Why are nuclear engines used when arcjet engines provide more heat?

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Newtype
12/19/08 03:22 PM
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I remember reading in my encyclopedia that a nuclear rocket engine can provide 2 to 3 times more heat than a solid or liquid propellant but an arcjet rocket can provide 3 to 4 times more heat than a solid or liquid propellant. The more heat the quicker the fuel gas expansion and thus more thrust. So why aren't arcjet engines used instead of nuclear engines?
CrayModerator
12/20/08 07:48 AM
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Quote:

I remember reading in my encyclopedia that a nuclear rocket engine can provide 2 to 3 times more heat than a solid or liquid propellant but an arcjet rocket can provide 3 to 4 times more heat than a solid or liquid propellant. The more heat the quicker the fuel gas expansion and thus more thrust. So why aren't arcjet engines used instead of nuclear engines?




That's a trick question. BT does not use "nuclear rockets" of the type you're reading about, which are solid core fission engines. (Those are the only nuclear rockets with lower specific impulses than arcjets.)

Arcjet's specific impulse is 1000-2000 seconds, which is better than solid core FISSION engines (specific impulse 1000). Arcjets also have awful thrust-to-weight ratios. The largest arcjets built only generate about a gram of thrust while massing tons.

BT's fusion rockets have a minimum specific impulse of 24,000 on 20-ton aerospace fighters, reach 240,000 ton 100-ton aerospace fighters, and go past the billions on large BT spacecraft operating in strategic fuel mode. (Compare the thrust of BT spacecraft to fuel consumption per second.) BT fusion also have very high thrust-to-weight ratios, usually better than 15 tons of thrust per ton of engine on large spacecraft.

Why aren't arcjets used in BT? Because they completely suck compared to fusion rockets. Arcjets can be millions of times colder than BT fusion rockets, less efficient, and have billions of times less thrust.

And in the real world, a good solid core fission engine design, like the Timberwinds developed for Reagan's Strategic Defense Initiative, can deliver better than 10 tons of thrust per ton of engine while sustaining 1000 seconds impulse. In other words, a fission engine can be used to launch an object off a planet and enable high acceleration maneuvering in space, while an arcjet can't lift itself off the ground and in deep space an arcjet would show very, very low acceleration. Arcjets have a fuel efficiency advantage in the long run, but not in BT, and they're inappropriate for many real world applications due to their low thrust.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Newtype
01/20/09 09:25 AM
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How do spacecraft that use CBT fusion rockets keep from melting if they're million times more hotter than arcjet rockets?
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Zandel_Corrin
01/20/09 02:48 PM
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Heat sinks..... duh!
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His_Most_Royal_Highass_Donkey
01/20/09 03:17 PM
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No, not the heat sinks. The hydrogen fuel its self is the coolant. No matter how good the heat sinks are they can not deal with a nuclear reactor that is at the heat levels that rockets require.

You let the reactor heat up to almost meltdown and push hydrogen into the reactor to expand and create thrust at the same time it will cool down the reactor from melting down. Its a very sensitive balancing act between having the reactor going into melt down and having the reactor becoming to cold to create enough thrust.
Why argue if the glass is half full or half empty, when you know someone is going to knock it over and spill it anyways.

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Zandel_Corrin
01/21/09 02:59 PM
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hey they use cooling coils too......

yeah i know it's not just heat sinks.... but it sounded appropriate...

ask a stupid question you should get a stupid answer.
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His_Most_Royal_Highass_Donkey
01/21/09 03:39 PM
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You expect something other from newtype?

Cooling coils can only do so much and with any nuclear rocket its just way to little to cool it. Also why wast heat on cooling coils if you can use that heat on the fuel.
Why argue if the glass is half full or half empty, when you know someone is going to knock it over and spill it anyways.

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CrayModerator
01/25/09 07:59 PM
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Quote:

How do spacecraft that use CBT fusion rockets keep from melting if they're million times more hotter than arcjet rockets?




Look up fusion reactors, HeroChip. The trick is not to let the plasma touch the walls of the engine.
http://www.ece.unm.edu/~plasma/Fusion/index.html

"The most common way to confine the hot plasma used to make fusion energy is to use strong magnetic fields. The plasma can't be confined by the material walls, because the plasma is millions of degrees Celsius. (Actually, the problem is the reverse: the vessel walls are so cold, that the cool the plasma and prevent fusion.) Physicists have been working since the 1950s on making better "magnetic bottles," a problem which has been compared to holding jello (the plasma) with rubber bands (the magnetic field)."

****************************

Quote:

The hydrogen fuel its self is the coolant.




BT fusion rockets produce so much heat that no amount of hydrogen would keep them cool if the hydrogen was being used as a coolant as in typical hydrogen-fueled chemical rockets. A 100-ton aerospace fighter generates about as much heat energy as all of the power industry of 2008AD Earth, and it uses an inefficient engine - smallcraft, DropShips, and larger vessels produce vastly more heat.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Newtype
02/21/09 12:06 PM
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This webpage seems to explain fusion reactors and magnetic "bottles".
http://library.lanl.gov/cgi-bin/getfile?00285870.pdf
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Prince_of_Darkness
02/21/09 03:19 PM
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Quote:

This webpage seems to explain fusion reactors and magnetic "bottles".
http://library.lanl.gov/cgi-bin/getfile?00285870.pdf




Well, I didn't really need to know the basics of cow-tipping, but thanks for the link nonetheless!
His_Most_Royal_Highass_Donkey
02/23/09 05:44 AM
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Quote:

Quote:

This webpage seems to explain fusion reactors and magnetic "bottles".
http://library.lanl.gov/cgi-bin/getfile?00285870.pdf




Well, I didn't really need to know the basics of cow-tipping, but thanks for the link nonetheless!




But you need to know the basics before you can go onto advanced cow-tipping!
Why argue if the glass is half full or half empty, when you know someone is going to knock it over and spill it anyways.

I was a Major *pain* before
But I got a promotion.
I am now a General *pain*
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CrayModerator
02/23/09 03:49 PM
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Quote:

This webpage seems to explain fusion reactors and magnetic "bottles".
http://library.lanl.gov/cgi-bin/getfile?00285870.pdf




So did the link in my previous post, and my link wasn't 26 years out of date.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Newtype
02/27/09 02:12 PM
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Yeah, so when the exhaust in a rocket pushes in all directions against magnetic bottle field (up exhaust pushes rocket up), can the field redirect the exhaust particles movements in different directions? Or do the up moving exhaust particles simply bounce off the magnetic bottle field and go down through the exhaust nozzles?
CrayModerator
02/28/09 06:40 AM
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Quote:

Yeah, so when the exhaust in a rocket pushes in all directions against magnetic bottle field (up exhaust pushes rocket up), can the field redirect the exhaust particles movements in different directions? Or do the up moving exhaust particles simply bounce off the magnetic bottle field and go down through the exhaust nozzles?




I'm not getting into a recirculating fuel rocket discussion, HeroChip.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Newtype
03/02/09 02:17 PM
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I understand. Sounds like you don't want to get into a discussion in which a magnetic field U "track" bottle continuously bounces exhaust particles forward and backward against each other and its end and curve pieces. An arcjet's exhaust, using U halves tunnels, could, as long as the metal structure tunnels don't melt, provide a little bit of dual forward thrust (the two thrusts cancel each other out when they hit the U tunnel midsection). A dual bop drive could provide dual forward thrust (the two rounded cubes cancel each other out when they hit the U tunnel midsection).
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His_Most_Royal_Highass_Donkey
03/03/09 02:33 PM
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Quote:

I understand. Sounds like you don't want to get into a discussion




That is not it at all, he wants to keep his brain from being swept through a shit pile. To which it would be, talking to you about your stupid ideas.
Why argue if the glass is half full or half empty, when you know someone is going to knock it over and spill it anyways.

I was a Major *pain* before
But I got a promotion.
I am now a General *pain*
Yay for promotions!!!
Newtype
04/21/09 09:03 AM
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Also, why is it that World Book encyclopedia indicates that a rocket engine is several thousand more times powerful than an automobile engine of the same size yet aerounits don't move very far per thrust point in the atmosphere (seems like a 400 rated fusion engine can move a 100 ton tank four 30 meter hexes per 10 seconds while an identical 400 rated fusion engine can provide an aerofighter with a safe thrust of 6 moving it to move six ground mapsheets (about one hundred two 30 meter ground hexes). That's not providing thousands of more power.
His_Most_Royal_Highass_Donkey
04/21/09 09:32 AM
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They wanted to keep things simple. So people that have a simple minds, like yours, could play the game.
Why argue if the glass is half full or half empty, when you know someone is going to knock it over and spill it anyways.

I was a Major *pain* before
But I got a promotion.
I am now a General *pain*
Yay for promotions!!!
Newtype
04/21/09 09:48 AM
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Faster aerounits are fine with me, just increase their BV.
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CrayModerator
04/21/09 11:34 AM
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Quote:

Also, why is it that World Book encyclopedia indicates that a rocket engine is several thousand more times powerful than an automobile engine of the same size yet aerounits don't move very far per thrust point in the atmosphere (seems like a 400 rated fusion engine can move a 100 ton tank four 30 meter hexes per 10 seconds while an identical 400 rated fusion engine can provide an aerofighter with a safe thrust of 6 moving it to move six ground mapsheets (about one hundred two 30 meter ground hexes). That's not providing thousands of more power.




HeroChip, the hexes used are at different scales. Read the aerospace movement rules, low altitude and high altitude movement (Aerotech 2R or Total Warfare). For a fighter, 9 thrust points means it can achieve mach 18 in the atmosphere. A tank will only achieve 64kph on the same engine.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Newtype
04/23/09 05:49 PM
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Nine thrust points means 340.29 meters multiplied by 9 * 2=6125.22 meters/s. So a thrust of 9 in ten seconds means the aerofigher is flying at 612.522 m/s or about twenty 30 meter ground hexes per turn.
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CrayModerator
04/24/09 12:01 PM
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Quote:

Nine thrust points means 340.29 meters multiplied by 9 * 2=6125.22 meters/s. So a thrust of 9 in ten seconds means the aerofigher is flying at 612.522 m/s or about twenty 30 meter ground hexes per turn.




Your basic assumptions are wrong. A thrust point is equal to 300 meters per second. Try again, and this time cite the page numbers of BattleSpace, AT2R, or Total Warfare that gives you the scales you're referring to.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.


Edited by Cray (04/24/09 12:02 PM)
Christopher_Perkins
04/24/09 09:37 PM
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AeroTech 2 Aero combat on BT Maps
9*16*30=
144*30=
4,320 meters in 10 seconds
432 Meters per Second

1*16*30=
16*30=
480 meters in 10 seconds
48 Meters per Second



AeroTech 2 Aero Maps
9*17*30=
153*30=
4,590 meters in 10 seconds
459 Meters per Second
(most often rounded to 4500 m per turn & 450 m/sec)

1*17*30=
17*30=
510 meters in 10 seconds
51 Meters per Second
(most often rounded to 500 m per turn & 50 m/sec)


AeroTech 2 Space Maps
9*18,000=
162,000 meters in 60 seconds
2,700 Meters per Second

18,000 meters in 60 seconds
300 Meters per Second
Christopher Robin Perkins

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CrayModerator
04/27/09 06:19 AM
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Numbers without explanations and definitions don't explain much, Chris.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Newtype
04/27/09 09:39 AM
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Cray, Mach18 is 18 times the speed of sound. The speed of sound is 340.29 m/s.
http://www.google.com/search?hl=en&q=speed+of+sound&aq=f&oq=
So an aerofighter doing a thrust of 9 (mach 18) in 10 seconds is going at 612.522 m/s.
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CrayModerator
04/27/09 01:42 PM
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Quote:

Cray, Mach18 is 18 times the speed of sound. The speed of sound is 340.29 m/s.
http://www.google.com/search?hl=en&q=speed+of+sound&aq=f&oq=
So an aerofighter doing a thrust of 9 (mach 18) in 10 seconds is going at 612.522 m/s.




No, Chip, you don't divide speed by time to get speed. Dividing speed by time gives you acceleration.

Mach 18 is 6125.22 meters per second. That's all speed is: distance divided by time. You don't divide velocity by time unless you want acceleration. If you maintain your speed at 6125.22 meters per second for 10 seconds, your speed is still 6125.22 meters per second. If you maintain it at 6125.22m/s for 1000 seconds, your speed is still 6125.22m/s because you did not change your speed.

Now, the distance you covered in 10 seconds at mach 18 would be (6125.22 m/s) x (10 s) = 61,252.2 meters. (You multiply speed by time to get distance.) But the speed is still 6125.22 meters per second.

Or if you accelerated from 0 to mach 18 in 10 seconds, then your acceleration would be (6125.22 m/s - 0 m/s) / (10s) = 612.522 m/s/s. (Like I said, divide speed by time to get acceleration.) But the speed is still 6125.22 meters per second.

You should've realized something was wrong with your math when you said mach 18 (6125.22m/s) was equal to mach 1.8 (612.522m/s) right after defining mach 1 as 340.29m/s.
Mike Miller, Materials Engineer

Disclaimer: Anything stated in this post is unofficial and non-canon unless directly quoted from a published book. Random internet musings of a BattleTech writer are not canon.
Newtype
04/28/09 03:14 PM
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Quote:

Or if you accelerated from 0 to mach 18 in 10 seconds, then your acceleration would be (6125.22 m/s - 0 m/s) / (10s) = 612.522 m/s/s. (Like I said, divide speed by time to get acceleration.) But the speed is still 6125.22 meters per second.



Yeah, that's what I was trying to indicate.
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