Reaction Drives fuel consumption in MGT

[Sturn]

Reaction Drives, per MGT High Guard, use twice as much fuel. Were these numbers based upon forumalae from FFS or previous versions of Traveller? Is the 2.5% consumption rate realistic for a game (a.i. in the ballpark) or should it be something much greater such as 4%, 10%, etc.

I don't need a complex run-down on Newtonian physics, just wondering how much of Traveller Reaction Drives are based upon science and how much is based upon purely a game mechanic.

 

[hector]

I suspect it's in the ballpark; the drive is most likely to use about the same amount of fuel as the power plant as both are likely to be in constant use.

By the way, a tiny bit of Newtonian science you may wish to remember for the sake of flavour is that halfway to its destination an object in space needs to turn around. This is because assuming constant acceleration, the ship needs just as much time to brake as it did to accelerate and the only way to brake in space is to turn around and use the same force that got you to your current speed to slow you down.

 

[Sturn]

With Reaction Drives there shouldn't be constant acceleration (as with non fuel-required standard MD's), but just equal acceleration at each end. Not enough fuel carried to have constant to mid-point. I'm aware of this and have created an Excel sheet (posted it here someplace iirc) to help do the math during game.

I'm just concerned that the "2.5%" requirement is too little.

 

[atpollard]

As a general rule of thumb, the hotter the exhaust - the more efficient the drive.

at 'melting titanium' temperatures (like the space shuttle) 10 minutes of thrust at 4G will require about 80% of the ship to be fuel.

At 'surface of the sun' temperatures (which will melt the rocket ... but that is another issue), you might get the 10 min at 4G burn down to 25% of the rocket.

Hours at even 1G is in the realm of pure handwave.

REALISTIC is 0.001G acceleration - but that isn't fun.

 

[simonh]

Where does the 2x fuel requirement come into play? i.e. what is is 2x of?

From what I can tell Reaction Drives have their own fuel requirements formula based on the thrust and the number of hours the fuel lasts at that thrust rating. This doesn't seem to be double anything else.

Simon Hibbs

 

[simonh]

Hours at even 1G is in the realm of pure handwave.

REALISTIC is 0.001G acceleration - but that isn't fun.

The rockets in HG sure aren't anything we've invented so far. I'm guessing some kind of magic-tech drive using exotic particles or gravity waves or some such for propulsion. They're not chemical rockets anyway, and they aren't claimed to be.

Has anyone checked to see if the HG reaction drive happens to match the performance characteristics of HEPLAR? That would be amusing. I don't have a rules set with HEPLAR drives in them available to check.

Simon Hibbs

 

[DFW]

The rockets in HG sure aren't anything we've invented so far.

And, ship missiles are definitely NOT TL 6 devices...

 

[simonh]

And, ship missiles are definitely NOT TL 6 devices...

The MGT ship design rules are pitched at a fairly high level of abstraction. I've no problem accepting that a TL 6 society could produce a basic space-to-space missile. Assuming that it would have the same characetristics and be interchangeable with later TL missiles is a bit of a stretch stretch, but but MGT simply doesn't deal with the nitty gritty of different msissile makes, compatibility between different manufacturers launchers or different TL launchers, etc. At TL 6 you can make msissiles, here's a missile launcher, off you go. Bish-bosh, job done. Traveller has always abstracted away implementation details below this level of detail.

In the context of an OTU campaign, I've no problem with many TL 6 worlds being able to produce basic missiles suitable for a standard launcher. They'd may need to import certain specialised components to build into them from off world, or you could just assume that their local TL is a little higher when it comes to space-to-space missile weaponry. The latter option is cheating because then they're not realy TL 6 missiles, but the core rules provide plenty of suport for the idea of importing higher tech components as and when necessary.

So the rules do provide some wiggle room on world tech levels and tech cross-pollination. I have no more problem accepting missile compatibility between TLs than I have accepting universal missile and launcher compatibility between worlds, manufacturers and even civilizations in general.

Simon Hibbs

 

[DFW]

I've no problem accepting that a TL 6 society could produce a basic space-to-space missile if they wanted to.

No way at TL 6 you get 1 hour of 10 G acceleration from a missile the size of a Trav ship missile. Sorry, not possible.

 

[Sturn]

Where does the 2x fuel requirement come into play? i.e. what is is 2x of?

Simon Hibbs

Sorry, error in my original post. Corrected it.

 

[Sturn]

Has anyone checked to see if the HG reaction drive happens to match the performance characteristics of HEPLAR? That would be amusing. I don't have a rules set with HEPLAR drives in them available to check.

Simon Hibbs

I quickly glanced at some notes for T4 Heplar. T4 craft design is much different then MGT, but it appears the Heplar requires around 1/2 the fuel for the same thrust/hours.

I think I will hand-wave, like it appears T4 did, and call my Reaction Drives, "Triple H's" for Hall-effect High-velocity Hydrogen-burning Ion Thrusters.

 

[simonh]

I've no problem accepting that a TL 6 society could produce a basic space-to-space missile if they wanted to.

No way at TL 6 you get 1 hour of 10 G acceleration from a missile the size of a Trav ship missile. Sorry, not possible.

If we're talking about a single completely isolated TL6 world with no off world trade then you're absolutely right. It's out of the question.

However if the question is, in the OTU (or another simmilar setting) is it possible to buy standard ship missiles on most TL 6 worlds with appropriate starports, the answer should quite reasonably be yes. They have basic space technology themselves, they have facilities for their own ships and for high tech offworld visitors. They have trade links with many other high tech planets. Why wouldn't they import decent gravitic propulsion systems and fit them in their own localy manufactured misssiles? They'd be stupid not to. Is the result strictly a TL 6 missile? No. Will most TL 6 planets then players will visit have these? Yes.

For TL 5 worlds, the answer should be very different. They don't have any local space technology to bring to the table, so any space weaponry is going to have to be all-import.

I'm not saying that the author thought through the consequences of making these missiles TL 6 at all, or had these justifications in mind at the time. There's a large portion of fudge involved in this, but I don't see it as being all that big a deal. I'm happy with the rules on this as they are.

Simon Hibbs

 

[DFW]

However if the question is, in the OTU (or another simmilar setting) is it possible to buy standard ship missiles on most TL 6 worlds with appropriate starports, the answer should quite reasonably be yes.

Buy, not manufacture. No more than a TL 8 world can manufacture a JD. The rules state that it is a TL 6 item. NO way. Sorry, broken rule that is internally inconsistent as it isn't even a TL 7 (present day) item.

 

[Rikki Tikki Traveller]

Has anyone checked to see if the HG reaction drive happens to match the performance characteristics of HEPLAR? That would be amusing. I don't have a rules set with HEPLAR drives in them available to check.

Simon Hibbs

I quickly glanced at some notes for T4 Heplar. T4 craft design is much different then MGT, but it appears the Heplar requires around 1/2 the fuel for the same thrust/hours.

I think I will hand-wave, like it appears T4 did, and call my Reaction Drives, "Triple H's" for Hall-effect High-velocity Hydrogen-burning Ion Thrusters.

I did some number crunching a while back and decided that the HG rule of 2.5% per G-Hour was "Close enough". High Tech but not too high tech.

I called it a Plasma Drive.

I then created a Fusion Drive which was more efficienct and used 1% Fuel per G-Hour; which is closer to TNE's numbers; but still easy to calculate.

 

[simonh]

Buy, not manufacture. No more than a TL 8 world can manufacture a JD. The rules state that it is a TL 6 item. NO way. Sorry, broken rule that is internally inconsistent as it isn't even a TL 7 (present day) item.

I agree it's a faulty rule. I'd also agree it's not internally consistent. It's an oversight.

However I don't think it's broken because the resultant effect - you can buy standard missiles on TL 6 worlds in 'standard' Traveller settings - is reasonable.

I'm hard pressed to imagine how this would actualy cause a problem in a game. A better way to handle it would be to say it's a TL8 item, but commonly available on worlds from TL6 onwards.

Simon Hibbs

 

[Rikki Tikki Traveller]

OR, create a low Tech version of the missile that uses a Reaction Drive, has a 2-3 turn burn time and a lower acceleration...

 

[atpollard]

I did some number crunching a while back and decided that the HG rule of 2.5% per G-Hour was "Close enough". High Tech but not too high tech.

I called it a Plasma Drive.

I then created a Fusion Drive which was more efficienct and used 1% Fuel per G-Hour; which is closer to TNE's numbers; but still easy to calculate.

Would you care to share your numbers?
Even one G-hour is a lot of thrust for a lot of time.

The last time that I applied Conservation of Momentum to Classic Traveller Maneuver Drive performance, I got exhaust velocities faster than the speed of light and a bust of several decimal places for any reasonable reaction drive (even using core of the sun fusion temperatures).

 

[Rikki Tikki Traveller]

Here is what I posted last June under the discussion of the HePlaR drive:

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

Where things get wonky is in the assumption of ship density (converting volume to mass). But from my caluclations, it seems that the Reaction Drive in MGT is "good enough".

 

[DFW]

Where things get wonky is in the assumption of ship density (converting volume to mass).

Yes, the shuttle has a tissue paper hull (comparatively) + the fusion drives, etc., will be very dense.

 

[atpollard]

Here is what I posted last June under the discussion of the HePlaR drive:

[snip]

Where things get wonky is in the assumption of ship density (converting volume to mass). But from my caluclations, it seems that the Reaction Drive in MGT is "good enough".

Thanks.
That works with the assumptions given and I would agree that 2.5% vs 3.6% is inconsequential.

The mass per volume seems ok - the Apollo and the Shuttle are closer to 4 tonnes per dT, but much more fragile than Traveller Starships.

Containing 10x the temperature of a nuclear explosion involves some handwaving, but no more than many other Traveller rules (any/every edition).

It is a good analysis.
So 2.5% works with the basic Traveller Technology assumptions, but is several orders of magnitude beyond 'realistic' thermal rocket engines (ISP = 20,000 is roughly the limit of any reasonably realistic theoretical material - ion drive with no grav focusing).