[HIgh Guard] Reaction Drives

[Mithras]

I'm really pleased that HG includes rules for reaction drives, even more pleased they gave us stats for the space shuttle!

I may not be reading it right though, but I'm not sure I understand fuel consumption for a normal two weeks of operation. The rules discus number of burns of Thrust during combat, but not typical travel times insystem, which would be extremely valuable to me!

Can anyone extrapolate from the rules, or are they already there...?

 

[Rikki Tikki Traveller]

Travel times are math, not rules.

Assuming continuous thrust you can use the following formulas for travel times:

Constant acceleration from a standing start:

T = Squareroot (2*D/a)

For constant acceleration to midpoint and then deceleration to endpoint (ending velocity is essentially zero, like starting velocity):

T = 2*squareroot(D/a)

Where:

T is time
D is Distance
a is acceleration

You need to make sure that your distance and acceleration are in the same unit system so that you can get a good number for the distance.

If you are looking for a "close enough" unit conversion, you can use the following for interplanetary travel:

T in DAYS
D in Astronomical Units (AU)
a in Gees of acceleration

You will get an answer within about 10% of doing all the unit conversions.

Hope that helps.

 

[BP]

Just to clarify - Rikki Tikki Traveller's formulas are from CT (Book 2 - I still use them myself)...

These appear to be the same for MGT, though I sadly could not find any reference to them...

For MGT Core pg 145 covers Travel Times with a table (Interplanetary Transit Times Table). This matches the formulas and is basically the CT Typical Travel Times table updated to include the Typical Distances table's terms (Surface to Orbit... Far Gas Giant). Note that this table is computed from the formula's Rikki Tikki Traveller (that's a mouthful ) provided.

 

[Mithras]

Well, I know the CT LBB2 formulae , but HG posits only an initial acceleration of a few hours with the rest of the journey coasting.

Perhaps I can turn to TransHuman Space for guidance...?

 

[Rikki Tikki Traveller]

Just to clarify - Rikki Tikki Traveller's formulas are from CT (Book 2 - I still use them myself)...

These appear to be the same for MGT, though I sadly could not find any reference to them...

For MGT Core pg 145 covers Travel Times with a table (Interplanetary Transit Times Table). This matches the formulas and is basically the CT Typical Travel Times table updated to include the Typical Distances table's terms (Surface to Orbit... Far Gas Giant). Note that this table is computed from the formula's Rikki Tikki Traveller (that's a mouthful ) provided.

No, my formulas are physics. They are printed in CT and other games, but they are math and physics, not tied to a specific game. Also, my edition of LBB1-3 (first edition) has an error in this formula that was later corrected.

 

[qstor]

Perhaps I can turn to TransHuman Space for guidance...?

There is some discussion of Reaction drives in GT Starships too.

Mike

 

[Rikki Tikki Traveller]

Typical Travel times to the 100D limit for planets:

ASSUMPTION 1: Constant acceleration from orbit to 100D limit (no turn around)

Travel time is in HOURS

Size   1g    2g    3g    4g    5g    6g

 1    0.79  0.56  0.46  0.40  0.35  0.32
 2    1.12  0.79  0.65  0.56  0.50  0.46
 3    1.37  0.97  0.79  0.69  0.61  0.56
 4    1.59  1.12  0.92  0.79  0.71  0.65
 5    1.77  1.25  1.02  0.89  0.79  0.72

 6    1.94  1.37  1.12  0.97  0.87  0.79
 7    2.10  1.48  1.21  1.05  0.94  0.86
 8    2.24  1.59  1.30  1.12  1.00  0.92
 9    2.38  1.68  1.37  1.19  1.06  0.97
10    2.51  1.77  1.45  1.25  1.12  1.02

SGG   4.69  3.32  2.71  2.35  2.10  1.92
LGG   6.64  4.69  3.83  3.32  2.97  2.71

ASSUMPTION 2: Starting and ending velocity is ZERO (Constant thrust to midpoint then turn around and decelerate to endpoint). This is most common method.

Time is in HOURS

Size   1g    2g    3g    4g    5g    6g

 1    2.2   1.6   1.3   1.1   1.0   0.9
 2    3.2   2.2   1.8   1.6   1.4   1.3
 3    3.9   2.8   2.2   1.9   1.7   1.6
 4    4.5   3.2   2.6   2.2   2.0   1.8  
 5    5.0   3.6   2.9   2.5   2.2   2.1 

 6    5.5   3.9   3.2   2.8   2.5   2.2
 7    5.9   4.2   3.4   3.0   2.7   2.4
 8    6.3   4.5   3.7   3.2   2.8   2.6
 9    6.7   4.8   3.9   3.4   3.0   2.8
10    7.1   5.0   4.1   3.6   3.2   2.9

SGG  13.3   9.4   7.7   6.6   5.9   5.4
LGG  18.8  13.3  10.8   9.4   8.4   7.7

ASSUMPTION 3: Interplanetary travel. Distance is in Astronomical Units (AU). 1 AU = 150,000,000 km = 93,000,000 miles

Travel time is in DAYS.

AU     1g    2g    3g    4g    5g    6g 

0.1   0.63  0.45  0.37  0.32  0.28  0.26 
0.5   1.41  1.00  0.82  0.71  0.63  0.58 
1.0   2.00  1.41  1.15  1.00  0.89  0.82 
2.0   2.83  2.00  1.63  1.41  1.26  1.15 
5.0   4.47  3.16  2.58  2.24  2.00  1.83 

10    6.32  4.47  3.65  3.16  2.83  2.58 
20    8.94  6.32  5.16  4.47  4.00  3.65 
50   14.1  10.0   8.16  7.07  6.32  5.77 
100  20.0  14.1  11.6  10.0   8.94  8.16

{Edited table 3 to correct an error in my equation. }

All values are rounded and approximate. They do not take into account any delay for space traffic control or orbital insertion burns.[/code]

 

[atpollard]

ASSUMPTION 3: Interplanetary travel. Distance is in Astronomical Units (AU). 1 AU = 150,000,000 km = 93,000,000 miles

Travel time is in DAYS.

Don't leave us hanging, what about PARSECS for interstellar travel?

 

[Mithras]

Of course! Was it that obvious? Simply working out the travel time to 100D distance ... why didn't I think about it a bit more

Many thanks - With Rikki's tables and HG I can make reaction drive rockets!

Typical Travel times to the 100D limit for planets:

 

[Rikki Tikki Traveller]

ATPollard: parsecs wont work with these formulas since you will get to relativistic speeds and these formulas don't work above about 80% of the speed of light (but you knew that right?).

Mithras, check out the thread where I have gone through the thought process for developing a realistic Reaction Drive.

Short version: 3 tons of fuel per day of thrust at 1g for a 100 ton ship. Scale linearly as desired. I also recommend that you drop the requirement for PP fuel and assume refueling every annual maintenance. Now your endurance is in DAYS of thrust, not weeks of Power. You might even be able to keep some of the Standard Designs.'

1 day of thrust at 1g will get you to Mars (at closest approach), so most Traveller designs will need a lot less than that most of the time.

 

[atpollard]

ATPollard: parsecs wont work with these formulas since you will get to relativistic speeds and these formulas don't work above about 80% of the speed of light (but you knew that right?).

Yes, but we've ignored so many other problems like:

1. High Thrust and High Efficiency mean engine temperatures hotter than the sun.
2. The 6G exhaust will probably punch small holes through the planet.
3. Efficiency decreases as the ship's speed approaches exhaust velocity (a problem for the longer interplanetary trips).
4. the exhaust is a weapon (Fusion Gun, Particle Accelerator or Laser depending on assumed technology and temperatures).

Don't take me wrong, it is right to ignore these things. Accurate rocket design is for NASA and not a Science Fiction game. All things considered, ignoring the speed of light seems like just another handwave to make the game interesting.

(Pseudo) Bussard Ramjet Interstellar Traveller anyone?

 

[lastbesthope]

3. Efficiency decreases as the ship's speed approaches exhaust velocity (a problem for the longer interplanetary trips).

Would it? The exhaust velocity will still be the same relative to the ship won't it, I mean the fuel/engine/etc onboard will all be accelerated up to the same velocity as the ship while it's on board won't it?

LBH

 

[Rikki Tikki Traveller]

3. Efficiency decreases as the ship's speed approaches exhaust velocity (a problem for the longer interplanetary trips).

Would it? The exhaust velocity will still be the same relative to the ship won't it, I mean the fuel/engine/etc onboard will all be accelerated up to the same velocity as the ship while it's on board won't it?

LBH

Yes, it would. There is a nice little curve that explains this, remember this is actually calculus, not linear math. At velocities well below Vexh, the curve is essentially linear (which is what I assumed), but once your speed is about half of your exhaust velocity then the efficiency starts going way down. It is essentially impractical to accelerate once your speed reaches Vexh. Bussard Ramjets are an exception here since they don't carry their fuel with them.

The amount of fuel necessary to accelerate a ship to a velocity equal Vexh starts approaching infinity.

I also ignored relativity which on longer journeys would become a factor.

BUT, considering that any trip that takes more than 7 days to complete will probably be done via Jump Drive, we have a self limiting speed built into the setting.

YES, you CAN accelerate a ship to near light speed (Near C rocks) but it will take so much fuel and so much time that it really is impractical. Much easier to find an asteroid that in another system that is already moving in the opposite direction of your target and Jump it to the 100D limit of your target planet. Anyway, I digress...

ATPOLLARD, I didn't mean to dismiss your objections, they are valid. But as a first pass at coming up with a Reaction Drive that can be used in Traveller, I have given the numbers so that someone can design what they want.

Jame Rowe is using the PP fuel as reaction mass for 1 week of acceleration, so he can use existing designs. A quick check shows that the Exhaust Velocity of his drive is essentially at light speed (but not greater than light speed). So, it is unlikely, but not impossible. As a game solution, I don't have a problem with it, even if I would personally make it 1 day of maneuvering rather than 1 week.

 

[atpollard]

ATPOLLARD, I didn't mean to dismiss your objections, they are valid. But as a first pass at coming up with a Reaction Drive that can be used in Traveller, I have given the numbers so that someone can design what they want.

I didn't actually have any objections. I like what you came up with. I was simply pointing out that the next logical stop along the 'more realistic reaction drives' road would be a 'no jumpspace' interstellar travel table based on the assumptions of the simplified travel formula.

I was suggesting that violating the speed of light seemes no worse than many other simplifying assumptions ... only engineers would care and nothing makes us happy. :wink:

 

[Rikki Tikki Traveller]

Well, in an exercise in silliness, I can do that. Let me play a bit and I will post something tomorrow.

You are right, we Engineers are never happy. :wink:

 

[GypsyComet]

As one buddy puts it, you can pose a problem with multiple solutions to an engineer, and he'll pick the most complex one "because it would be cool".

As someone who works with a couple Civil Engineers, however, I see that Mad Science tendency reduced to the occasional speculative gleam in the eye once said engineer becomes a Registered Professional.

 

[Rikki Tikki Traveller]

CRAP! I just realized that my spreadsheet used the wrong formula when I calculated the interplanetary distances!

Below are the corrected values.

ASSUMPTION 3: Interplanetary travel. Distance is in Astronomical Units (AU). 1 AU = 150,000,000 km = 93,000,000 miles

Travel time is in DAYS.

AU     1g    2g    3g    4g    5g    6g 

0.1   0.63  0.45  0.37  0.32  0.28  0.26 
0.5   1.41  1.00  0.82  0.71  0.63  0.58 
1.0   2.00  1.41  1.15  1.00  0.89  0.82 
2.0   2.83  2.00  1.63  1.41  1.26  1.15 
5.0   4.47  3.16  2.58  2.24  2.00  1.83 

10    6.32  4.47  3.65  3.16  2.83  2.58 
20    8.94  6.32  5.16  4.47  4.00  3.65 
50   14.1  10.0   8.16  7.07  6.32  5.77 
100  20.0  14.1  11.6  10.0   8.94  8.16

All values are rounded and approximate. They do not take into account any delay for space traffic control or orbital insertion burns.[/code]

For fun, here are some interstellar distances assuming no time dilation etc.

Using the Pollard Effect, it is possible to exceed the speed of light in normal space. The Pollard Field Generator (same size as a Jump Drive btw) creates a field around the ship that keeps space-time uncurved. Therefore, Newtonian physics can be used to travel FTL.

Below is a table of interstellar distances and the necessary travel times.

FYI: Ships pass the speed of light after 355 days of acceleration at 1g.

As before, it is assumed that the ship accelerates for 1/2 the trip and then decelerates for 1/2 the trip at a constant rate.

Time in DAYS:

 AU     1g   2g    3g   4g   5g   6g
1000    63   45    37   32   28   26
5000   141  100    82   71   63   58
10000  200  141   116  100   89   82

TIME IN YEARS:
(1 LY = 63,241 AU)

 LY    1g    2g    3g    4g    5g    6g
 1    1.4   1.0   0.80  0.69  0.62  0.56
 2    1.9   1.4   1.1   1.0   0.87  0.80
 3    2.4   1.7   1.4   1.2   1.1   1.0

TIME IN YEARS:
(1 PC = 3.26 LY)

PC    1g   2g   3g   4g   5g   6g
 1   2.5  1.8  1.4  1.2  1.1  1.0
 2   3.5  2.5  2.0  1.8  1.6  1.4
 3   4.3  3.0  2.5  2.2  1.9  1.8
 4   5.0  3.5  2.9  2.5  2.2  2.0
 5   5.6  3.9  3.2  2.8  2.5  2.3
 6   6.1  4.3  3.5  3.0  2.7  2.5
100  61   43   35   31   27   25

As you can see, the Pollard Drive is not an effective interstellar transportation means for a game setting.

 

[atpollard]

Technically, the rules do not prohibit FTL speeds with the Grav MD, so the Pollard Generator might be unnecessary in some cases.*
From MgT High Guard we know that up to 16G is possible.

We are still talking interstellar trips of over a year, but they might be possible before TL 9 and there are no limits to distance (except fuel duration for the PP).

*[EDIT: If you tell me how reactionless drives work , I might be able to give a better answer. ]

 

[lastbesthope]

3. Efficiency decreases as the ship's speed approaches exhaust velocity (a problem for the longer interplanetary trips).

Would it? The exhaust velocity will still be the same relative to the ship won't it, I mean the fuel/engine/etc onboard will all be accelerated up to the same velocity as the ship while it's on board won't it?

LBH

Yes, it would. There is a nice little curve that explains this, remember this is actually calculus, not linear math. At velocities well below Vexh, the curve is essentially linear (which is what I assumed), but once your speed is about half of your exhaust velocity then the efficiency starts going way down. It is essentially impractical to accelerate once your speed reaches Vexh. Bussard Ramjets are an exception here since they don't carry their fuel with them.

I'll believe you, it's been many years since I last studied the equations, but it all sounds familiar.

LBH

 

[Rikki Tikki Traveller]

Maybe the Pollard Drive would allow additional acceleration, say increase the effective acceleration by a factor of 10, but can only be use outside of a gravity well (say 10AU from a typical G2V star), that way it doesn't affect interplanetary travel, but can be used for interstellar travel.

If you do that, then you get shorter trip times (still not short enough for an RPG though).

For example:

TIME IN DAYS

       10g  20g  30g  40g  50g  60g
  LY
  1    159  112   92   80   71   65
  2    225  159  130  112  101   92

  PC
  1    287  203  166  144  128  117
  2    406  287  235  203  182  166
  3    498  352  287  249  222  203
  4    574  406  332  287  257  235
  5    642  454  371  321  287  262
  6    704  498  405  352  315  287

Of course to achieve this kind of acceleration, we are back into using Unobtainium with Armwavy science, so I don't know that it is any better than reactionless drives.