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Old 09-19-2020, 05:07 AM   #1
Pectus Solentis
 
Join Date: Nov 2018
Location: Live in Seoul, Korea and I have never been abroad.
Default [Spaceships 1] A question about Ram-Rocket Engines

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Ram- Rockets
The nuclear thermal rocket, antimatter thermal rocket, fusion torch, super fusion torch, antimatter plasma torch, and super antimatter plasma torch drives may have an auxiliary air- breathing air- ram mode for atmosphere. They suck in air with a turbine, heat it using their onboard drive reactor, and expel it for thrust. Thus, they don’t require fuel while flying in an atmosphere (breathable or otherwise) provided it is “very thin” or greater density (see p. B429). ultiply cost by 5.
It is the description of Ram Rocket Engine. But There is no mention if Ram Rocket's Acceleration is affected by Engine's and Reaction Mass's type or not.

Quote:
*SM +4, Loaded Mass 10ton, TL11.
*Front
**System 1 : Armor, Metallic Laminate, dDR 1.
**System 2 : Cargo Hold, 0.5ton
**System 3 : Cargo Hold, 0.5ton
**System 4 : Cargo Hold, 0.5ton
**System 5 : Cargo Hold, 0.5ton
**System 6 : Cargo Hold, 0.5ton
**Core System : Control Room : C7 Computer installed. Basically automatic, but 1 person is able to board this vessel.
*Central
**System 1 : Armor, Metallic Laminate, dDR 1.
**System 2 : Cargo Hold, 0.5ton
**System 3 : Cargo Hold, 0.5ton
**System 4 : Cargo Hold, 0.5ton
**System 5 : Cargo Hold, 0.5ton
**System 6 : Cargo Hold, 0.5ton
**Core System : N/A
*Rear
**System 1 : Armor, Metallic Laminate, dDR 1.
**System 2 : Fusion Torch Ram Rocket (water reaction mass, 1.5G)
**System 3 : Cargo Hold, 0.5ton
**System 4 : Cargo Hold, 0.5ton
**System 5 : Cargo Hold, 0.5ton
**System 6 : Cargo Hold, 0.5ton
**Core System : Fuel Tanks, 0.5 tons of Water (24km/s delta-V)
*Costs 1.015M$
It is my Foldable SSTO which is used by my planet explorer to lift their specimens on to orbitting Exploration Mothership (that has Major Labs to research the specimens explorer got).
Since this SSTO has a Ram Rocket it maneuvers in the atmosphere without Reaction Mass, but the Acceleration of Fusion Torch varies by which Reaction Mass it use. Basically my SSTO uses water as Reaction Mass and gets 1.5G Acclerations, originally its engine uses hydrogen as Reaction Mass and gets 0.5G Accelerations. But what if it uses air as Reaction mass?

I can't find the mention which Accleration will the Ram Rocket get when its usable Reaction Mass and gettable Accleration varies. This is my question.

Last edited by Pectus Solentis; 09-19-2020 at 05:10 AM. Reason: grammar correction
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Old 09-19-2020, 08:01 AM   #2
AlexanderHowl
 
Join Date: Feb 2016
Default Re: [Spaceships 1] A question about Ram-Rocket Engines

Ram-Rockets have the same amount of base thrust, there is no stated change in their description, so they are really only useful for NTRs in realistic campaigns (and their greater cost makes that questionable). By the way, the cost for a SM+4 fusion torch ram rocket is $1M, so your cost is off a little.

Last edited by AlexanderHowl; 09-19-2020 at 08:07 AM.
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Old 09-19-2020, 08:05 AM   #3
Varyon
 
Join Date: Jun 2013
Default Re: [Spaceships 1] A question about Ram-Rocket Engines

To my knowledge, this isn’t addressed anywhere. From what I understand, acceleration is a function of molecular mass. Water (H2O) has roughly 10x the molecular mass of molecular hydrogen (H2), and 3x the acceleration, implying that, if we go off the Size and Speed/Range table, every +2 SSR to molar mass is +1 SSR to acceleration. The average molecular mass of dry air is around 15x that of molecular hydrogen, or +7 SSR. That works out to +3.5 SSR, which can be approximated as around x4 acceleration.

Of course, I may be completely off. I think there are rules for using ammonia, so you may want to see if the relationship to molecular mass is the same there.
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Old 09-19-2020, 08:13 AM   #4
Pectus Solentis
 
Join Date: Nov 2018
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Default Re: [Spaceships 1] A question about Ram-Rocket Engines

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Originally Posted by AlexanderHowl View Post
Ram-Rockets have the same amount of base thrust, there is no stated change in their description, so they are really only useful for NTRs in realistic campaigns (and their greater cost makes that questionable)
Well...
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Originally Posted by AlexanderHowl View Post
By the way, the cost for a SM+4 fusion torch ram rocket is $1M, so your cost is off a little.
Thanks for letting me know my error. Then the price of SSTO should be recalculated.
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Old 09-19-2020, 08:20 AM   #5
Pectus Solentis
 
Join Date: Nov 2018
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Default Re: [Spaceships 1] A question about Ram-Rocket Engines

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Originally Posted by AlexanderHowl View Post
By the way, the cost for a SM+4 fusion torch ram rocket is $1M, so your cost is off a little.
Recalculated that, but I got 1.368M$. Anything except Fusion Torch Ram Rocket was so cheap.
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Old 09-19-2020, 10:43 AM   #6
AlexanderHowl
 
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Default Re: [Spaceships 1] A question about Ram-Rocket Engines

Yes, that sounds about right. As I said, it was off by a little.
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Old 09-23-2020, 03:06 PM   #7
cvannrederode
 
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Default Re: [Spaceships 1] A question about Ram-Rocket Engines

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Originally Posted by Varyon View Post
To my knowledge, this isn’t addressed anywhere. From what I understand, acceleration is a function of molecular mass.
Not quite...Specific Impulse is inversely proportional to molecular (or molar) mass by way of exhaust velocity, so it directly affects the delta-v numbers. (real hydrox engines burn very fuel rich to keep the average molar weight of their exhaust low for this reason among others)

However, two similar designs: one with water and one with H2...the water one will have a much high mass flow rate through the engine, since liquid flow will be more volume limited. (A gallon of liquid hydrogen masses about 9 ounces). That will generally give it higher thrust.

The authors seem to have combined both effects into one, and given them the same number to simplify things. A torch throwing water has 3X thrust but 1/3x delta-v compared to one using H2. I'm not sure how much of a hand Dr Kromm had in SS, but the attention to real rock science just drips off the book.

(Of course, I could be just as off base)
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Old 09-23-2020, 03:43 PM   #8
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Default Re: [Spaceships 1] A question about Ram-Rocket Engines

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Originally Posted by cvannrederode View Post
The authors seem to have combined both effects into one, and given them the same number to simplify things. A torch throwing water has 3X thrust but 1/3x delta-v compared to one using H2.
You can get there by assuming that the temperature of the drive core is fixed, perhaps limited by the temperature at which the parts melt or sublime. That gives you a fixed thermal energy per molecule, which the rocket nozzle turns into an exhaust velocity inversely proportional to the average molecular weight of the exhaust species.
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