For reference, accellerating a 1000 metric ton mass by 1g for one hour requires about 36 billion newton-seconds (not counting additional impulse required to accellerate the fuel required to produce that much impulse). The above done in my head at this hour, and I may have done something horribly wrong - if so, I'm sure someone will set me straight.
So it seems that, if anything, FF&S has the fuel consumption significantly lower than should actually be the case.
wildstar@quark.qrc.com
I've been watching this discussion for a few issues, thinking about jumping in, but have been too lazy. However, I have nothing better to do today, since the flood of May 8 is lapping over our front steps and I'm not going anywhere.If you are thinking about the low tech thrusters I think that they are good enough. If you consider the Shuttle it need large fueltanks just to get it into orbit. Low tech fuels are not efficent if you consider the weight and volume, but they are all we have at the moment.WRONG!! The shuttle is only 66.7% fuel tank and it can hit orbit Under FFS it would have to be 1000% fuel tank to hit orbit. The FFS thruster rules are broken. Feel free to prove me wrong though, post a Tech 7 design that can hit orbit (with annotations) under The realistic thrusters rules from a size 8 world under 1g and I'll eat this post.
callahan@crash.cts.com
The thrusters ARE broken. The fuel consumption is somewhat *better* than the real-world, but the thrust-to-weight ratio is pathetically low. As an illustration, they give a TL5 liquid fuel rocket 6 tonnes-thrust per ton of motor; in the Real World (tm), the V-2 rocket motor (a TL5 LF rocket) developed 20-30 tonnes-thrust per ton of motor.
GDW also requires absurdly high Delta-V numbers to reach orbit: they claim Earth requires 0.64 G-hours, which equals 22.5 km/sec delta-V. In the Real World(tm), we require less than 10 km/sec (0.3 G-hours) to reach low Earth orbit.
Finally, GDW's formulas for calculating G-hours are broken. Here's the Real World formulas that apply to the Real World space program:
Mass Ratio MR = Wl / Wef
Wl = Loaded Weight
Wef = (Loaded Weight - Reaction Mass Weight)
also: MR = e ^ (dV/Ve)
Exhaust Velocity Ve = 36,000 / FC
FC = Fuel consumption in tonnes/hour
(yes, the number from FF&S)
Ve is in m/sec
G-hours = ln(MR)/FC
Delta-V = ln(MR)*Ve
Delta-V is in m/sec
Hohmann Transfer Orbit:
dVh = Ve1 * sqrt( R2/(R1+R2) ) + Ve2 * sqrt( R1/(R1+R2) )
dVh = total delta-V (m/sec) of Hohmann transfer orbit
Ve1 = Escape velocity (m/sec) of start body
Ve2 = Escape velocity (m/sec) of end body
R1 = Radius of start body's orbit
R2 = Radius of end body's orbit
Orbital Velocity
Vo = sqrt( R * G)
Vo = orbital velocity (m/sec)
R = Radius of orbit (m)
G = G (gravity) at altitude of orbit (m/sec^2)
Escape Velocity
Vee = sqrt (2*R*G)
Delta V to achieve Orbit is, ideally, a Hohmann Transfer Orbit from
ground (which has the rotational velocity of the planet's surface as its
"orbital velocity") to Orbit, plus some extra for getting above the
atmosphere, say +1-2 km/sec.
Because of the ridiculous numbers for thrust-to-weight ratios in FF&S, I arbitrarily changed them to be more in line with the Real World(tm); see below:
Chapter 9: Sublight (Maneuver) Drives, Page 70 (change): In the Self-Contained Thrusters table, use the following values for thrust-to-volume/weight ratio instead: TL Type Th ---------------------------- 5 LF Rocket 20 6 LF Rocket 40 7 HF Rocket 40 8 AZHRAE (Rocket) 40Final note: George Herbert's corrected tables for MegaT's Hard Times low-tech rockets are also fairly accurate (in spite of he and Steve's quibbling over a few Isp's here and there), if you want a more detailed treatment of different types of sub-TL9 rockets.
-- Cynthia
Brief Glossary:
Traveller's thrusters and such are all much better than 700s. There's no reason for TL9+ vehicles to prefer the equator. Starports should be near population centers, but not too near.
-george
ccjoe@showme.missouri.edu (Joseph Heck) asked:
question, where would I find the tidbits for that info? I'd like to be able to get some better ideas of what a rocket _should_ be able to do (yeah, and me not being a rocket scientist), but I don't know where to look up the info.Is there an equation that relates newton-seconds to mass x accel x time?
KELLOGG@thorin.uthscsa.edu (Scott "2G" Kellogg) answered:While quite correct (and the basis for the entire branches of physics!), I thought this explanation might be a little terse. :-) So I thought I'd explain in a bit more detail. This sort of basic analysis comes in very handy when working with FF&S, so you can judge what's absurd and what's not.
F=M*A
Force = Mass * Acceleration
The force required to accelerate 1 kilogram to 1 meter/second is 1 newton.
Units: For convenience, it's easiest to work in the SI (metric) system of units. The fundamental units of this system are:
Mass: the kilogram (1000 grams), abbreviated kg. About 2.2 pounds.
Distance: the meter (1000cm), abbreviated m. About 3.2808 feet.
Time: the second, abbreviated s.
There are also some derived units:
Velocity: meters per second, abbreviated m/s. About 2.2369 MPH.
Accelleration: meters per second squared, abbrev m/s^2. About 0.102g.
Force: newtons (kg*m/s^2), abbreviated N. About 0.2248 pounds.
In addition, a useful measure of a rocket's ability to shove something
around is it's total impulse. If you were to graph the thrust (force) of
a rocket over time, the _area_ under the curve would represent the total
impulse. The unit is newton-seconds (N-s), 1 newton-second represents a
thrust of 1 newton for one second (and also two newtons of thrust for a
half second, or a quarter-newton of thrust for 4 seconds).
In an airless, gravityless environment, all rocket engines with the same total impulse would (eventually) accellerate identical test loads to the same final velocity - they would simply take different amounts of time to do so, and subject the test load to different accellerations.
To figure out the 1000 ton example above, first I had to do some conversions:
1000 tons:
Assuming a "ton" is a metric ton, 1 metric ton is 1000kg.
Thus, mass is 1,000,000kg.
1 G-hour:
An accelleration of 1G for 1 hour.
To simplify calculations in my head, I assumed 1G = 10 m/s^2.
1 hour is 3600 seconds.
So, the force required to make 1G of accelleration is:
Force = Mass times Accelleration (from Scott's F=MA above).
Force = 1,000,000 kg * 10 m/s^2 = 10,000,000 kg-m/s^2 = 10,000,000 N.
To make this thrust continue for a whole hour, we need:
Total Impuse = Force * Time
Total Impules = 10,000,000 N * 3,600 s = 36,000,000,000 N-s.
The equivalent units in FF&S Spacecraft construction are:
Mass: 1 Ton = 1,000 kg.
Time: 1 hour = 3600 s; 1 "turn" = 1800 s.
Accelleration: 1G = 9.80665 m/s^2 (though for rough work 10 m/s^2 is OK).
And now for the sticky units: GDW seems to have a very loose grasp
of the difference between weight (which is a _force_: the force exerted
by a planet's gravity on an object) and mass (which is an inherent
property of any object).
Force: 1 Ton (thrust or weight) = 1000 kg * 9.80665 m/s^2 = 9806.65 N
or if you approximate = 10,000 N
In Real Life, a rocket's mass changes drastically over it's flight time;
that's because a VERY large fraction of it's mass is conposed of fuel,
so that while the rocket's thrust (generally) remains constant, the
accelleration experienced will change dramatically.
For a good example, a fully-loaded and fueled Saturn V massed 2,913,000 kg. when ready for ignition. In Earth's gravity, that's 28,570,000 N; it's five first-stage engines produced 33,000,000 N of thrust, for a lift-off accelleration of 0.155g (yes, that's less than 1/6th of a g). However, the accelleration increased during flight, as fuel was burned.
Hope this helps ...
wildstar@qrc.com
Note #1: TL 8- rockets should be designed by the kilo not liter. Volume doesn't matter nearly as much until later TL rockets. rev 5 : 5/20/97 Table entries are: TL Tech Level Type Rocket Engine Type Ftype Fuel Type Fmass Fuel Burned (tonnes(mass)/hr) Fcons Fuel Cons (kl/hr) AThr/VThr Breakdown of: AThrust Thrust at 1atm (Sea Level) VThrust Thrust in a vacuum (outer space) Isp Specific Impulse (sec) sea level / vacuum Power Power used (#) or produced (+#) per ton of engine MCr Megacredits per cubic meter Examp Sample real-world engine All values are per cubic meter of engine (mass = 1.0 tonne / cubic meter) Power output on all liquid motors: Mw = 0.1 * AThrust delta V = 10 * Isp * ln(Mr) Mr = (Mfuel+Mrocket)/Mrocket HIGH THRUST ROCKETS TL Type Ftype Fmass Fcons AThr/ Isp MCr Examp VThr -- ---- ----- ----- ----- ------- -- --- ----- 2 Solid Solid 600* 600* 12/15 70/90 0.25 3 Solid Solid 600* 600* 15/17 90/100 0.25 4 Solid Solid 600* 600* 20/22 120/130 0.15 5 Solid Solid 600* 600* 30/32 180/190 0.125 5 Liq Cryo 385 385 24/30 220/280 var xptl 6 Solid Solid 600* 600* 42/47 255/280 0.1 Minuteman1 1st 6 Hybrid Hybrid 600* 600* 43/48 260/290 0.1 Firebolt 6 Hyp Liq Hyp 1040 1040 74/85 255/295 5.5 RS-87 6 Liq Cryo 580 580 42/50 260/310 4.5 MA-5 6 HDLiq Perox 780 560 55/65 250/300 4.5 Black Arrow 7 Solid Solid 600* 600* 43/48 260/285 0.05 SRB 7 Hybrid Hybrid 600* 600* 45/50 270/300 0.05 7 Hyp Liq Hyp 1130 1130 80/93 255/295 3.5 VikingV 7 Liq Cryo 1010 1010 75/85 265/305 2.0 F-1 7 HDLiq Perox 1300 925 95/108 260/300 1.75 7 LH Liq LHCryo 550 1650 -/65 -/425 6.0 J-2 7 NTR LH 34 475 -/8 -/850 25.0 NERVAus 8 Solid Solid 600* 600* 44/49 265/295 0.04 T4B SRMU 8 Hybrid Hybrid 600* 600* 46/54 275/325 0.05 AMROC 8 Hyp Liq Hyp 1500 1500 120/132 285/315 3.0 RD-253 8 Liq Cryo 820 820 70/77 310/340 2.5 RD-180 8 HDLiq Perox 1400 1000 117/125 300/320 3.0 8 LH Liq LHCryo 580 1750 60/73 365/455 8.0 SSME 8 NTR LH 42 590 6/10 500/850 30.0 NERVA 8 AdvNTR LH 36 500 -/12 -/1200 35.0 TimberW 9 GCNTR LH 9 -/5 -/2000 45.0 GasCore 9 FusRkt DtELH Fuels: (tm = ton mass) Type Density m^3/tm Cr/tm Notes Hypergolics 1.0 1 600 Nitrogen Tetroxide + Hydrazine Cryo 1.0 1 250 LOX/Kerosene Perox 1.4 0.7 1,200 Hydrogen Peroxide/Kerosene LPCryo 0.50 2 250 LOX/Propane or Methane LHCryo 0.33 3 400 LOX/LH2 LH 0.07 14 500 Liquid Hydrogen Solid - - - Mass, Cost are engine cost * Solids have nominal burn duration of 6 sec, hence 600 Fc
Steve asks:The SSME has an Isp(vaccum) of about 454, though they're hoping to hit 460 by the late 1990's. However, at sea level, that's closer to 390-400. IMHO, without explaining the whole specific impulse variation with altitude theory, the only reasonable way is to average and simplify.You give a TL8 LH rocket a specific impulse of about 428. Why? The SSME has a specific impulse of 450+.
Maybe I should state that thrust goes up 5 tons for all liquid rockets operating purely in vaccum. (this is off the top of my head... let me analyze it a bit first 8-) That should help correct things a bit.
The resistojet has a specific impulse of 18 here. Again, why? There is not really any upper limit to such with enough power, and according to my steam tables, the steam at 232C should have a specific impulse approaching 190, depending on the nozzle design.I was presuming a really low-tech and low-heat resistojet. I can't imagine wanting to use one at a higher TL (well, ok, maybe, but the chart can't get infinitely big 8-). However, you have a point, which I'll explain in a bit...
Your fusion rocket (experimental) has an Isp of 51,400+. Your TL9 version has an Isp of 50,100+. Why does the production version have LOWER Isp than the experimental? And why are the performances so low? Admittedly, a fusion rocket is pretty open-ended as far as performance, but I have seen studies that concluded that a fusion rocket could have an Isp in the range of 1,000,000 or more.Oops. 8-) You're right, the specific impulse ought to go up a bit at TL9. As for why did I set the values that low... well, I've talked to several people who do things like design theoretical fusion engines, and they don't think that specific impulse of fusion rockets will get higher than about that (50,000) for any near-term (i.e. one we can understand, if not build, now) designs. Those values of 1,000,000 were from physicists, not engineers. 8-)
And why are you using liquid Xenon and such for your ion drives, instead of cadmium, which has a density of 8.65?Because last time I checked, the people working with testing ion drives were having reliability problems with the ones that didn't use noble gases. I don't want to presume using something that may or may not work.
For that matter, why do you assign a density of 1.0 for rocks? Very few rocks float, even fewer have neutral bouyancy. Most seem to have a density in the range of 2-3.You need to stack round rocks in non round fuel hoppers, and more importantly, rocks don't pack that well unless you make them into cubes or such. If someone wants to complain about this, well, if you think you can pack them in more, go right ahead 8-)
Ok, now here's the one question that I've been avoiding talking about: "why didn't I do anything about putting Ion drives in?"
Well, there's a good reason for this. In real ion and MPD drives, there is a strong variation in specific impulse and thrust among the various drive options. More importantly, the specific impulse and thrust of a particular engine type varies with sort of inversely... you can get higher thrust at lower Isp and visa versa. There are power supply weight considerations that require a careful look at the MegaTraveller power supply options before any reasonably accurate simulation can be done in these rules.
WHile mostly applying to the Ion and MPD engines, this also will affect things like the resistojet, which could theoretically get a whole lot hotter, the mass driver, etc. I need to go back and re-read several J.Propulsion and J.Spacecraft papers to take a look at how the tradeoffs work in detail. Until I've done that, I am not willing to make a guess as to how to accurately simulate these items in MegaTraveller... 8-)
Health warning: I'm an observational astronomer by trade, not a nuclear or plasma physicist, so please don't take this as gospel :-)
Two related notes: How big would the reaction mass "flame" behind a Heplar or fusion equipped ship be. Since the reaction mass IIRC is around 10^6 C, shouldn't it be visible from a great distanc, like multiple AU?
The flame would be very hot, but when travelling at a speed of 67 km/s (4hexes per turn) or a similar order of magnitude, the reaction mass expended would be spread out across such a large area that it would not be easily visible. The exhaust near the ship would be hotter, but the greatest signiture would come from the end of the ship heating up, which would accumulate heat, rather than the exhaust itself.
This is something which has bothered me about TNE for some time. Any ship with an operating fusion drive will create a clearly defined 'drive plume' visible across a wide spectrum (soft X-ray to radio) and likely to be tens of thousands of km long. In effect: I'm here --- shoot me! :)
To achieve the kinds of specific impulse described for TNE drives, the reaction mass emitted by the ship would have to be travelling "backwards" at a healthy fraction of the speed of light. If it was otherwise unconfined, the plasma trail would expand "sideways" at its internal sound speed, which would be considerably less than the exhaust velocity...giving a narrow "drive plume". Magnetic effects may make the plume even narrower (an effect called "flux-freezing" would drag magnetic field from the drive's magnetic confinement 'bottle' out into the plume and wrap the field around it).
If I was looking for a fusion drive plume, I'd look at UV wavelengths (free-free emission from the ultra-hot plasma), optical (line emission from recombining hydrogen and helium), and radio (synchrotron emission from charged particles in the magnetic fields of the plume). Radio interferometry especially would be able to pinpoint the ship very nicely (fractional-arcsecond accuracy) :)
I'd have to do some guesstimating, but I reckon that high-tech passive EMS sensors could _image_ (not just detect) a drive plume as far out as the orbit of Jupiter.
And second, would using a fusion drive in the region from the upper atmosphere to LEO generate an EMP?
HEPlaR is not radioactive, so it wouldn't. Fusion rockets would certainly emit a lot of electromagnetic radiation, but it probably wouldn't be an EMP. The energy from the fusion rocket is output more gradually than from a nuclear explosion, so there would be a constant wash of radiation rather than a pulse. This radiation might be enough to disrupt electronics depending on the size of the rocket and the range involved.
*Blink*...so far as I'm aware, radioactivity isn't a prerequisite for an EMP. An EMP as I understand it arises from a large mass of ionised material moving very rapidly (moving charges radiate EM radiation). If the HEPlaR produces an ionised exhaust, then it should produce EMP-like effects (but probably only a fairly weak, continuous effect, as stated above). More dangerous would be flying _through_ the magnetic "drive plume" of another ship...if you were moving quickly enough that could well blow all your electronics :-)
Oh well, just my Cr0.02...
Duncan
From: Brendan O'Donovan writes:
I suggest the following thruster as a TL9 solution for planetary landingsTL Type Th MCr MaxT FC FT Airframe 9 HeatJet 8 0.5 - 2.00 Lhyd HyperThe heat jet passes liquid hydrogen around a fusion reactor, heating it and boiling it, expelling it as a gas. This is a precursor to the HEPlaR drive, but the hydrogen does not reach a plasma state. Although considerably inferior to the fusion rocket which becomes available at the same time, it is a cleaner alternative, and can be used within planetary atmospheres.
From: Neil Taylor with its own HePLAR drive and sensors cou by using lots of reaction mass. If you had big enough this is almost how the fusion drive works anyway. I think the Trav version has the Lhyd dumped into the fusion core; but all realistic proposals (eg from NASA) have in fact used your plasma drive, fueled by the reactor heat.
This answers my question about realistic HePLAR. I'll just pick an exhaust temperature I like and work out performance from there.
I said:
From: Duncan Law-GreenTwo related notes: How big would the reaction mass "flame" behind a Heplar or fusion equipped ship be. Since the reaction mass IIRC is around 10^6 C, shouldn't it be visible from a great distanc, like multiple AU?
This is something which has bothered me about TNE for some time. Any ship with an operating fusion drive will create a clearly defined 'drive plume' visible across a wide spectrum (soft X-ray to radio) and likely to be tens of thousands of km long. In effect: I'm here --- shoot me! :)stuff deleted
Radio interferometry especially would be able to pinpoint the ship very nicely (fractional-arcsecond accuracy) :)
I'd have to do some guesstimating, but I reckon that high-tech passive EMS sensors could _image_ (not just detect) a drive plume as far out as the orbit of Jupiter.
Wow! Thanks for the technical detail. What this means in game terms I think is that ships will be able to spot bogies from a long way off. They might not be able to get a targeting lock, but they will know someone is out there, maybe even system-wide. This might not matter much if most combats take place within the 100d limit of the mainworld, but it also means that there will be little chance of hiding elsewhere in the system if you ever turn on your drive.
Hmmm... What about stealth drives? A ship could intentionally run its drive "cool" by using extra reaction mass, but this would probably not reduce the plume signature signifigantly until fuel consumption become impossibly high. What about exotic drives useful for their low sig but not otherwise practical?
The other thing this brings up is the possibillity of intercepting intruders with "cruise" missiles which have their own HePLAR drives and sensors. Because they could take higher G's than a human-crewed vessel, they could intercept the target more quickly, and since bogies can be detected so far off they would provide a stand-off attack capability. Having their own drive would also allow them to carry a much larger warhead. Any ideas on the max G tolerance of missile components?
Thanks again to everyone who responded to my question.
-- M
Also, in my apologising post I made another mistake - 'heterolytic' should be
'homolytic'.
-- Brendan
Lambda(max) * T = 0.29 cm*K
Given the high temperature of the emitted reaction mass (which I don't have handy), I suspect you'd get quite short-wavelength radiation, very possibly even in the X-ray range, which should be FAR easier to detect than mere thermal (infrared) emissions, and also less characteristic of "harmless" natural phenomena. I know *I'd* perk up if a raging X-ray source streaked into the solar system!
I suspect this, along with thermal radiation from those huge fusion plants, is exactly what is used in the "lock-on" from Brilliant Lances et alia. This doesn't help for contact missiles (which it sounds like some people are re-proposing), since Traveller lasers are such awesome point-defense weapons in interplanetary space. (Would that we had a harder-sf universe without gravitic focussing, but then it wouldn't be Traveller any more... :-) Detonation-laser missiles, of course, would use the same targetting and tracking information as larger vessels do: they can just afford to get closer to deliver their punch.
From: Peter L. BergholdYou're both moving around the galactic center, so you use that for reference, subtract out its motion through space, then just adjust for individual variances. Since stars are unlikely to arbitrarily change their direction of motion, you make sure you have an adequate base of observations of proper motion, red/blue shift of spectral lines, etc. so that you can predict the destination star's movements. Yes, it might get tricky if we were going from here to the Larger Magellanic Cloud, or between different spiral arms all in one hop, but for distances of one to six parsecs, I doubt it makes that much of a difference. Perhaps you just have to make a little extra boost at the end of the trip to "catch up" on the star's (or planet's) orbital motion.
Subject: Relativity and its effects on navigation.Given that everything in the universe is moving (quickly!) how do you calculate where things are in space? The light you see from a distant star system may or may not really give you a clue as to its location. In the time it takes for light or other radiation from a star system to get to you the object has moved from the point it was when the light was emitted. Same for your perspective in space. The place you are observing from is *also* moving so that changes the "value" for the observation even more...
So what is a navigator to do? This makes navigating in space REAL interesting...
----------------------------*-------------------------*----------------------- Joseph L. "Chepe" Lockett | "Nullum magnum ingenium | GURPS fan, Amiga user, jlockett@hanszen.rice.edu | sine mixtura dementiae | Shakespearean scholar, http://www.io.com/~jlockett | fuit." -- Seneca | actor and director. ----------------------------*-------------------------*-----------------------
Could you post it again for me? That or just mail it to me. Thanks!Done, plus free bonus items.
Item TL Vol Mass Th MCr MinT FC ----------------------- ---- ------- ---- ---- ------- ---- ------- Fusion Ram-jet 10 1 1 9 0.35 10 0.00035 Fusion Ram-jet 10M 0.7 0.49 9 0.35 10 0.00035 Fusion Rocket 10M 0.7 0.49 9 0.35 10 0.00035
What about a SD and BSD version? What are the material volume figures you're using? Just assume the thing is a cylinder of a some size? What thickness (or I guess armor value would be the thing here)?The density of the rockets lead one to believe that most of the volume is supporting structure, of whatever shape, thus it makes a passible estimate to use armor values as a mode of comparison.
I haven't done SD or BSD versions, because these were developed for Steve's FF&S Trillion Credit Squadron-by-email game, which starts at TL10. He's still got openings, if you are interested...
I kinda like the idea of big military ships using fusion drives... makes large marshalling areas away from the major world(s) make sense. In CT really big ships usually didn't land (like above 5000tons, right?). That might be Imperial military standards... ships 5kt and below are fitted with HEPlaR for operating near commercial traffic. Big ships would have a small HEPlaR as a backup, and for close ship operations.Well, we've always argued that HEPlaR are just a big a torch as a Fusion Rocket, only less efficient. So... for docking, my ships carry very small chemical rocket packs for docking maneuvers, and otherwise just watch where they point their torches. For atmospheric operations, that's what the above free item (Fusion Ram-Jet) was developed for. In GURPS Vehicles, it is known as the Fusion Air-Ram... A small fusion reactor heats intake air and spits a superheated air-jet out the back, along with its own exhaust. The exhaust is mildly radioactive--not dangerous, but trackable with a Geiger counter or similar detector... and is about as hot as a jet engine of similar thrust.
-- Cynthia
---------------------------------------------------------------- I've just passed the I-10/610 split, the road is empty in front of me, I've a 54-mile commute, 3000 pounds of Detroit steel at my command, Judas Priest's "Freewheel Burning" is in the stereo... DRIVE OFFENSIVELY! -- cyhiggin@usa.pipeline.com
According to FFS, 1 MW HePlaR thruster consumes 0.25 m3 of liquid hydrogen in one hour. This amount can be converted to (0.25 m3 * 0.07 ton/m3 *1000 / 3600 s =>) 0.0049 kg/s.
Thrust: F = v * m/t [Newtons]
Power consumed by generated thrust: P = 0.5 * m/t * v^2 [Watts]
m/t = 0.0049 kg/s
Thrust in Newtons can be converted to tons by dividing the Newton value by (9.81 m/s^2 * 1000 =>) 9810.
With 1 MW input power and 100% efficiency, the exhaust velocity
of plasma is:
v = sqrt(P/(0.5 * m/t)) = 20284 m/s
Thrust produced by this plasma stream is:
F = v * mt / 9810 = 0.0101 tons = 10.1 kg thrust
However, according to FFS, this thrust should be 20 tons, which is 1980.2 times the calculated value.
While thrust is proportional to exhaust velocity, the power consumption is proportional to the square of this exhaust velocity. Because of this, it is more efficient to have heavy exhaust with lower speed than light exhaust in high speed.
Derek Wildstar (wildstar@qrc.com) says:
Yes. Fixing the exhaust mass problem would require that the thruster consume even more fuel that it does now - which would be a problem.
One practical example of this realization is the difference between turbojet (TJ) and high-bypass-turbojet (HBT) engines. TJ engine sucks air and ejects it at high velocity jet, while HBT engine directs its exhaust into a turbine that rotates a ducted fan. (that is, gas turbine can be considered to be a turbojet with additional power turbine fixed at exhaust port)
This ducted fan moves larger volume (and therefore mass) of air than TJ, but with lower velocity. The end result is greater thrust with the same power input.
Some old TJ engines can be converted to HBT engines by adding this turbine/ducted fan assembly at the rear end of engine, (these converted turbojets are called Aft-fan -engines) but today most HBTs have build-in ducted fan, and are externally identical to TJs.
I think that the problem with HePlaR engine is that it appears to be working as a resitojet. This means that the hydrogen fuel itself does not give any extra energy to the exhaust jet, and is not used as propellant but as ejectant. The thrust would be much better if the thruster used rocket fuel that gives off extra energy when it is heated by the engine (EAPlaC), or if the hydrogen fuel would undergo fusion (Fusion Rocket).
Antti Lahtinen : Justice is Only a Wish of a Weak
al76188@cs.tut.fi :
I'm not really up on the physics enough to figure this out. How would one calc the Isp from a rocket given the stuff that GDW presents in it's tables?
The short answer is, using the FF&S "Self Contained Thrusters" table, the Isp == (3600 / FC) seconds. Extending the table for HEPlaR and MW required gives (with the Fusion Rocket for reference):
TL Type Thrust MCr FC FT AF MW req. ------------------------------------------------------- 9 Fusion 9 tons 0.35 0.000350 LHyd Hyper (- 0.18) 10 HEPlaR 200 tons 0.01 0.000875 LHyd Hyper + 10.0
The fusion plant needed by the HEPlaR drive has minimal impact on the fuel usage of the drive. FC is measured in metric tons of fuel per ton of thrust per hour, which may be confusing. (Everything else uses figures based on one cubic meter of engine -- in my opinion, they should have used metric tons or cubic meters of fuel per cubic meter of the rocket per hour.)
The long answer is a bit more convoluted. Specific impulse is a measure of fuel efficiency. Officially,
Specific Impulse == (Force * Time) / (Weight of fuel to apply force)
However, I believe that the acceleration factor in the force and in the weight are the same in this situation and can be canceled to get the equation I gave you above. Since FC is expressed in metric tons of fuel per hour per "metric ton accelerated to 1 gee", we need a factor of 3600 to convert from hours to seconds. Normally, force and weight are given in Newtons and time and specific impulse are given in seconds. To get an acceleration of 1G (9.8 meters per second squared) on one metric ton (1000 kg) you need a force of 9800 N, and under one gee of acceleration, one metric ton of fuel weighs 9800 N. Plug these numbers, and the FC for HEPlaR into the equation above, and you get:
Isp HEPlaR == (9800 N * 3600 sec/hr) / (9.8 m/s^2 * 0.875 kg/hr)
== (9800 N * 3600 sec/hr) / (8.575 N/hr)
== about 4.11 million seconds
...or using the quick FF&S equation,
Isp HEPlaR == 3600 ton-seconds/hr / 0.000875 ton/hr
== about 4.11 million seconds
Which is absurd, but this is SF. Even the modern rockets in FF&S are optimistic in regard to Isp -- I believe that the LOx/LHyd rocket has an Isp of 800 seconds or so, but a real rocket of this type, like the SSME (space shuttle main engine) gets only about 450 seconds.
Steve Bonneville
|
Traveller is a registered trademark of Far Future Enterprises. Portions of this material are Copyright ©1977-1996 Far Future Enterprises. |
Joseph Heck
(joe@mu.org)
21 August 2000