HePLaR Drives

[TheDS]

No no, HEPlaR is not fusion. Wherever you read that, that's a typo or you are misremembering.

At it's simplest, HEPlaR would be using reactor coolant as reaction mass. A reactor makes a LOT of heat. You need to cool it. Whatever you cool it with is going to get HOT too. The expansion of the coolant when heated can be turned into motive power by allowing it to expand someplace outside the ship in a directed nature, like say a thrust nozzle at the back.

Your exhaust should be mostly clean, since it's NOT fusing, it's just really hot, but it may have picked up some radioactivity from passing near the reactor; it all depends on how close it got, or how much shielding there was; basically, on design.

For a better illustration, look up "nuclear lightbulb", or go here to read up on a potential realworld example. (You can skip the first page linked, as it just explains some of the terminology.)

The hotter you can make the exhaust, the more thrust you can get from a given unit of fuel. You are supposedly limited by the speed of light, but considering that as things get faster they get heavier, I don't have a problem with realtivistic exhaust velocities myself, so long as I'm not doing it in an atmostphere, but other people do. I don't know how fact-founded these concerns are; it's hard to tell unimaginitive scientists from hacks sometimes.

 

[TheDS]

I like the CONCEPT of limited turns of action, but it does add a lot of recordkeeping. I rarely keep stock of the arrows I've loosed or the bullets I shoot or the grenades I launch/throw, so why would I want to keep track of that? (Generally, I carry enough ammo that I can fight for a while, and then place a good guess on what needs to be replaced and deduct some money from my bankroll... not that I keep much track of that either...)

However, as neato as the concept is, its just not what Traveller players expect. It removes wilderness refueling as the primary source of fuel for players, forcing them to go to the mainworld most of the time. It takes loads of game time to get to places within a system, so if you need to go interplanetary, you wind up being forced to microjump (and if you're doing it for free fuel, you're just wasting the fuel you'd be getting).

I took advantage of this once to create a fortress system, which had a huge red supergiant as the primary world. The jump barrier (aka jump shadow) went out to about the 13th orbital slot (Pluto is orbit 9, remember) and the mainworld was at like orbit 11. Pioneer isn't even that far out yet!

Any limited-fuel ship that jumped in would never be able to invade the main world without drifting for YEARS, and be subject to attack the whole way. In system, they used fusion rockets. I had to stipulate that those would be improved upon AT LEAST as much as reactors were, but still, even under constant 1 G thrust, it could take a ship many months. I had an inner star surrounded by metallic debris; it was so hot that metal naturally occured in globs that needed only be scooped. This was about orbit 7, I think. The miners would then go to a factory in the Oort Cloud (orbit 15?) and drop off their scoopfuls of metal, then bring ice back to a smelting/purification facility close to the mining site. The whole round trip took about 2 years, including about 0.1 G when loaded with metal and I think 5 G when loaded with ice.

The mainworld had runs out to the factories in the Oort cloud for materials, and there was a jump barrier station where interstellar traffic bound for either place was expected to arrive.

Virus (or the Civil War) could smash the outer factories, but couldn't touch anything else in the system. I'm forgetting a few details, and probably getting a couple wrong by a small amount, but that's the gist of it.

 

[aramis]

Originally posted by Commander Drax:
Well if you're a merchant you shouldn't really need anything more than 4G hours of fuel, (leave the surface, break orbit, jump, arrive in system, travel to orbit and then surface again) with a few m3 left over for course corrections and emergencies.

Mercantile ships should therefore have very small fuel tanks, allowing them to dedicate more volume to cargo and passengers.

Actually, I'd put the requisite at about 8 G-hours, not 4...

you need 1GHour to orbit, and at least one half g-hour out to jump point. That's three. Plus, assume you need to be able to reverse and return course. so absolute flat minimum 4 G-hours. HOWEVER, few merchant ships will only use 1/2 g... time is money. under power, 2 additional G-hours each way provides a very reasonable time boost to 100 diams... 5 times faster...shaving over a day off per jump cycle. the more jumps one can make, the more money one can make, to a point.

In play, I've found 6-8 G-hours spent on accell/decell pairs is financially worth the added expense long-term... if one is spec trading.

(And before you get started attacking spec: Spec trade is stacked in the merchant's favor, otherwise merchant's wouldn't do it. Hence it's a profit oppotunity for every jump, slightly balanced in the merchant's favor. If you have a 1/100 chance of a MAJOR-BIG-SCORE, then an extra jump is pretty slim for a MBS, but all you need for a major score is about a 4- purchase and 10+ sale (1/6 * 1/6 = 1/36). so all you need is a single MBS and no MBL's; if you have a DM+2 broker, you will almost always avoid the MBL.... and make the MBS about in in 20 jumps...)

One can also see that, if one scraps the canon pricing, people might pay extra (a fuel share) for faster trips, which in TNE are mostly limited by N-space, not J-space.

 

[mike wightman]

Originally posted by TheDS:
Your entry about damper-catalyzed fusion, though, that's something I never noticed before, and it soulds to me like an EXCELLENT use of the technology.

Oh, it's been mentioned a few times before

 

[graden1]

Hmmm. If heplar isn't fusion, then its Isp may be even farther out of whack than I thought! Thanks for pointing that out for me, DS.

Anyways, just to get my story straight, I dug out my old copy of FF&S (should've scrounged it up to start with) and sure enough, nothing there about "damper mediated fusion". So where, exactly, did I get that idea? I'm sure I read about it somewhere. Must have been some other version of Traveller, or maybe another TNE title? Now I'll just have to find that reference to prove I'm not going nucking futs!

Hope I haven't totally confused the issue for anyone with these nigh-senile ramblings of mine. It still seems to me heplar makes more sense as a fusion drive, though.

XO

 

[TheDS]

Your entry about damper-catalyzed fusion, though, that's something I never noticed before, and it soulds to me like an EXCELLENT use of the technology. The effect would be to allow fusion to occur at much lower temperatures and pressures, reducing the minimum size for a reactor and the average density of the gear to protect the ship from its power source, but at the same time, it would reduce efficiency, since at the very least, dampers would suck down some of that power you're producing.

This means P-P fusion can be what's used in Traveller, once you get to TL12. Earlier TL shps will have to rely on magnetics and gravitics and perhaps the CNO cycle, or maybe just use deuterium exclusively.

 

[TheDS]

Thanks, Sigg! I went there and made a comment for them. Nonetheless, this is a recent example, so I can still say it's not been seen elsewhere, and I'm going to take it and RUN!

Glad I could help, Xavier. I don't know where you could have read it. I've read all the TNE stuff, most of the MT stuff, most of the T4 stuff, and lots of the GT stuff and don't recall seeing anything like that, but that's hardly conclusive. You might try looking at some Challenge articles, or whatever other magazines they had back then that I never saw.

There's lots of people who say that HEPlaR is way more efficient than is possible. Again, I don't have a problem with relativistic exhaust velocities, so long as we're talking about plasma recombustion and not fusion (well, considering the engine we're talking about; I have no problem for it with fusion rockets or muon rockets or whatever, but I do have a problem if it's a linear-accelerator launching rocks). What I care about is how much energy is being used and how much fuel.

 

[Rupert Boleyn]

Originally posted by Malenfant:

And also, the reaction mass is a bit odd. From what I understand, isn't it better to lob out a stream of HEAVY ions out of the back of the craft? Current/recent spacecraft (I think NEAR was an example) are using Xenon as a reaction mass, which is about 130 times more massive than hydrogen and therefore can impart more momentum to the spacecraft.

As I understand it you want your exhaust particles to be as low in mass as possible. I'm not sure exactly why, but it's why hydrogen-fluorine rockets have the best specific impulse of any chemical rocket. Pity about the exhaust, though (hydrofluoric acid).

Xenon, Argon, etc., are used by cold-gas motors because they store well, and the motors are easy to start & stop. They are primarily attitude and minor orbit adjustment engines - you don't see them used for earth-orbit work, or long-duration thrusting.

 

[aramis]

I think ther is confusion of Rocket exhausts with Particle Acceleration (Ion drive) exhausts.

For rockets, however, the maximum ISP is with lighter elements in combustion.

The basic idea is that you want your exhaust as fast as possible, in both cases.

Combustion: So the energy release of combustion must accelerate the waste materials; the more energetic the reaction, the more energy can move the particles; the lighter the particles, the easier it is to accelerate the waste; accelerating the waste faster means the ability to put more fuel through faster.

Ion: heavier particles need more charge differential to accelerate them to the target speeds. The ideal will be maximum mass throughput at the highest velocity. Higher acceleration differentials will create higher energy per unit mass...

Note, the FF&S Ion Drives are one TL's drive only, and IIRC, we have modern designs comparable. The field test drive, however, is the most reliable, not the most powerful, ion drive currently available. It is generating low double digit newtons of thrust, and masses in the low double or high single digit kilograms. (A person can hold it in both hands comfortably) and draws REALLY low power needs.

(Refresh my memory, please, how much is a newton? in G/M/s/s)

I'm beginning to think HEPlaR is NOT a thermal rocket after all... (Canon HEPlaR uses waste fusion product to heat LH2 to produce thrust.)

I'm thinking it might be a fusion product filled high-performance ion drive (Since plasma is often inherently ionized...)

 

[Malenfant]

F = ma

One newton is the force required to accelerate a 1 kg mass by 1 m/s2. Or a 0.5 kg mass by 2 m/s2, etc.

Dimensionally, one newton is (kg.m.s-2) ("kilogram metres per second squared")

 

[aramis]

that's what I thought it was, just wasn't sure, and my copy of Perry's is buried.

according to Nasa
486.3 kg ship, powered by ion drive, 90mN (0.09N), 2300W peak output; 20mN at 500W. REALLY low accel. Uses Xenon gas. in operational life, has used some 22kg of reaction mass, for 1300m/s of delta-V.

amazingly low, but the engine is not terrifically heavy.

 

[TheDS]

So it takes 10 Newtons per kg to make 1 G of thrust.

 

[aramis]

So it takes 10 Newtons per kg to make 1 G of thrust.

and 10kN (10000N) to make a ton of thrust. Yup.

And the current ion drive is pretty much 25-28W per 1mN
to a ton of thrust = 250MW/TT at TL8 "Real World"...

Hm... for deep space ships with fusion power, this might be a feasible in-system drive...

 

[MadGav]

In case anyone has not seen it, you might wish to look at Dave Nilsen's answer to my question concerning HEPlaR in the ongoing mega-thread.

http://www.travellerrpg.com/cgi-bin/Trav/CotI/Discuss/ultimatebb.cgi?ubb=get_topic;f=28;t=000208;p=15

To summarise, Dave confirms that all of the repercussions of replacing thruster plates with HEPlaR were intentional.

I struggled with HEPlaR when TNE came out and always had the nagging suspicion that no one at GDW had realised the full impact it had on the game. There is some comfort in knowing that it was all planned and that I wasn't labouring with rules purely due to a game designer's oversight!

 

[aramis]

I always suspected it was intentional, as well... the routine two-week trips under CT/MT were boosted to typically about 3-week trips, if not four in an expanded system. Which made the economics of starships that much nastier.

 

[robject]

I disliked the idea of HePLaR -- until today, when I read Dave Nilsen's post. Now I'm taken with the idea of a severely limited fuel supply, and the idea of holding G-hours of fuel sounds like a very useful (and meaningful) abstraction.

Even if we don't keep track of bullets in our magazines, we do keep track of fuel use (for jumps, at least). Adding cost to landings, launchings, and maneuver sounds pretty neat.