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MegaTraveller Discuss of the MegaTraveller ruleset and the Rebellion Milieu

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  #21  
Old March 6th, 2007, 08:18 PM
Liam Devlin Liam Devlin is offline
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The GDW TNE-FF&S TL9 Fusion Rocket (page 70) describes the following characteristics:

[img]graemlins/alpha.gif[/img] "Thrust: (in tonnes per cu.meter of engine) 9

[img]graemlins/alpha.gif[/img] Price (MCr, per cu.meter of engine) 0.35

[img]graemlins/alpha.gif[/img] Maximum Thrust (MaxT, as in tonnes, per engine installed, listed as a minimum per) 100

[img]graemlins/alpha.gif[/img] Fuel Consumption (FC/ in tonnes/ per hour) 0.00035.

[img]graemlins/alpha.gif[/img] Fuel Type (FT/ Fuel consumed) LHyd

[img]graemlins/alpha.gif[/img] Airframe (AF/ for vehicle required at these speeds achieved) Hypersonic.
"

That's a wee bit more than 5 liters..helpfully yours,
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  #22  
Old March 11th, 2007, 06:49 PM
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Fusion Rockets are incredibly dangerous, due to the radioactive exhausts. It has often been stated in various traveller materials that a single fusion engine could steralise entire continents due to its radioactivity, using them in crowded (local space) may even be considered an act of war in populous systems. This is one of the Few areas in which MegaTraveller falls down, the efficiency is way to high, the dangerous exhaust not considered and so on. In short later canon sources such as those found in TNE adressed the radioactivity, but by then I had created loads of MegaTraveller designs that had taken advantage of the high thrust/low fuel consumption.

Using Fusion Rockets in an Atmosphere is a definate no-no if you wish to keep it as realistic as possible.

What I want to know is what the real fuel consumption of a fusion rocket would be for a realistic unit of thrust.
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  #23  
Old March 11th, 2007, 06:49 PM
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Fusion Rockets are incredibly dangerous, due to the radioactive exhausts. It has often been stated in various traveller materials that a single fusion engine could steralise entire continents due to its radioactivity, using them in crowded (local space) may even be considered an act of war in populous systems. This is one of the Few areas in which MegaTraveller falls down, the efficiency is way to high, the dangerous exhaust not considered and so on. In short later canon sources such as those found in TNE adressed the radioactivity, but by then I had created loads of MegaTraveller designs that had taken advantage of the high thrust/low fuel consumption.

Using Fusion Rockets in an Atmosphere is a definate no-no if you wish to keep it as realistic as possible.

What I want to know is what the real fuel consumption of a fusion rocket would be for a realistic unit of thrust.
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  #24  
Old March 11th, 2007, 06:49 PM
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Fusion Rockets are incredibly dangerous, due to the radioactive exhausts. It has often been stated in various traveller materials that a single fusion engine could steralise entire continents due to its radioactivity, using them in crowded (local space) may even be considered an act of war in populous systems. This is one of the Few areas in which MegaTraveller falls down, the efficiency is way to high, the dangerous exhaust not considered and so on. In short later canon sources such as those found in TNE adressed the radioactivity, but by then I had created loads of MegaTraveller designs that had taken advantage of the high thrust/low fuel consumption.

Using Fusion Rockets in an Atmosphere is a definate no-no if you wish to keep it as realistic as possible.

What I want to know is what the real fuel consumption of a fusion rocket would be for a realistic unit of thrust.
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  #25  
Old March 12th, 2007, 03:26 AM
Wm_Humphrey Wm_Humphrey is offline
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Quote:
What I want to know is what the real fuel consumption of a fusion rocket would be for a realistic unit of thrust.
Be careful what you wish for!

All of the following is based on what I remember from my rocket propulsion class (the textbook is packed away somewhere), wich was almost 20 years ago now...

"Specific Impulse" is the measure of efficiency of any rocket. Divide thrust (in pounds or Newtons) by total propellant weight flow rate (in pounds or Newtons per second) and you get specific impulse (abbreviated Isp), in units of seconds.

The most efficient rocket engine of sizeable thrust to date is the space shuttle main engine (SSME) with an Isp of about 450 sec. Electric-ion thrusters, used for attitude control and station-keeping on some newer satellites, can have Isp values of something on the order of 10,000 sec.

We had a guest lecture in that class from a NASA engineer who described a fusion rocket system concept. I don't recall how the plasma was initially created at this point. I do recall that the plasma was to be exhausted through a magnetic nozzle and that in the pinch point at the throat of the nozzle, they were projecting that up to 3% of the plasma would undergo a fusion reaction. That would tremendously heat the exhasut just as it was being expanded and accelerated through the nozzle. Isp values on the order of 100,000 sec. were thought to be achievable.

So if you round up the Isp of a LHyd-LOx rocket to 500 sec. for convenience, then a fusion rocket would generate the same thrust with 1/200th of the fuel consumption.

In Traveller terms, a "ton" of thrust accelerates 1000 kg at one G. So that's actually 9.8 kN (let's round up to 10 kN for convenience). Based on an Isp of 100,000 seconds, a fusion rocket engine should consume 0.01 kg/s of hydrogen per ton (10 kN) of thrust. Let's call the motor NASA was talking about the TL-8 experimental fusion rocket from "Hard Times". (Sorry, I could find my copy of Hard Times, but not COACC.) That 150-ton thrust fusion rocket unit should consume 77 kL/hr of LHyd. The listed fuel consumption of 0.005 kL/hr gives a specific impulse of 1.5 BILLION seconds. Even if you decide that they meant kL/hr/ton of thrust for fuel consumption, the specific impulse is still 10 million seconds. I might buy faulty memory on my part or conservative estimates on the NASA guy's part if that number were one million seconds, but I hesitate at 10 million and just can't accept 1.5 billion seconds.

The problem isn't limited to the engines that don't yet exist, unfortunately. Assuming that the TL-6 "High Performance Rocket" is supposed to represent the SSME, it should (based on rounding both the thrust and Isp) consume about 2 kg/s of propellant per "ton" of thrust, or about 800 kL/hour for 39 tons of thrust. The listed fuel consumption of 24 kL/hr for 39 tons of thrust is a specific impulse of 16,700 seconds. If I remember correctly, the theoretical upper limit of Isp for a hydrogen-oxygen chemical rocket is around 800 seconds. If you assume that the listed fuel consumption is per ton of thrust, then the Isp of the high-performance rocket is a very reasonable 430 seconds.

I guess my twin suggestions would be to treat all fuel consumption numbers for the rocket engines in "Hard Times" as "per ton of thrust" and additionally to increase the fuel consumption of the fusion rockets by at least a factor of 10, if not 20, 50, or even 100.

Bill "Why, yes, I am (OK - was) a rocket scientist." H.
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  #26  
Old March 12th, 2007, 03:26 AM
Wm_Humphrey Wm_Humphrey is offline
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Quote:
What I want to know is what the real fuel consumption of a fusion rocket would be for a realistic unit of thrust.
Be careful what you wish for!

All of the following is based on what I remember from my rocket propulsion class (the textbook is packed away somewhere), wich was almost 20 years ago now...

"Specific Impulse" is the measure of efficiency of any rocket. Divide thrust (in pounds or Newtons) by total propellant weight flow rate (in pounds or Newtons per second) and you get specific impulse (abbreviated Isp), in units of seconds.

The most efficient rocket engine of sizeable thrust to date is the space shuttle main engine (SSME) with an Isp of about 450 sec. Electric-ion thrusters, used for attitude control and station-keeping on some newer satellites, can have Isp values of something on the order of 10,000 sec.

We had a guest lecture in that class from a NASA engineer who described a fusion rocket system concept. I don't recall how the plasma was initially created at this point. I do recall that the plasma was to be exhausted through a magnetic nozzle and that in the pinch point at the throat of the nozzle, they were projecting that up to 3% of the plasma would undergo a fusion reaction. That would tremendously heat the exhasut just as it was being expanded and accelerated through the nozzle. Isp values on the order of 100,000 sec. were thought to be achievable.

So if you round up the Isp of a LHyd-LOx rocket to 500 sec. for convenience, then a fusion rocket would generate the same thrust with 1/200th of the fuel consumption.

In Traveller terms, a "ton" of thrust accelerates 1000 kg at one G. So that's actually 9.8 kN (let's round up to 10 kN for convenience). Based on an Isp of 100,000 seconds, a fusion rocket engine should consume 0.01 kg/s of hydrogen per ton (10 kN) of thrust. Let's call the motor NASA was talking about the TL-8 experimental fusion rocket from "Hard Times". (Sorry, I could find my copy of Hard Times, but not COACC.) That 150-ton thrust fusion rocket unit should consume 77 kL/hr of LHyd. The listed fuel consumption of 0.005 kL/hr gives a specific impulse of 1.5 BILLION seconds. Even if you decide that they meant kL/hr/ton of thrust for fuel consumption, the specific impulse is still 10 million seconds. I might buy faulty memory on my part or conservative estimates on the NASA guy's part if that number were one million seconds, but I hesitate at 10 million and just can't accept 1.5 billion seconds.

The problem isn't limited to the engines that don't yet exist, unfortunately. Assuming that the TL-6 "High Performance Rocket" is supposed to represent the SSME, it should (based on rounding both the thrust and Isp) consume about 2 kg/s of propellant per "ton" of thrust, or about 800 kL/hour for 39 tons of thrust. The listed fuel consumption of 24 kL/hr for 39 tons of thrust is a specific impulse of 16,700 seconds. If I remember correctly, the theoretical upper limit of Isp for a hydrogen-oxygen chemical rocket is around 800 seconds. If you assume that the listed fuel consumption is per ton of thrust, then the Isp of the high-performance rocket is a very reasonable 430 seconds.

I guess my twin suggestions would be to treat all fuel consumption numbers for the rocket engines in "Hard Times" as "per ton of thrust" and additionally to increase the fuel consumption of the fusion rockets by at least a factor of 10, if not 20, 50, or even 100.

Bill "Why, yes, I am (OK - was) a rocket scientist." H.
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  #27  
Old March 12th, 2007, 03:26 AM
Wm_Humphrey Wm_Humphrey is offline
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Quote:
What I want to know is what the real fuel consumption of a fusion rocket would be for a realistic unit of thrust.
Be careful what you wish for!

All of the following is based on what I remember from my rocket propulsion class (the textbook is packed away somewhere), wich was almost 20 years ago now...

"Specific Impulse" is the measure of efficiency of any rocket. Divide thrust (in pounds or Newtons) by total propellant weight flow rate (in pounds or Newtons per second) and you get specific impulse (abbreviated Isp), in units of seconds.

The most efficient rocket engine of sizeable thrust to date is the space shuttle main engine (SSME) with an Isp of about 450 sec. Electric-ion thrusters, used for attitude control and station-keeping on some newer satellites, can have Isp values of something on the order of 10,000 sec.

We had a guest lecture in that class from a NASA engineer who described a fusion rocket system concept. I don't recall how the plasma was initially created at this point. I do recall that the plasma was to be exhausted through a magnetic nozzle and that in the pinch point at the throat of the nozzle, they were projecting that up to 3% of the plasma would undergo a fusion reaction. That would tremendously heat the exhasut just as it was being expanded and accelerated through the nozzle. Isp values on the order of 100,000 sec. were thought to be achievable.

So if you round up the Isp of a LHyd-LOx rocket to 500 sec. for convenience, then a fusion rocket would generate the same thrust with 1/200th of the fuel consumption.

In Traveller terms, a "ton" of thrust accelerates 1000 kg at one G. So that's actually 9.8 kN (let's round up to 10 kN for convenience). Based on an Isp of 100,000 seconds, a fusion rocket engine should consume 0.01 kg/s of hydrogen per ton (10 kN) of thrust. Let's call the motor NASA was talking about the TL-8 experimental fusion rocket from "Hard Times". (Sorry, I could find my copy of Hard Times, but not COACC.) That 150-ton thrust fusion rocket unit should consume 77 kL/hr of LHyd. The listed fuel consumption of 0.005 kL/hr gives a specific impulse of 1.5 BILLION seconds. Even if you decide that they meant kL/hr/ton of thrust for fuel consumption, the specific impulse is still 10 million seconds. I might buy faulty memory on my part or conservative estimates on the NASA guy's part if that number were one million seconds, but I hesitate at 10 million and just can't accept 1.5 billion seconds.

The problem isn't limited to the engines that don't yet exist, unfortunately. Assuming that the TL-6 "High Performance Rocket" is supposed to represent the SSME, it should (based on rounding both the thrust and Isp) consume about 2 kg/s of propellant per "ton" of thrust, or about 800 kL/hour for 39 tons of thrust. The listed fuel consumption of 24 kL/hr for 39 tons of thrust is a specific impulse of 16,700 seconds. If I remember correctly, the theoretical upper limit of Isp for a hydrogen-oxygen chemical rocket is around 800 seconds. If you assume that the listed fuel consumption is per ton of thrust, then the Isp of the high-performance rocket is a very reasonable 430 seconds.

I guess my twin suggestions would be to treat all fuel consumption numbers for the rocket engines in "Hard Times" as "per ton of thrust" and additionally to increase the fuel consumption of the fusion rockets by at least a factor of 10, if not 20, 50, or even 100.

Bill "Why, yes, I am (OK - was) a rocket scientist." H.
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  #28  
Old March 12th, 2007, 05:36 AM
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Hi !

An pretty easy approach is to use the simple formula for thrust based on reaction:

Thrust[N] = Mass-Flow[kg/s-1] * Exhaust velocity [m/s]

or
Needed Exhaust velocity [m/s] = Thrust/Massflow

Result is in Newton, meaning you would have to div by 10 to get a related thrust value in kg and div by another 1000 to get thrust in tons (in earth g environment!).

You can use this one to check which values given in the rulesets are ok or just bullshit.
A mass-flow of 5 l/hour is just around 0,0001 kg/s (different result here Humphrey ?). For a rocket this is near to nothing.
The needed exhaust velocities to get 130 tons of thrust are beyond reality, namely

1300000 N / 0.0001 kg/s-1 = 13371428571 m/s

As you might see, the result is a bit unphysical [img]smile.gif[/img]

E.g, if I would try to get the 130 tons of thrust and assume a reasonable exhaust velocity around 100000 m/s, this results a a fuel usage of 655 kl/h.

Just play a bit with the numbers ... [img]smile.gif[/img]

regards,

TE
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  #29  
Old March 12th, 2007, 05:36 AM
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TheEngineer TheEngineer is offline
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Hi !

An pretty easy approach is to use the simple formula for thrust based on reaction:

Thrust[N] = Mass-Flow[kg/s-1] * Exhaust velocity [m/s]

or
Needed Exhaust velocity [m/s] = Thrust/Massflow

Result is in Newton, meaning you would have to div by 10 to get a related thrust value in kg and div by another 1000 to get thrust in tons (in earth g environment!).

You can use this one to check which values given in the rulesets are ok or just bullshit.
A mass-flow of 5 l/hour is just around 0,0001 kg/s (different result here Humphrey ?). For a rocket this is near to nothing.
The needed exhaust velocities to get 130 tons of thrust are beyond reality, namely

1300000 N / 0.0001 kg/s-1 = 13371428571 m/s

As you might see, the result is a bit unphysical [img]smile.gif[/img]

E.g, if I would try to get the 130 tons of thrust and assume a reasonable exhaust velocity around 100000 m/s, this results a a fuel usage of 655 kl/h.

Just play a bit with the numbers ... [img]smile.gif[/img]

regards,

TE
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  #30  
Old March 12th, 2007, 05:36 AM
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TheEngineer TheEngineer is offline
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Hi !

An pretty easy approach is to use the simple formula for thrust based on reaction:

Thrust[N] = Mass-Flow[kg/s-1] * Exhaust velocity [m/s]

or
Needed Exhaust velocity [m/s] = Thrust/Massflow

Result is in Newton, meaning you would have to div by 10 to get a related thrust value in kg and div by another 1000 to get thrust in tons (in earth g environment!).

You can use this one to check which values given in the rulesets are ok or just bullshit.
A mass-flow of 5 l/hour is just around 0,0001 kg/s (different result here Humphrey ?). For a rocket this is near to nothing.
The needed exhaust velocities to get 130 tons of thrust are beyond reality, namely

1300000 N / 0.0001 kg/s-1 = 13371428571 m/s

As you might see, the result is a bit unphysical [img]smile.gif[/img]

E.g, if I would try to get the 130 tons of thrust and assume a reasonable exhaust velocity around 100000 m/s, this results a a fuel usage of 655 kl/h.

Just play a bit with the numbers ... [img]smile.gif[/img]

regards,

TE
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