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 02-27-2020, 03:02 AM   #1
Pectus Solentis
 
Join Date: Nov 2018
Default [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

When I played Kerbal Space Program, only 1.2G-1.6G was sufficient to escape Kerbin. And I know that TWR of real-world Saturn V is 1.15G.

But at GURPS Spaceships p21, it says that TL7 Chemical Rocket provides 3G accelerations. It seems that it is an astounding overspec. It is greater than real-world TL7 Chemical Rockets, and it is greater than even TL9 HDEM Chemical Rocket (it provides 2G).

Last edited by Pectus Solentis; 02-27-2020 at 03:06 AM.
Pectus Solentis is offline   Reply With Quote
Old 02-27-2020, 03:56 AM   #2
Rupert
 
Rupert's Avatar
 
Join Date: Aug 2004
Location: Wellington, NZ
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Quote:
Originally Posted by Pectus Solentis View Post
When I played Kerbal Space Program, only 1.2G-1.6G was sufficient to escape Kerbin. And I know that TWR of real-world Saturn V is 1.15G.
The initial acceleration might have been that low, but maximum acceleration for the Saturn V's first stage was about 3.5G. What's more, the system was designed to not have too high an acceleration - both the first and second stages cut fuel to the centre rocket well before they did to the outer ones to keep acceleration within set amounts.

Also, the F-1 rocket used in the first stage has a thrust/weight ratio of 94-to-1 on its own, so Spaceships is actually being conservative - an F-1 weighing 8.4 tons in a ship weighing 168 tons (twenty times as much) would have an initial acceleration of 4.7G.

Likewise, the 2nd stage J-2 had a 73-to-1 thrust/weight ratio.

Russian launch rockets have thrust/weight ratio of anywhere from 75:1 up to at least 137:1.

I'm not sure how generous (or not) Spaceships is with delta-vee, but it's conservative with rocket engine accelerations.

As for the TL9 HDEM rocket - it trades raw thrust for efficiency. I doubt anyone would think it very unreasonable for you to decide that chemical and HDEM rockets can use the High Thrust option available to most other reaction engines (x2 acceleration, x1/2 delta-vee per tank).

Now, if you want numbers that don't make much sense, consider jet engines. Being generous a modern jet manages a thrust/weight ratio of 5:1, or 8:1 with afterburning, which should give 0.25G per system (0.4G per system with afterburner). Also, they should burn an entire system of fuel per quarter hour of normal thrust and every 7.5 minutes on afterburner.
__________________
Rupert Boleyn

"A pessimist is an optimist with a sense of history."
Rupert is offline   Reply With Quote
Old 02-27-2020, 04:03 AM   #3
Ulzgoroth
 
Join Date: Jul 2008
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Just to make it explicit, that means that if you want a chemical rocket with less thrust, the natural approach is simply to use a smaller system as described in Spaceships 7.
__________________
I don't know any 3e, so there is no chance that I am talking about 3e rules by accident.
Ulzgoroth is online now   Reply With Quote
Old 02-27-2020, 04:13 AM   #4
Pectus Solentis
 
Join Date: Nov 2018
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Quote:
Originally Posted by Rupert View Post
The initial acceleration might have been that low, but maximum acceleration for the Saturn V's first stage was about 3.5G. What's more, the system was designed to not have too high an acceleration - both the first and second stages cut fuel to the centre rocket well before they did to the outer ones to keep acceleration within set amounts.

Also, the F-1 rocket used in the first stage has a thrust/weight ratio of 94-to-1 on its own, so Spaceships is actually being conservative - an F-1 weighing 8.4 tons in a ship weighing 168 tons (twenty times as much) would have an initial acceleration of 4.7G.

Likewise, the 2nd stage J-2 had a 73-to-1 thrust/weight ratio.

Russian launch rockets have thrust/weight ratio of anywhere from 75:1 up to at least 137:1.

I'm not sure how generous (or not) Spaceships is with delta-vee, but it's conservative with rocket engine accelerations.

As for the TL9 HDEM rocket - it trades raw thrust for efficiency. I doubt anyone would think it very unreasonable for you to decide that chemical and HDEM rockets can use the High Thrust option available to most other reaction engines (x2 acceleration, x1/2 delta-vee per tank).

Now, if you want numbers that don't make much sense, consider jet engines. Being generous a modern jet manages a thrust/weight ratio of 5:1, or 8:1 with afterburning, which should give 0.25G per system (0.4G per system with afterburner). Also, they should burn an entire system of fuel per quarter hour of normal thrust and every 7.5 minutes on afterburner.
Thanks for your generous explanation. But then, Is the G that Reaction Drive provides not an initial G? Then, How can I set initial G for my Reaction-Drive-Driven vessels?
Pectus Solentis is offline   Reply With Quote
Old 02-27-2020, 04:18 AM   #5
Ulzgoroth
 
Join Date: Jul 2008
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Quote:
Originally Posted by Pectus Solentis View Post
Thanks for your generous explanation. But then, Is the G that Reaction Drive provides not an initial G? Then, How can I set initial G for my Reaction-Drive-Driven vessels?
It is an initial acceleration, and also a final acceleration, because while Spaceships doesn't entirely ignore the way decreasing rocket mass in flight affects delta-V, it does entirely ignore the way it affects acceleration.
__________________
I don't know any 3e, so there is no chance that I am talking about 3e rules by accident.
Ulzgoroth is online now   Reply With Quote
Old 02-27-2020, 04:21 AM   #6
Pectus Solentis
 
Join Date: Nov 2018
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Quote:
Originally Posted by Ulzgoroth View Post
it does entirely ignore the way it affects acceleration.
well... understood.
Pectus Solentis is offline   Reply With Quote
Old 02-27-2020, 04:32 AM   #7
Aldric
 
Join Date: Nov 2015
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Quote:
Originally Posted by Ulzgoroth View Post
It is an initial acceleration, and also a final acceleration, because while Spaceships doesn't entirely ignore the way decreasing rocket mass in flight affects delta-V, it does entirely ignore the way it affects acceleration.
Could you calculate the acceleration every time you empty a fuel tank for example?
Aldric is offline   Reply With Quote
Old 02-27-2020, 04:40 AM   #8
Rupert
 
Rupert's Avatar
 
Join Date: Aug 2004
Location: Wellington, NZ
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Quote:
Originally Posted by Aldric View Post
Could you calculate the acceleration every time you empty a fuel tank for example?
You could, though if you're doing that you should probably also do a more precise and correct calculation of the delta-vee in each tank, as it increases as the vessel gets lighter too, and Spaceships treats is as constant just as it does acceleration.

At some point you may as well just do all the calculations from scratch, and use real rocket science. (But not me - that's too much effort given the minimal return I'd get.)
__________________
Rupert Boleyn

"A pessimist is an optimist with a sense of history."
Rupert is offline   Reply With Quote
Old 02-27-2020, 04:41 AM   #9
Pectus Solentis
 
Join Date: Nov 2018
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Quote:
Originally Posted by Aldric View Post
Could you calculate the acceleration every time you empty a fuel tank for example?
Well, I understood that there will be a lot of calculation problem for computing accelerations of every times.
Pectus Solentis is offline   Reply With Quote
Old 02-27-2020, 05:10 AM   #10
Agemegos
 
Agemegos's Avatar
 
Join Date: May 2005
Location: Oz
Default Re: [Spaceships] It seems that TL7 Chemical Rocket's acceleration is an overspec.

Quote:
Originally Posted by Pectus Solentis View Post
When I played Kerbal Space Program, only 1.2G-1.6G was sufficient to escape Kerbin. And I know that TWR of real-world Saturn V is 1.15G.



But at GURPS Spaceships p21, it says that TL7 Chemical Rocket provides 3G accelerations. It seems that it is an astounding overspec. It is greater than real-world TL7 Chemical Rockets, and it is greater than even TL9 HDEM Chemical Rocket (it provides 2G).
Are you taking into account that the reaction engines in Spaceships are 5% of the mass of the entire ship including propellant?

A Saturn V all-up launch weight was 3,270 US tons. Its first-stage engines, five Rocketdyne F-1, totalled 46.25 US tons mass. That's one seventieth of the vehicle mass (1.4%), not one-twentieth of vehicle mass (5%).

In Spaceships' terms a Saturn V had less than one third of a system of reaction engine, chemical rocket (TL7). Build it as a smaller system using the rules in Spaceships 7 on page 4 — then you will be complaining that the TL7 chemical rocket is actually slightly under-spec¹ and that a Saturn V cannot take off until it has burned part of its fuel and if you write house rules for the changing acceleration as ship mass diminishes.

___________

¹ TL7 chemical rockets in Spaceships have a thrust-to-weight ratio of 60 g₀, Rocketdyne F-1 engines had a thrust-to-weight ratio of 84 g₀.
__________________
© copyright Brett Evill
FLAT BLACK discussion group at tekeli.li
On-line texts at flatblack.wikidot.com

Last edited by Agemegos; 02-27-2020 at 03:18 PM.
Agemegos is offline   Reply With Quote
Reply

Tags
spaceships

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 03:41 PM.


Powered by vBulletin® Version 3.8.9
Copyright ©2000 - 2020, vBulletin Solutions, Inc.