Just What Is the Reaction Drive?

[Mithras]

I think I've finally settled on the various details of a Sol System campaign, but one thing still intrigues me, what are the reaction drives supposed to be?

Rated in Gs: 1-6. That's powerful! Moreso than estimates for NTR or fusion drives I've read about which postulate more limited accelerations (generally less than 1G seems typical).

They use hydrogen propellant, although it does not say so explicitly in High Guard. This means they are probably fusion or NTR rockets.

Thrust-hours are generally not very high considering the fuel tankage required. The ships in Transhuman Space have long burn times, 50, 70 or over a hundred hours. Coupled with accelerations of less than a G for that settings common drive, the nuclear pulse drive (using nuclear pellets as a reaction mass).

Since I'm not using grav plates or compensators, I will have to reduce G ratings by a tenth for human comfort. This will allow me to have extra thrust hours and still gain the delta-V I need to reach the outer planets in a timely fashion.

Although the nature of the HG reaction drive does puzzle me, my own under-rated version will probably resembles a high thrust fusion torch burning liquid hydrogen.

 

[rust]

I think I've finally settled on the various details of a Sol System campaign, but one thing still intrigues me, what are the reaction drives supposed to be?

There seems to be only one short note on this in High Guard, in the small
craft design chapter, that describes reaction drives as "rockets" - which
is not exactly very helpful, I think.

 

[Rikki Tikki Traveller]

The standard Maneuver Drive in the TMB is NOT a reaction drive, it is Reactionless (call it Gravitic and wave your hands).

The Reaction Drive mentioned in HG is probably a Plasma Rocket or perhaps a Fusion Rocket. It could also be an advanced Ion Drive (Twin Ion Engines anyone?). Since it burns hydrogen for fuel, it is probably not anything more exotic than that. No Nuclear Pulse drives etc.

Personally, I would call it a Plasma Drive and move on.

 

[atpollard]

Reaction Drives are not explicitly defined, but anything close to traditional MD performance will break some law of physics.
The alternative (a realistic rocket performance) will mean complex orbits and days to reach Jump Limit and months to reach other planets ... boring.

Go with whatever description you like and accept that it is at least 50% handwavium, 50% unobtanium and 50% plot device.
I like the Triton NTR - google it if you like - which is a LH2 Nuclear Torch with a LOX afterburner (developed by Pratt & Whitney).

 

[EDG]

The Reaction Drive mentioned in HG is probably a Plasma Rocket or perhaps a Fusion Rocket. It could also be an advanced Ion Drive (Twin Ion Engines anyone?). Since it burns hydrogen for fuel, it is probably not anything more exotic than that. No Nuclear Pulse drives etc.

IIRC Hydrogen would be pretty much the worst fuel ever for an Ion Drive - you need heavy ions to throw out of the engine, which is why current Ion Drives use Xenon as fuel.

 

[Mithras]

Yes, atpollard, I think you suggested the Triton in a previous post - very nice link!

I did read somewhere that most, some, or all nuclear thermal (ie fission) drives produce radioactive exhaust. I don't know, and would like to know which ones don't.

 

[Mithras]

Not so! If we keep the speeds up, maybe introducr ALIENS style cryogenic /low berths for crews, the game is as good as ever.

And orbits are no longer a headache, not when you can easily calculate a rough Delta-vee using a simple formula in Transhuman Space, and then download the fast, easy, and FREE software application called ORBITS 2.2.2 http://nemtos.ouvaton.org/orbits/orbits.php

Its fantastic. And gives me travel times based on Delta V to anywhere, for any calendar date. Fast.

Reaction Drives will mean complex orbits and days to reach Jump Limit and months to reach other planets ... boring.

 

[Mithras]

I'll pass on that Triton link: http://www.nuclearspace.com/PWrussview_fin.aspx

I particularly liked this idea from the Triton designer: using the reactor en-route to power ion thrusters, to continue accelerationg even after main engine burn.

It's what I call the "Hybrid approach" It is where I 'size' the reactor for the levels needed for propulsion mode and I would run the power mode at much higher output levels approaching 1 megawatt, I'd probably would design it to produce 100-500kW electrical power for let's say, maybe 8 or more large Ion thrusters, or Hall effect thrusters and use them along the trajectory for pushing me faster to the planets, not simply for station-keeping. I'd use it to further reduce trip times en route to ultra long destinations like Jupiter because once I've done my initial 1 hour long escape burn in the typical nuclear thermal propulsion mode at high thrust, I simply go to power mode and pump electrical power to those electrical thrusters and keep accelerating to reduce the heliocentric transit times and reaching the vicinity of my destination and turn around to present myself for orbital insertion to my destination.

 

[daniel_t]

I think I've finally settled on the various details of a Sol System campaign, but one thing still intrigues me, what are the reaction drives supposed to be?

Rated in Gs: 1-6. That's powerful! Moreso than estimates for NTR or fusion drives I've read about which postulate more limited accelerations (generally less than 1G seems typical).

They use hydrogen propellant, although it does not say so explicitly in High Guard. This means they are probably fusion or NTR rockets.

Thrust-hours are generally not very high considering the fuel tankage required. The ships in Transhuman Space have long burn times, 50, 70 or over a hundred hours. Coupled with accelerations of less than a G for that settings common drive, the nuclear pulse drive (using nuclear pellets as a reaction mass).

Since I'm not using grav plates or compensators, I will have to reduce G ratings by a tenth for human comfort. This will allow me to have extra thrust hours and still gain the delta-V I need to reach the outer planets in a timely fashion.

Although the nature of the HG reaction drive does puzzle me, my own under-rated version will probably resembles a high thrust fusion torch burning liquid hydrogen.

I'm working on something along the same lines as you. Here is what I have discovered so far... If you include the mass of the power plant with the maneuver drive, and assume the High Guard option of reaction drive engines. A Traveller engine is about as efficient as a Gurps Spaceships high thrust fusion torch, which is considered a "A limited superscience high-performance fusion rocket."

Now, if we go with just the information in Mongoose Traveller... A type S Scout holds 34 tons of fuel and has a 2g trust rating. That means it has 6.8 hours of maximum thrust available, or 68 G/Turns worth of delta-V. The power plant consumes 1 G/Turn every 84 hours. So, how far can this ship go?

1 G/Turn accelerates the ship to 9.8 mps (meters per second,) so assuming you want to stop at the end of your trip, the fastest you can go is about 328 mps and it would take you about 3 hrs, 20 minutes for acceleration to max and deceleration, which leaves a 77 hrs, 20 minutes cruise time which is only 91,000 km or so. Considering that 1 AU = 150,000,000 km, that isn't very far at all.

Anybody care to check my math?

 

[Mithras]

What type of a powerplant are you rating the Type S with for this experiment? What-ever it is, its pretty pathetic isn't it? Give it a fission pile and watch it fly to Saturn!

 

[daniel_t]

What type of a powerplant are you rating the Type S with for this experiment?

The Scout in the core rule book has a powerplant A.

However, I just realized that I was off on my initial calculations. 1 G/Turn is good for 9.8 mps times 6 minutes, or 3,528 mps so the fastest the scout can go is 118 kps times 77.2 hours equals 32,794,560 km. To that, we have to add in the distance travelled while accelerating and decelerating which is another 1,425,347 km for a total of 34,219,907 km. A much bigger number but still only about 0.23 AU and when you get to the destination, you will be out of fuel.

Of course, the ship only needs to travel to the jump point. For the Earth, that would be 1,270,000 km which for the Scout would take a minimum of 4.5 hours at 2 g and use up 50 G/Turns of fuel. Not fun enduring 2 g of simulated gravity for 4.5 hours only to end up without enough fuel to jump!

So... The ship needs 20 G/Turns of fuel to jump, and the powerplant will use 2 G/Turns while in jump. That leaves 46 G/Turns, of which half is for getting to a jump point and half going from the jump point to another planet... That's 1.1 hrs of 2g acceleration, a 0g cruise of 6.6 hours and another 1.1 hours of 2g while decelerating.

 

[atpollard]

Interplanetary travel at 1G+ constant thrust will take rediculous amounts of reaction mass (even with ions). Your options are:
1. some magic drive that violates conservation of momentum,
2. Short burns and long coasts, or
3. ugly math involving accelerating particles to relativistic velocities and power levels closer to matter-energy-total-conversion than fusion.

With #3, the exhaust also becomes a Spinal Mount Weapon.

 

[Gentleman John]

Yes, atpollard, I think you suggested the Triton in a previous post - very nice link!

I did read somewhere that most, some, or all nuclear thermal (ie fission) drives produce radioactive exhaust. I don't know, and would like to know which ones don't.

If we are going with real-world physics, then thermal fission drives can be divided into direct cycle and indirect cycle drives.

In the direct cycle drives, the reaction mass fluid is passed directly through the reactor for heating. Thus the reaction mass is directly exposed to radiation from the core. This will then transmute some of the elements in the fluid, which will then decay giving off radiation. Even more worrying, it is possible for the reaction mass fluid to pick up fragments of radioactive material from the core (although this would require damage to the fuel plates). Voila. A radioactive exhaust.

In the indirect cycle drives an intermediate fluid is heated by the reactor. This intermediate fluid is then used to heat the reaction mass. Only the intermediate fluid is exposed to the reactor core and becomes activated. The reaction mass is not activated (or at least not as much as in a direct cycle drive), and any fuel released is kept in the intermediate fluid.

Both direct cycle and indirect cycle drives were built and tested in the 1950s and 1960s. Their remains can still be found on the White Sands test range.

If you want to go science fiction, then the so-called lightbulb drive has a non-radioactive exhaust. The lightbulb drive surrounds the reactor core with a material that is transparent to heat but opaque to radiation. The reaction mass can then be heated directly by the core, but not be activated. Of course, there are some problems with this concept - not the least of which is where to get the unobtainium required to build one.

 

[Corvus]

... download the fast, easy, and FREE software application called ORBITS 2.2.2

That is amazing Mithras. Can one use this for arbitrary orbits and trajectories?

 

[Mithras]

Yes I think so, in the notes it says you can input your own asteroids and other bodies. So potentially you could have space-stations and the like ...