Alternate Ship Design Systems

[Straybow]

Errr, rockets don't work. In this post (and the one following) I showed how fusion power can't generate rocket thrust for even 1 gee of acceleration for any useful length of time using Traveller fuel ratios (including jump fuel).

Rocketry requires greater mass of fuel than structure and payload, typically many times greater, and that doesn't work for Traveller.

Using grav tech to exchange momentum with the universe via handwave instead of exchanging momentum with ejected reaction mass is the only way to have Traveller fuel and acceleration values.

 

[TheDS]

While rockets are certainly not the cure-all magic bullet we'd like them to be, I think you are underestimating their capability. Nuclear reactions are orders of magnitude above chemical rockets, and it is entirely possible to impart enough energy into something (like your exhaust) to get it to relativistic velocities, else we would not have particle accelerators.

Unfortunately, I cannot atm compare with the fine quality of your other post, though I did posit a couple questions for you there.

 

[Anthony]

Well, there's thermodynamic issues with any form of high-thrust high-impulse rocket -- if, say, you have a rocket with 1000 tons of thrust that burns 1 ton of fuel per hour, it has an exhaust velocity of 36,000 km/sec and a power output of 180 terawatts (about 40 kilotons per second) -- but if you don't mind power densities a million times greater than anything seen in Traveller you could theoretically have a fusion rocket that produces 1 G for 10-12 hours per 1% of ship's mass that is fuel.

 

[TheDS]

Forgive my impudence, but where do those numbers come from? I am trying to understand this rocket-science stuff, but alas, I am no rocket scientist (pun obviously intended).

 

[TKalbfus]

Here is my list of Maneuver Drives, these drives are also power plants of the same type, they can either deliver 1 EP of energy or 50 tons of thrust per unit.

Maneuver Drives
Type_________________ TL Size____ Cost____ Fuel Usage
Fission (50 tons)____ 7_ 2 tons__ Mcr 6___ 2.9 tons/min
Fusion (50 tons)_____ 9_ 1.5 tons Mcr 4.5_ 7 tons/hour
Fusion (50 tons)_____ 13 1 ton___ Mcr 3___ 3.5 tons/hour
Fusion (50 tons)_____ 15 1 ton___ Mcr 3___ 0.18 tons/hour
Fusion (50 tons)_____ 16 1 ton___ Mcr 3___ 0.09 tons/hour
Antimatter (50 tons)_ 17 1 ton___ Mcr 1___ 0.015 tons/day antimatter + 0.015 tons/day hydrogen

I got these figures by consulting "The StarFlight Handbook" Chapter 3 Rocket Propulsion for Interstellar Flight.

Using the formula:

Delta-V = Exhaust-V ln (original_Mass / final Mass)

I used these given figures for specific impulse:

Solid Core Fission = 1000 sec -> exhaust Velocity = 10,000 m/sec

Fusion ranged from 2500 sec to 200,000 sec impulse so I divided it up into 4 parts assigning

Fusion TL 9 = 25,000 Sec -> 250,000 m/sec
Fusion TL 13 = 50,000 Sec -> 500,000 m/sec
Fusion TL 15 = 100,000 sec -> 1,000,000 m/sec
Fusion TL 16 = 200,000 Sec -> 2,000,000 m/sec

The Antimatter rocket was a pion drive with 50% efficiency, in my calculation I assumed total conversion to deterimine the amount of mass that needed to be converted into energy, and that is the total usage of antimatter required, but since the conversion is only 50% efficient, the rocket requires an equal amount of matter in the form of hydrogen to be used up at the same rate as the antimatter. The exhaust velocity in this calculation is the speed of light since light is its exhaust.

After determining the final velocity of each rocket type with an initial mass of 50 tons and a final mass of 49 tons, I assumed that each rocket type can produce 50 tons of force per unit, that is my handwave, but it is a lesser handwave than assuming gravitic technology. Inertial confinement fusion can achieve greater thrust than known magnetic confinement fusion, but perhaps some future technology would solve this problem and just use liquid hydrogen as fuel instead of pellets or bombs, so the spaceship can skim a gas giant for fuel. Hydrogen pellets or bombs can't be skimmed out of gas giant atmospheres and have to be manufactured, so I'm assuming that somehow you can get 50 tons of force out of a 1 ton fusion rocket. With that handwave I offer you these alternatives to traditional Traveller maneuver drives. If you want 1-g of acceleration simply add enough rocket units so that the force generated equals the starship's hull tonnage and we'll assume that the starship's average density is equal to that of liquid hydrogen. To get higher that 1 g acceleration add enough units so that the force generated is equal to a multiple of the hull tonages. These rockets are also power plants and each unit can also supply 1 EP to the ships systems if its rocket fuction is turned off, the rocket then simply contains the reaction and extracts energy from it. A scout ship would need 4 rocket units for instance to have 2-g acceleration, if the scout ship accelerated at only 1-g, it would use 2 units as rockets and would have 2 additional rocket units avialable as power plants able to supply 2 EP to the ship for weapons or whatever else. A dedicated power plant can also be included in adition to that. The rocket units require no energy inputs from the powerplant to operate. When the rocket units are rockets they have a greater fuel consumption than when they are used as power plants. When using the rockets as power plants, use the same rates of fuel consumption as the dedicated power plants in the T20 book.

Hows That?

 

[robject]

That's excellent, Tom. I can use that. Thank you for finding and crunching the numbers!

So it looks like your handwaves include the assumptions that size = volume = mass, and the units for 'tons of thrust' is G/ton?

Yeah, I like it.

Although, it looks like fission is terribly inefficient. Looks like fission-powered ships are going to take a leisurely pace every time. Imagine a 1000-ton insystem supply ship with fission reactors: if it only carries 20 minutes' worth of fuel (10 minutes accel, coast a long time, 10 minutes decel) that's 120 tons of fuel! Aigh!

I'm also worried that TL12 Fusion engines aren't very feasible either. It looks like impulse may have to be 100,000 on the low end.

 

[robject]

Just for the heck of it, I wrote a basic distance-time-acceleration calculator to figure out what fission could really do.

http://home.comcast.net/~downport/perl/travel.html

Assuming 20 minutes' worth of 1G is about the same as a day's worth of 0.01 G, said ship would travel 186,000 km, nowhere near a significant fraction of 1 AU. At 0.0015 G (a week), the ship would travel 1.3 million KM -- still less than 1% of an AU.

That ship can't go anywhere! At best it's a moon run, or maybe some kind of prospecting ship. Or just power for the lights.

Of course, that makes sense for TL 8.

However, I would hope that at TL 9, if fission is still used, that it would consume much less fuel. I mean, we need a sustained 0.1 G for 10 days just to make 1 AU.

 

[Anthony]

Originally posted by TheDS:
Forgive my impudence, but where do those numbers come from? I am trying to understand this rocket-science stuff, but alas, I am no rocket scientist (pun obviously intended).

Power output of a drive is no less than Exhaust Velocity (m/s) * thrust (N) * 0.5, or no less than 50,000W * thrust(tons) * Isp. To determine Isp, multiply thrust by burn duration(in seconds) and divide by fuel consumption. Thus, in my example, thrust is 1,000 tons, burn duration is 1 hour (3600 seconds), fuel consumption is 1 ton, so Isp is 3,600,000 and power output is 50,000W x 3.6 million x 1000 or 180 trillion watts.

For comparison, that's about 1/4000 of the total sunlight hitting the earth, and is about 14x the current total power consumption of humanity.

 

[Straybow]

Yep, and how exactly are you going to accelerate that exhaust to 0.01c or more??

A handwave toward gravtech is far more believable. 250MW (1 EP) can provide 1G to 100dT assuming a 30% efficiency and 5 ton mass/dT (see my calc in this post about half-way down the page).

 

[Anthony]

Originally posted by Straybow:
A handwave toward gravtech is far more believable. 250MW (1 EP) can provide 1G to 100dT assuming a 30% efficiency and 5 ton mass/dT (see my calc in this post about half-way down the page).

Well, they could if those calculations were correct, which they aren't. The power requirement for applying 5 million newtons thrust (500T mass at 1G) is 5 MW x velocity relative to what you are pushing on. Your solution is only correct if what you are pushing on has a zero velocity relative to the ship at the beginning of every second, which is a pretty unlikely state of affairs.

In order to get 5 million newtons thrust for 250 MW, you have to be moving no faster than 50 meters per second relative to what you're pushing on. This is moderately possible for CG craft, but will essentially never be true of spaceships, and will usually be false by 3+ orders of magnitude.

 

[Drakon]

The weird thing about grav tech is that you ain't pushing on anything. What you are doing is warping the underlying dimensional manifold itself. (This is all assuming that Einstein is correct, which so far seems to be the case)

For something to exist, to have form, it has to exist inside or on some kind of dimensional manifold. And such a manifold will determine or limit the kinds of forms available for the object to take. For example, knots and donuts cannot exist unless you have a minimum of 3 spatial dimensions.

The only method we know of to date to change this manifold (according to Einstein) is gravity. Which in general relativity is not a "force" as we are used to thinking, but a pseudo-force" the natural characteristic of objects to follow geodesic (shortest distance) path. In curved space, these geodesics are curved.

So by stretching and/or shrinking the manifold along the desired flight path, one can increase its distance from one point and cause the distance to the target to decrease.

Right now, all we know is that mass and energy make gravity. But the effect is incredibly weak. A while back, while looking at a paper on grazers, (gravitational lasers) I figured that in one design, by the time you got a gravity beam strong enough to be measured, your devise is already slag from all the energy it was being fed. So far, its incredibly inefficient.

Hmmm... this is giving me an idea. One of the projects I have been tinkering with is a conversion of Alcubierre's warp drive paper into Traveller game terms. (Well not exactly that paper, but the resultant discussion in the journals and on LANL about it have led to some interesting work arounds.) It just occured to me that tech level would be tied to how efficient a gravtech system works, how much energy is required to produce the desired gravitational effect.

As tech level goes up, "grav efficiency" goes up. This means that a warp drive becomes faster, and more fuel efficient. Your gravtech manuvering drives would get smaller, and require less power (which means more fuel efficiency)

Anyway, my original point is that until details of the gravtech are laid down, (even in game terms) comparing reaction thrusters in any form (chemical, nuclear, photonic) and gravtech is like comparing apples to oranges. 250 MW providing the equivalent of 1G to 100 tons might not be that out of bounds.

 

[TKalbfus]

The fission rocket I used the solid core reactor, the gas core reactor has an ISP of 3000, to 7000, perhaps this is what ought to be used as a fission rocket perhaps at TL8. Grav tech is no small handwave, what your talking about is warping space to push a spaceship. Anyway, I like spaceships to have firery plumes of rocket exhaust. A UFO-like spaceship that moves silently in atmospheres is kind of disappointing.

 

[Drakon]

Different strokes. To me, I prefer the quiet approach. Easier to sneak up on people

Rockets do require reaction mass, and that fuel requirement has got to play havoc with your ship size. Also, you are not gonna go too far with them. (although nuclear rockets like you are talking about do have far smaller fuel needs.)

In any case, you ain't gonna travel the stars with rockets. Silent gravity propulsion may be boring, but it might get the job done.

 

[TKalbfus]

The only problem is that the first Traveller Manuever drive appears at TL7 which is today. In case you haven't noticed we don't now have reactionless maneuver drives. If we did, we'd probably be travelling in space in large numbers. Try making a TL7 spaceship using fission reactors and a 2-G TL7 manuever drive and you'll wonder why we don't already have a Moonbase. At least with rocket engines, you'll understand why space travel is hard. Also the lowest Fusion rocket engine can get you to 100 planetary diameters, perhaps you can't accelerate all the way, but the important thing is to reach that distance so you can use the Jump Drive.

 

[mike wightman]

In CT, LBB1-3, the air/raft appears at TL8, starhip drives at TL9, and grav vehicles at TL10 - all according to the TL charts in book 3.
In MT the maneuver drive is TL9 (anti-grav) or TL11 (thruster plates), inertial compensators and grav plates are TL10.

Yet another broken table in High Guard that was never corrected???

(and one that T20 has just copied without looking at the TL chart???)

 

[Anthony]

Treating traveller M-drives as a variant alcubierre warp drive does potentially work, but has some weird side effects (such as the fact that the ship stops if you turn off the drive, and is surrounded by a force field while in movement), but even then we can place upper limits on performance based on conservation of energy concerns, which limits are fairly underwhelming (250MW for a 500T mass ship will allow moving at 50 m/s against a 1G resistance, and about 80 km/sec against solar gravity while in space near earth).

 

[mike wightman]

Hmm, similarities with (and possibly an alternative explanation of) the T2300 StutterWarp drive
Could make entering planetary orbit kind of interesting

 

[TKalbfus]

I kind of like the movements of spaceships to follow real world physics. You accelerate and decelerate to get somewhere. Warp Drives turn starships into boats. You have a steering wheel, and a gas pedal. Take your foot off the accelerator and the ship slows down. You have a shift stick which you shift into warp 1 when you want to go faster, but if that's not fast enough you shift into warp 2. Really sporty models can go as high as warp 5 or 6, but don't let the interstellar traffic cops catch you!

 

[Jamus]

I kind of like the movements of spaceships to follow real world physics. You accelerate and decelerate to get somewhere.

Amen. personally i find the whole hand wave to grav teck in traveller to be next to unbelievable. I had always invisioned M-drives as using the power of the ship to creat charged particles or ion which pushes the ship forward like a rocket. IMTU M-drives are ion drives.

I dont know how plausible Ion drives are but it seems more possible than "magic" grav thruster plates.

 

[far-trader]

Well if its realism you want then you're going to have to accept your alternate maneuver drives are maybe 10% as powerful as thruster plates, and that's being generous. So you'll be putting around at fractions of a Gee.

Now fractions of a Gee can build up a good percentage of light speed with enough time and fuel so interstellar travel is possible, either as a generational ship or everybody hibernates for the whole trip (ala Alien movies, and others).

And of course you don't have artificial gravity either so your ship will need spin gravity or the crew and passengers will develop severe health problems on long trips.

Lets not forget about all that radiation exposure either, better have lots of meds.

I'm not saying don't play that way, just that there's a reason for a few "magic" items in any game. In Traveller (like StarTrek) the Jump drive is a plot device to move the story forward by getting to new worlds without it taking decades or centuries. Same thing with thrusters (and ST impulse engines), they let the ship get where its going in reasonable times to keep the plot moving.