s twice.
There is no math to be done. You're violating the laws of physics to produce a propulsion ssytem with the characteristics you want. Try out several sets of numbers first but in the end there is only what you want.

In particular you should probably abandon any thoughts of "dry mass" or "mass ratio". Spaceships drives have only accelration per number of systems and Delta-V per tank.

You could try and describe the mass reduction system as a multiplier for the conventional drive. You could start with a 2x mass reducer. That would give you 2x as much acceleration and Delta-V and so on.

You'll never end up with anything as good as a reactionless drive so I suspect you'll end up with something close to a Total Conversion Drive which si the next best thing.
__________________
Fred Brackin
Fred Brackin is offline   Reply With Quote
Old 07-12-2020, 09:36 AM   #7
AlexanderHowl
 
Join Date: Feb 2016
Default Re: [Spaceships] Mass reduction system

I think that what you are imaging is actually an inertial screen, a superscience device that reduces the inertia required to move an object. For example, a 90% inertial screen would reduced effective mass by 90%. As for the effect on the rocket formula, it gets complicated because the fuel only gains in 10% of the inertia that it otherwise would, meaning that the energy contained within the reaction mass is much less than suggested by the rocket formula. At velocities above 100 meters per second, that starts becoming significant, especially with chemical rockets (at 3 km/s, the reaction mass contains the equivalent 4.5 MJ of energy in inertia alone).

While the math is likely wickedly complicated, I would suggest that ignoring the multiple for fuel ratio would probably be a good approximation. Since thrust (and therefore delta-v) would go up by 1/inertial divisor though, it would be a net benefit. For example, a chemical rocket with a 90% inertial screen would have a thrust of 30g and a delta-v of 1.5 mps.
AlexanderHowl is offline   Reply With Quote
Old 07-12-2020, 11:38 AM   #8
the-red-scare
 
Join Date: Sep 2014
Default Re: [Spaceships] Mass reduction system

Quote:
Originally Posted by Fred Brackin View Post
In particular you should probably abandon any thoughts of "dry mass" or "mass ratio". Spaceships drives have only accelration per number of systems and Delta-V per tank.
Of course, but that’s only because the math was already done by David Pulver. Delta-v per tank is just the delta-v of a drive with a given specific impulse and a mass ratio of 20/19. The fuel tank multiplier table is just a simplified rocket equation. I don’t think it is unreasonable to try to do the same math under altered conditions to figure out what the delta-v per tank would be under those conditions.

But it is not something real, so the answer may indeed just be to make it up.
the-red-scare is offline   Reply With Quote
Old 07-13-2020, 09:59 AM   #9
Donny Brook
 
Donny Brook's Avatar
 
Join Date: Aug 2014
Location: Snoopy's basement
Default Re: [Spaceships] Mass reduction system

I'm no expert, but wouldn't it be simplest to just calculate the delta-V as if the propellant was normal and simply generically reduce the mass variable of the vessel being propelled by whatever amount of mass the propelant does not weigh when inside?
Donny Brook is offline   Reply With Quote
Old 07-13-2020, 10:27 AM   #10
AlexanderHowl
 
Join Date: Feb 2016
Default Re: [Spaceships] Mass reduction system

You have reduced kinetic energy in the reaction mass, which changes the assumptions of the formula (the rocket formula assumes that the fuel possesses a certain amount of kinetic energy at each velocity, which is not the case with an inertial screen).
AlexanderHowl is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Fnords are Off
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -6. The time now is 01:15 AM.


Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2024, vBulletin Solutions, Inc.