TNE HePlaR - Efficient???

[Rikki Tikki Traveller]

So, I am wondering about the TNE M-Drive system of HePlaR.

I can do all the calculations, but the drive seems very inefficient to me. It uses a lot more fuel than I would expect a Plasma Drive to use.

MGT uses the same fuel costs for it's Reaction Drive in HG, so it does apply to MGT.

IS the fuel usage for this drive too high? I was thinking it should be 2.5% per G-DAY, not G-Hour, but maybe that is too efficient?

THOUGHTS from those that have played it out?

 

[apoc527]

I have no basis for this, but I think fuel usage is too high also. I don't know if I'd go 2.5% per G-DAY, but maybe .5% per G-hour...

 

[far-trader]

Depends what you mean by efficient I suppose.

If I recall correctly TNE HEPlaR, like pretty much all sci-fi drives, depends on handwavium to work. I seem to recall the calculated exhaust values being much faster than light to achieve the required performance, which of course is not reality. So in that context it is exceedingly efficient. Naturally it has to be to make it work for the game, hence the handwave. If the "efficiency" seems too low for your game go ahead and fudge it. I've even seen some who simply ignore fuel entirely as below their level of detail. The ship just goes.

 

[Rikki Tikki Traveller]

It was actually fuel usage that I was concerned with when I said efficiency.

Guess I need to break out those old Rocket Design books from college and do the math myself.

There goes an evening! (But it will be fun!)

 

[Rikki Tikki Traveller]

I don't have a problem with squinting one eye (or even two) for playability, but it just looked wonky to me when I looked a the fuel usage.

Plasma and Fusion drives are supposed to be the best thing until gravitics come around and 2.5% of the ship for 1 G-hr seems really high.

BUT, I haven't done the math so maybe my expectations are wrong and it should actually be higher.

 

[apoc527]

Here's a question: let's say I have a 6G reaction drive and I want 3 g-hours of fuel. It takes 6x2.5%x3=45% of my hull in fuel, correct? Now, what if I just want to cruise at 1G acceleration? Is my increase in efficiency simply linear or is it somehow better?

For the above ship, I have 3 G-hours of 6 G acceleration, so would that mean I have 18 G-hours of 1G acceleration?

Regardless, that seems to make fuel a prime consideration in the games, which I'm not sure I like. I kind of like my 0.5% per G-hour per G of acceleration.

 

[Rikki Tikki Traveller]

You are SORT OF right, but you got your wording wrong and they is confusing things a bit.

You wanted 3 hours of acceleration at maximum thrust. (not 3 G-Hours, 3 HOURS).

In your calculation, you figured 6*2.5%*3 = 45% That is actually 18 G-Hours of fuel.

You calculated 3 hours at 6-Gs of acceleration.

So, yes, fuel consumption is linear. With 18 G-Hours of fuel, you have the following endurances:

3 hours at 6-Gs
3.6 hours at 5-Gs
4.5 hours at 4-Gs
6 hours at 3-Gs
9 hours at 2-Gs
18 hours at 1-G


And yes, fuel is a HUGE consideration when using Reaction Drives, which is why I was wondering if the fuel usage was right. It FEELS really high for a Plasma/Fusion drive, but too low for a chemical drive (like what we have now).

 

[Rikki Tikki Traveller]

Sorry if this gets a bit technical, but all of the equations I use are in Wikipedia along with decent explanations.

DANGER! DANGER WILL ROBINSON! MATH AHEAD!

OK, I did some digging and I want to present my numbers here for those that care...

First, for simplification I will use 10 m/s2 for G instead of 9.8.

So, lets go with a 100 Dton ship accelerating at 1G. (this makes calculating percentages very easy)

First, we have to assume a density of the ship. Lets say it is about 10 times the density of LHyd (about like the US Space Shuttle).

So using F=ma, we get F=(1000000)*(10) = 10000000 Newtons (10 MN)

Fuel Flow rate is F/(g*Isp) = M-dot in kg/sec
g is force of gravity, again using 10
Isp is the specific Impulse of the plasma drive in seconds

Using the concepts from Project Orion (Pulsed Plasma Drive) they had a theoretical Isp of 100,000 seconds. But we have gravitic focusing and all those other handwavium ideas, so lets say it should be about 10 times that number for our HePlaR drive.

So Fuel Flow Rate (M-dot) is:

M-dot = (10000000)/(10*1000000) = 1.0 kg/sec

Converting that to Dtons/hr we get:

M-dot = 3.6 Dtons/Hr

Or in Traveller terms: 3.6% per G-Hour

The only thing we really can play with is the Specific Impulse of the drive and the density of the ship. An Isp of 1000000 is really high by current standards, but very doable in a proposed universe with gravitics and all the other wonderful handwaviums that we have there.

SO, I guess 2.5% per G-Hour is pretty close, well within any reasonable margin of error.

Since others have claimed that HePlaR as presented wouldn't work, I would like to know if anyone can see what I did wrong?

MATH MODE OFF

 

[apoc527]

Thanks RTT, I appreciate the math effort. It seems that base HG isn't that far from reality after all.

This gives me some wiggle room if I want to introduce antimatter doped plasma drive systems. Some quantity of antimatter (probably waaaaaay less than a ton) would give a new "efficiency" (or rather fuel usage) of 1% or even .5% per hour of full thrust.

It's rough, but it could work!

 

[apoc527]

As an aside, these kinds of reaction drives would work even better in a setting that used a warpdrive or hyperdrive, rather than the fuel-hungry jump drive system. Hmmm...intriguing. I feel yet another iteration of my really long running homebrew project coming up!

 

[Rikki Tikki Traveller]

In my ATU, I have delayed the introduction of Gravitc Thrust Plates until TL12, so HePlaR is my drive from TL 8-11.

I am going to use 2.5% for TL 8-9 and 1% for TL 10-11. due to material and other design efficiencies. Since there are no Inertial Dampers, accelerations are limited to 1G for civilians and 3G for military (combat use only).

I have made a little spreadsheet table with various thrust times and final velocities so that I can figure out how long it takes to get from point A to point B.

At TL12, the Gravitic drive is introduced and the thrust go up to 6G and the fuel goes away and all kinds of things have to change in the setting.

My game plays at the cusp of TL12 so I can have both types of drives around at the same time.

 

[apoc527]

FWIW, with proper acceleration protection (couches, augments, etc), humans can probably deal with up to 6Gs of "undamped" acceleration for short periods of time. At least, that's what it seemed like in Revelation Space, which seemed to be as scientifically plausible as possible.

 

[Rikki Tikki Traveller]

I thought about that, but I was going with being able to function (at least minimally) during combat situations.

3Gs of combat maneuvering seemed like a reasonable compromise. While higher Gs are survivable for short periods of time (I think trained pilots can take about 10Gs for a few seconds), with 6-minute combat rounds, I wanted to use a reasonable limit over that time period.

 

[apoc527]

Definitely. And besides, "survive" is not the same as "be functional." At 6Gs, any humans would be struggling to breathe, and only their equipment would keep them alive. Not useful for combat, unless the ships are computerized.

 

[ColHut]

I thought HePlar was more like whollyunobtanium and you might as well use gravitics. The best solid propellants have an Isp of about 240, liquid propellants I think up to 400 plus. Acceleration is of course not constant even if the thrust is due to the changing mass ratio. And unless you have multiple chambers or variable geometry nozzles or some such there will be all sorts of inefficiencies/inability to control thrust (basically on/off). As others will have mentioned elsewhere the Saturn V has way less than 1G-hour of thrust, but as rockets don't really fly straight up, but cant over after clearing the denser parts of the atmosphere and accelerate to a velocity where gravity is equalled by centripitel force this is not a problem. The old TML and Missouri archive covered this I think.
For more fun use the classical rocket equations to determine velocity!


regards

 

[Rikki Tikki Traveller]

Well there is a bit of handwavium, but the Nuclear Pulsed Propulsion system designed (on paper at least) for the Daedalus project back in the 1970s had an Isp of 30,000 to 300,000.

I took their highest number and multiplied it by 10. After all, the HePlaR of Traveller DOES have gravitic technology available to help it out as well as some advanced materials that we don't have right now.

I thought it was a "reasonable handwave" rather than a "close your eyes handwave".

Nothing is perfect, but the design characteristics as presented in TNE (and in MGT) are not impossible, just unlikely. Personnaly I can live with that.

 

[rinku]

Electric propulsion thrusters already operate in Isp's in the multi-1000s range, even up to 10,000s (i.e. VASMIR, Hall Effect). I realise that these have sub-G thrusts, but it shows that solid, liquid or even atomic fuel is NOT the limit of the discussion.

HEPLaR may well be pie-in-the-sky, but it's not unreasonable to assume SOME kind of breakthrough that might combine multi-G thrust with very high Isp. (Though it might need a new standard model of physics to work. Good thing they're working on that... )

 

[Rikki Tikki Traveller]

The old handwave of gravitic control helps too.

Even TNE allowed some gravitic control and that can have a PROFOUND effect on exit velocities.

Imagine a chamber where the mass of the propellant is reduced by 90%. It is then accelerated to a very high velocity and shoved out of the chamber. As it exits the chamber, it gains it's mass back, but it has all of this velocity as well... Now you have very high exit velocities, which means very high Isp.

Of course, it all depends on what you think happens to an object that leaves a Gravitic Control Field... does it retain it's velocity or not? Conservation of Energy would say that it should slow down as the energy stays the same, the mass increases, so the velocity should go WAY down. BUT, gravitic control already violates Conservation of Energy to some extent (at least as we understand it right now), so who's to say. Gravitic control will require a fundemental redefinition of several basic principles of the universe. NOT saying that it is impossible, but until we understand how gravity, the strong and weak nuclear forces and String Theory all tie together, then we don't really understand what would happen.

 

[Felonmarmer]

Ummm, 1 displacement ton has a mass of 1 ton - the density is irrelevant, so you don't need to multiply the impulse by 10 to get the same result.

 

[Rikki Tikki Traveller]

That is one of the big sticking points for me too. I wasn't sure what to use for the density of the ship.

I picked the Space Shuttle as "close enough" and went from there. I think any value between 5 and 10 times the density of LHyd is just fine and doesn't change things too much.

I was just looking for a "ballpark" estimate anyway to see if I really needed to dig and change the fuel consumption rate or if it was OK. I decided, against my intuition, that it was OK, so I will leave it as is.

I AM adding a more efficient version at a higher TL that uses less fuel, but that is strictly MTU.