Up by the bootstraps or, Interstellar Empire sans gravitics

[AndrewW]

Also Trek uses ION manuever drives - which at the time where pure speculation, but today are actively used in the newest DirectTV satellites and some space probes.

The use of Ion propulsion started with Deep Space 1 which was a testbed for various technologies.

 

[BP]

Yep - the first of the 2 NASA technology testbed platforms - space probes whose primary mission was to probe applied space technologies (and not any specific solar bodies/features).

It is interesting to note that ION drives have been used more in commercial applications than research...

 

[simonh]

However, the thing about these handwaves that Traveller does, is one should at least try to posit some sort of fix with reality. Thereby avoiding the worst excesses of Space Opera.

I'll have a stab at another 'fix' gravitic drives that might make them more interesting and constrained.

Suppose the gravitic drive emits gravitons. In this model, the drive would use up matter which is converted into the graviton emissions. Even assuming ideal conversion of mass to gravitons this would place practical limits on the maximum speed a ship could reach, based on the mass available for conversion. For example to reach 10% of the speed of light, the ship would have to set aside 10% of it's total mass for conversion matter.

Essentially this just means the drive is actually a form of reaction drive, but a super-efficient one. For most normal ship designs, you could even assume that the conversion mass is subsumed within the current design system, with some small fraction of the manoeuvre drive mass being set aside for conversion mass, say a nominal 0.1% of the mass of the ship per Gravity of acceleration. For a 1G drive this gives a cruising speed about 150,000 m/s or 1 AU per 6 days. For a 6G drive this goes up to 900,000 m/s and 1AU per day, ignoring acceleration and deceleration times.

Allowing for containment, I'd probably require double size for additional reaction mass.

Simon Hibbs

 

[BP]

...For example to reach 10% of the speed of light, the ship would have to set aside 10% of it's total mass for conversion matter. ...

Doesn't really fit well in the Traveller setting - why not just consume fuel (hydrogen) and use distances and masses for solar bodies?

 

[Rikki Tikki Traveller]

BP, that is perfectly doable, but you run into the Mass vs Volume issue. Traveller used Volume for convenience, but a reaction drive will need MASS to calculate acceleration. Also, if you don't completely fill the cargo hold, or fuel tanks, you would be able to accelerate faster. Traveller simplified all those assumptions (remember calculators were only just starting to show up in 1977 and they were EXPENSIVE). Today, with PCs and such, these calculations would be essentially trivial, but not in 1977.

I think someone needs to take some time and do an OGL Reaction drive system that would replace the Mdrive but not destroy the ship designs.

Maybe do away with PP fuel (assume 1 year of fuel internal to the units) and then add Reaction Mass. The calculations would have to be fairly simple, perhaps tabular and then good travel time and speed tables could be made (simple HS math in tabular form).

 

[Ishmael]

I think someone needs to take some time and do an OGL Reaction drive system that would replace the Mdrive but not destroy the ship designs.

I don't think that's really going to be possible. Existing ship designs would be destroyed.
FFS tried and gave us HePlar which has all the problems that simonh said; not possible/unrealistic and greatly reduces standard TU ship performance.
At best, it is halfway between the two extremes in the hard vs. space opera balance.

At least it 'sounds' better than handwaved "dark matter ramjets"
( hmmmm jet intakes?... tech to use dark matter the same way airplanes use air? banking and swooshing? )

I'll stick to HePlar* and fractional G thrust and limited burn transits

*and other more realistic rockets like chemical boosters, ion thrusters, etc.

 

[BP]

BP, that is perfectly doable, but you run into the Mass vs Volume issue. Traveller used Volume for convenience, but a reaction drive will need MASS to calculate acceleration. Also, if you don't completely fill the cargo hold, or fuel tanks, you would be able to accelerate faster. Traveller simplified all those assumptions (remember calculators were only just starting to show up in 1977 and they were EXPENSIVE). Today, with PCs and such, these calculations would be essentially trivial, but not in 1977.

Oh -I quite agree - I was referring only to a 'better' fit for the scenario given - and mass from volume is no problem since we are dealing with a homogeneous volume (density = hydrogen) and the calcs would be arbitrary (i.e. unit = 1x).
Calcs would have been by tables back then (and slide rules - used them even after calculators due to precision issues in early calculators )

I think someone needs to take some time and do an OGL Reaction drive system that would replace the Mdrive but not destroy the ship designs.

Maybe do away with PP fuel (assume 1 year of fuel internal to the units) and then add Reaction Mass. The calculations would have to be fairly simple, perhaps tabular and then good travel time and speed tables could be made (simple HS math in tabular form).

Quite - and I believe it is doable.

The travel and speed parts do add other problems (not to mention atmo type, density issues - but I would keep gravitics when close in to gravity wells). The system generation needs a few missing elements (i.e. - determination of 'initial' orbital positions - and calcs to current positions). And the math is definitely more challenging - but can be 'tabled' even if a bit crudely (its all a swag anyway )

The real challenge is making it playable. By that I mean without an electronic calculator/computer (and in my experience most people cannot consistently generate correct results with a calculator :? ).

Though, I have always thought a simple dual wheeled prop would come in handy here - one would be turned to indicate % and the other would be used for multi-table lookups (i.e. the calcs). I think this would make it easy (no batteries/wires/bulk/weight/cheap) and interactive (fun prop - tech looking without real complexity of use).

 

[atpollard]

I find that I can do 'realistic' and I can do 'simple' but trying to create 'simple and realistic' tends to give me a headache.

 

[BP]

I find that I can do 'realistic' and I can do 'simple' but trying to create 'simple and realistic' tends to give me a headache.

Thanks for simplifying what I was trying to say

'Simple and Realistic' is an art. (And I know I'm no artist - just trying to do a little finger painting here and seeing if anyone will pick up the palette!)

 

[lastbesthope]

Traveller simplified all those assumptions (remember calculators were only just starting to show up in 1977 and they were EXPENSIVE). Today, with PCs and such, these calculations would be essentially trivial, but not in 1977.

That's why the Great Maker invented slide rules, my Dad taught me how to use one for the basics, man I love those things, weirds people out when you use one and beat them using a calculator.

LBH

 

[Rikki Tikki Traveller]

I learned on a slide rule in middle school (grades 7 and 8 ), but got my first calculator in High School (grade 9). My dad bought it for me. Add/Subtract/Multiply/Divide - It also did Square-roots! No memory at all. It cost $100 in 1976.

 

[Rikki Tikki Traveller]

OK, here are some numbers to consider. I apologize in advance for the math/science lesson and the equations. For those that are not interested, skip to the table at the end...

Several things related to propulsion are tied up in the idea of Specific Impulse (Isp). Essentially it is a measure of how powerful your rocket is.

For example, liquid fuel rockets have an Isp of about 500, while the most advanced nuclear-electric rocket we can believably create has an Isp of about 100,000.

Another important concept, related to Isp is Exhaust Velocity (Vexh). This is the speed of the rocket stuff shooting out the back. The faster the exhaust, the more powerful the engine and the higher the Isp. Ironically, exhaust velocity also determines the fuel flow rate (FFR). The higher the exhaust velocity, the LOWER the FFR for the same thrust.

So, lets make some assumptions as see what happens.

1. Basic Rocket: If we assume that the fuel is liquid Hydrogen (LH2) then the Traveller displacement ton and the mass are the same (1 ton = 1Dton of LH2). We will also assume that the rocket as an overall density of LH2 for convenience.

2. Simple Math: While calculating fuel usage is an integration problem since as fuel is used the mass of the ship goes down etc etc... We will assume that the mass of the ship is essentially constant. This assumption is pretty good until our fuel volume exceeds about 50% of our ship mass. This is a CAUTION... the following calculations will NOT work when you calculated fuel needed exceeds about 50% of your ship mass.

3. Standard Ship: I will use a 100 ton ship (scout ship). All of the values given can be scaled linearly for any other mass of ship. If you want to figure the numbers for a 500 ton ship, multiply the fuel per hour by 5. Simple. Additionally, we will assume a 1g thrust. If you want higher thrust, multiply the fuel used by the the G value.

So, first some assumptions about future Fusion Drives. These are SWAGS (scientific wild ass guesses) that I have made trying to keep things semi-realistic. I have also tried to put things in Traveller TL terms. These are MY assumptions, YMMV.

Advanced Nuclear-Electric Propulsion (NEP) becomes available at TL 8. It is basically Nuclear Pulse Propulsion. The idea is that you take small nodules of hydrogen, zap them with a laser to cause fusion and kick the hot exhaust out the back for thrust. Exhaust velocity is about 1,000 kilometers per second (kps). These are numbers from real-world studies like the Orion project and the Daedalus Project.

Basic Fusion Drive (BFD) is available at TL 9, using gravitic focusing technology, exhaust velocities can be increased to about 1% speed of light (1%c) or 3,000 kps

Standard Fusion Drive (SFD) is available at TL 12 when microgravitic control is possible. This results in a 10 fold increase in exhaust velocity to 10%c (30,000 kps)

Advanced Fusion Drive (AFD) is TL 15 and increases exhaust velocity to 50%c (150,000 kps)

Very Advanced Fusion Drive (VFD) is TL 17 and assumes that the exhaust velocity is essential light speed (300,000 kps).

So, given these technologies and our standard ship at 1g acceleration, here are the estimated fuel consumptions:

Drive   Vexh (kps)    Isp (sec)    FFR (tons/hr)
NEP       1,000         100,000     3.62
BFD       3,000         300,000     1.21
SFD      30,000       3,000,000     0.12
AFD     150,000      15,000,000     0.02
VFD     300,000      30,000,000     0.01

Now, to give you an idea of how much fuel it would take to travel to different planets, lets assume a constant acceleration at 1g (accelerate half way then decelerate at 1g the other half). For a trip to Mars the minimum distance is 0.5 AU, so lets use that as our starting point. At 1g, it will take about 24 hours to travel 0.5 AU. (Yes 1 day to Mars at 1g!)

Drive  Fuel (Dtons)
NEP      86.9
BFD      29.0
SFD       2.9
AFD       0.6
VFD       0.3

Because of Assumption 2, the NEP numbers are NOT accurate, so these assumptions would not work for that drive at interplanetary ranges (you could use lower thrust ratings).

So, the Standard Fusion Drive would use 3 tons of fuel per day of thrust (1g). For the advance drive round it to 0.5 tons per day (why not?).

Assume the Fusion Drive has the same size and cost as the M-Drive in the TMB, ignore PP fuel and calculate M-Drive fuel based on how many days you want to be able to accelerate. You can use the travel times posted on other threads (or in the book) to determine how far you can travel in a given time at a given acceleration. This WILL increase your fuel, but not really that much. Now instead of spending 2 tons of fuel per drive letter for PP fuel for 2 weeks, you spend 3 tons of fuel per ship size letter for 1 day of acceleration at 1g. Many of the standard designs could probably be left alone and just convert the PP fuel to Mdrive fuel and calculate the new endurance.

This is all done with a lot of assumptions, but should give people that want to have realistic reaction drives in their TU a way to calculate M-Drive fuel pretty easily

Fuel = (Ship Size/100)*Thrust (in Gs)*Days of Thrust*3

 

[DCAnsell]

For my game, I'm pretty much going with a very simple workaround inspired by James Rowe, using the following assumptions:

1) Gravitics provide lift, but not thrust. Thrust is provided by gravitically focussed fusion drives using tiny amounts of hydrogen as reaction mass on starships and spacecraft. This follows through for all grav-assisted vehicles.

2) The machinery for artificial gravity is generally built into the hulls of spacecraft, starships, and any other structure that requires it.

3) Spacecraft generally take off from planets using gravitic lift, with only small amounts of thrust required.

4) Power plants do not require hydrogen fuel to function, instead having sufficient internal fuel to operate for one year. This fuel on a starship or spacecraft is replenished during annual maintenance. As a corollary, this means that you are far more likely to die of starvation or lack of oxygen on a out of fuel spacecraft then you are to freeze to death from lack of power.

5) Each week of 'power plant' fuel on current designs equates to one week of operation at the rated thrust of the M-Drive. This allows standard designs from the book to be used without revision. Reserves of hydrogen fuel meant for a jump drive can be tapped to extend this time, though usually if you need this much continuous thrust, a micro-jump will get you there faster.

This obviously does not cover the potential for near-c velocity issues to arise, but it does work well enough, and simply enough, to cover the needs of my players and I.

 

[Infojunky]

For my game, I'm pretty much going with a very simple workaround inspired by James Rowe, using the following assumptions:

1) Gravitics provide lift, but not thrust. Thrust is provided by gravitically focussed fusion drives using tiny amounts of hydrogen as reaction mass on starships and spacecraft. This follows through for all grav-assisted vehicles.

2) The machinery for artificial gravity is generally built into the hulls of spacecraft, starships, and any other structure that requires it.

3) Spacecraft generally take off from planets using gravitic lift, with only small amounts of thrust required.

4) Power plants do not require hydrogen fuel to function, instead having sufficient internal fuel to operate for one year. This fuel on a starship or spacecraft is replenished during annual maintenance. As a corollary, this means that you are far more likely to die of starvation or lack of oxygen on a out of fuel spacecraft then you are to freeze to death from lack of power.

5) Each week of 'power plant' fuel on current designs equates to one week of operation at the rated thrust of the M-Drive. This allows standard designs from the book to be used without revision. Reserves of hydrogen fuel meant for a jump drive can be tapped to extend this time, though usually if you need this much continuous thrust, a micro-jump will get you there faster.

This obviously does not cover the potential for near-c velocity issues to arise, but it does work well enough, and simply enough, to cover the needs of my players and I.

I think you have it. I am going to steal this for my technical assumptions.

Though I believe I am going to add gravity displacement also such that I can justify Grav skimmers.

 

[Dave Chase]

I have only one add though a small one

Gravity compension or in other words, gravity control allows both for thrust and creating Zero-G & or artificial gravity.

If you don't have a compension fact involved with gravity control just lifting off the planet the G forces will still apply to the 'front' side of the thrust.

Hope that was clear. <shrug>

Dave Chase

 

[atpollard]

Several things related to propulsion are tied up in the idea of Specific Impulse (Isp). Essentially it is a measure of how powerful your rocket is.

While your rules seem a workable compromise, ISP is closer to a measure of 'Fuel Economy'. Thrust is the measure of how powerful your rocket is. The two tend to be inversely related (low thrust = high ISP, high thrust = low ISP) but that is of no use in Traveller which requires high thrust and high ISP.

 

[DCAnsell]

I have only one add though a small one

Gravity compension or in other words, gravity control allows both for thrust and creating Zero-G & or artificial gravity.

If you don't have a compension fact involved with gravity control just lifting off the planet the G forces will still apply to the 'front' side of the thrust.

Hope that was clear. <shrug>

Dave Chase

Very clear indeed. I think I still prefer a lack of thrust for grav systems, though Zero-G and inertial compensation, as well as very close range repulsion (something like Star Wars landspeeders and so forth) should definitely be part of this.

 

[simonh]

1) Gravitics provide lift, but not thrust. Thrust is provided by gravitically focussed fusion drives using tiny amounts of hydrogen as reaction mass on starships and spacecraft. This follows through for all grav-assisted vehicles.

Small amounts of hydrogen means very high exhaust velocities.

What happens to the penetration of a weapon when you decrease the cross sectional area you are hitting, but increase the energy you're putting into it?

I'm guessing you're saying this so you can hand-wave away the rocket exhaust as being safer, when in fact it makes it a much more effective weapon.

Now that's fine, I've run games in the Known Space universe based on Larry niven's stuff where fusion drives are common and extremely dangerous, but you realy can't have fusion drives or anything like them in your setting without taking into account that they are very dangerous. I'm just saying.

Simon Hibbs

 

[DCAnsell]

1) Gravitics provide lift, but not thrust. Thrust is provided by gravitically focussed fusion drives using tiny amounts of hydrogen as reaction mass on starships and spacecraft. This follows through for all grav-assisted vehicles.

Small amounts of hydrogen means very high exhaust velocities.

What happens to the penetration of a weapon when you decrease the cross sectional area you are hitting, but increase the energy you're putting into it?

I'm guessing you're saying this so you can hand-wave away the rocket exhaust as being safer, when in fact it makes it a much more effective weapon.

Now that's fine, I've run games in the Known Space universe based on Larry niven's stuff where fusion drives are common and extremely dangerous, but you realy can't have fusion drives or anything like them in your setting without taking into account that they are very dangerous. I'm just saying.

Simon Hibbs

Quite dangerous indeed. Which is why ships take off and land using gravitic effect and micro thrust, and why starports and their associated startowns tend to not be in the center of the planet's main city. It can also make the lives of vacc-suited boarding parties short and incandescent, so you'd probably want to make sure the target's drives weren't functional before you tried it.

The handwaving wasn't so much directed at the danger and weapons potential of the exhaust, as the ability to get a ludicrously large amount of thrust over time out of a relatively small supply of reaction mass.

Anytime you do something like that, you have a lot of things that may or may not follow on. I'm honestly not scientist enough to know what all of those are, until they are pointed out by folks who have thought through such things.

Getting back to the high energy exhaust, it will give some potential for cleverness and evil on the part of any starship owner, but I think that's a good thing, not a bad one.

 

[Rikki Tikki Traveller]

So, the fusion drive has an equivalent weapon factor of 1 per drive letter. Range is limited to CLOSE since it disperses faster than a standard weapon.

Attacks are made via the Pilot's skill (taking one of his actions for the combat round) and done at a -2 DM to hit on all other weapons being used on that ship, since the pilot will be aiming the drive, not helping the gunners, in fact, he will be hurting the gunners.

That makes the drive a weapon of last resort, so it doesn't really impact the existing designs.