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Bolthole location

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Re: Bolthole location
Post by kzt   » Fri Nov 08, 2019 3:02 pm

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ThinksMarkedly wrote:
Let's not discuss The Battle That Shall Not Be Named... perpetual motion devices abound.

I really liked how it proved that Manticore was really destroyed when the multiple reactors on the orbital platforms exploded and set fire to the entire facing hemisphere. So everything after the OB strike is really Honor on drugs in the burn ward.
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Re: Bolthole location
Post by fallsfromtrees   » Fri Nov 08, 2019 3:24 pm

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kzt wrote:
ThinksMarkedly wrote:
Let's not discuss The Battle That Shall Not Be Named... perpetual motion devices abound.

I really liked how it proved that Manticore was really destroyed when the multiple reactors on the orbital platforms exploded and set fire to the entire facing hemisphere. So everything after the OB strike is really Honor on drugs in the burn ward.

Given that Honor was in Haven at the time of the OB strike, this seems unlikely.
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Re: Bolthole location
Post by TFLYTSNBN   » Fri Nov 08, 2019 7:40 pm

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ThinksMarkedly wrote:
TFLYTSNBN wrote:The fusion rockets used by sunlight starships would be detectable at hundreds of light years. Keep in mind that the power output for a million ton colony ship would be comparable to the insolation of a habitable planet. The same space based telescopes that would detect and evaluate planets for potential colonization would be able to observe and track the colony ship not only during initial boost but during decelleration.


When the ship is accelerating away from Earth, sure, it would be a bright source of light, close by, and directly pointed at you. When it's decelerating 200 light-years away, it's pointed directly away from you. The thermal bloom of the rocket may not be visible.

As for the second journey, it's not pointed at Earth but neither is it pointed directly away. So the thermal bloom may be observable, but it's much, much fainter.

Another point is that there's a very, very bright source of light at the exact direction you need to be looking at: the star.

You might recall that there was once an unappreciated genius known as "NAMELESSFLY" who posted on this board years ago who pointed out that Honor's ambushing of the Havenite ships by sneaking up on them using fusion rockets rather than Impeller drive required a rather absurd obtuseness by the victims. I am informed that David Weber himself was so impressed that he sent a PM to this NAMELESSFLY asking for details of the calculations including the mass ratio (fuel portion) of the ships doing the ambushing.


Let's not discuss The Battle That Shall Not Be Named... perpetual motion devices abound.


The orientation of the exhaust from a fusion rocket has little effect on the ability to detect it. The plasma has an exhaust velocity of "only" 1/10 Cee. You are not detecting the exhaust particles directly. You detect the X-rays, UV, visible, IR and radio emitted by the plasma as it cools.
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Re: Bolthole location
Post by ThinksMarkedly   » Fri Nov 08, 2019 8:12 pm

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TFLYTSNBN wrote:The orientation of the exhaust from a fusion rocket has little effect on the ability to detect it. The plasma has an exhaust velocity of "only" 1/10 Cee. You are not detecting the exhaust particles directly. You detect the X-rays, UV, visible, IR and radio emitted by the plasma as it cools.


I'll take your word for it, but if you have links for more information, I'd like to read more. About the fusion rocket system, that is, I know how plasma works.

In fact, having the plasma pointed sideways to you would make the length of the plasma bigger, as opposed to the cross-section of its expansion in the directions perpendicular to the motion of the ship. On the other hand, a head-on plasma is consistently at the same location in space, so the effects are additive, whereas a sideways one is spread over a distance as the ship is moving.
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Re: Bolthole location
Post by TFLYTSNBN   » Sat Nov 09, 2019 5:41 pm

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Robert_A_Woodward wrote:(I snipped out the rest of the original and kept this reference to the Cerberus action)

TFLYTSNBN wrote:
You might recall that there was once an unappreciated genius known as "NAMELESSFLY" who posted on this board years ago who pointed out that Honor's ambushing of the Havenite ships by sneaking up on them using fusion rockets rather than Impeller drive required a rather absurd obtuseness by the victims. I am informed that David Weber himself was so impressed that he sent a PM to this NAMELESSFLY asking for details of the calculations including the mass ratio (fuel portion) of the ships doing the ambushing.


I wonder how much of that was cribbed from my comments from September 1998:

"Half an hour under thrusters and not detected? How is the gas accelerated? If the thrust is coming from thermal expansion, the exhaust of those rockets should be a plasma hot enough to radiate X-rays in order to get that much delta-V. It will then, eventually, cool down enough to radiate in infrared and radio. If accelerated by some sort of mass driver, then the gas will not be hot, but would still be a very fast moving plume that would interact (noisily in radio) with the interplanetary plasma. In chapter 29 of In Enemy Hands, Shannon, at tens of millions of miles away, was able to see that the "Tepes" had lost air and pieces of its hull. An cruiser squadron under reaction thrusters should be more obvious (even at hundreds of million of miles)."


I can guarantee you that the NAMELESSFLY did not crib anything from you although it appears that you also understood then point.

All of the ribbing by the NAMELESSFLY as well as TFLYTSNBN aside, it is very conceivable that Honorverse warships would become as fixated on gravity sensors as modern AWACs are fixated on radar.
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Re: Bolthole location
Post by TFLYTSNBN   » Sat Nov 09, 2019 6:11 pm

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ThinksMarkedly wrote:
TFLYTSNBN wrote:The orientation of the exhaust from a fusion rocket has little effect on the ability to detect it. The plasma has an exhaust velocity of "only" 1/10 Cee. You are not detecting the exhaust particles directly. You detect the X-rays, UV, visible, IR and radio emitted by the plasma as it cools.


I'll take your word for it, but if you have links for more information, I'd like to read more. About the fusion rocket system, that is, I know how plasma works.

In fact, having the plasma pointed sideways to you would make the length of the plasma bigger, as opposed to the cross-section of its expansion in the directions perpendicular to the motion of the ship. On the other hand, a head-on plasma is consistently at the same location in space, so the effects are additive, whereas a sideways one is spread over a distance as the ship is moving.



We are still struggling to develope viable fusion reactors. The power to mass ratio of any prospective devices is on the order of a Gigawatt per 10,000 tons. Fusion rockets with high specific impulse would have to have power densities several orders of magnitude higher to give us even a few hundredths of a gee acceleration. My comments about Petawatt or even Exawatt power levels are based on actual calculations rather than wild ass guesses.

A few hints.

Thrust = Mass Flow Rate * Exhaust Velocity

Power = 1/2 * Mass Flow Rate * (Exhaust Velocity)^2

The theoretical Exhaust Velocity varies with the particular fusion fuel and reaction utilized, but 3x10^7 m/s or 1/10 Cee is about right.

The bottom line is that your power density needs to be on the order of 100 Megawatt per Kilogram to give your starship an asccelleration of one Gee.

I had some interesting correspondence with the late Dr Jerry Pournelle about Light Sails and THE MOTE IN GODS EYE. It turns out that the power density needed for a light sail is only about an order of magnitude higher than for a fusion rocket, and the Sun provides the power. Pournelle and Niven took artistic license to make the Moties extremely powerful launching lasers necessary. Lightsails are probably the most plausible technology for starships. Even if the mass to surface area of the sail is not as low as desired, massive Fresnell lenses to concentrate solar power can easily enable acceleration of one tenth gee for a month to give a transit velocity of 1% Cee.
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Re: Bolthole location
Post by ThinksMarkedly   » Sun Nov 10, 2019 12:43 am

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TFLYTSNBN wrote:A few hints.

Thrust = Mass Flow Rate * Exhaust Velocity

Power = 1/2 * Mass Flow Rate * (Exhaust Velocity)^2

The theoretical Exhaust Velocity varies with the particular fusion fuel and reaction utilized, but 3x10^7 m/s or 1/10 Cee is about right.

The bottom line is that your power density needs to be on the order of 100 Megawatt per Kilogram to give your starship an asccelleration of one Gee.


Got it. A reaction-based rocket needs to obey the laws of conservation of energy and momentum. The energy of the exhaust cannot be lower than the kinetic energy acquired by the ship. Otherwise, we get to the perpetual motion engine problem.

Applying some handwavium magic technology to patch the holes in the story:

We don't know how the gravity plates work, but we do know that they were able to nullify somehow the effects of the acceleration as perceived by the crew. They probably operate on the same principle as the inertial sump of the wedge. And because of that and because we know that ships under impeller drive are extracting energy from the alpha band, it's not impossible to conceive that the gravity plates also function as a source of kinetic energy for the ship.

That way, the mass of the ship as "felt" by the engine is a tiny fraction of its real mass. That would allow the rocket principle to accelerate it a much higher factor than it otherwise could.
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Re: Bolthole location
Post by TFLYTSNBN   » Sun Nov 10, 2019 12:55 pm

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The NAMELESSFLY actually shared his calculations regarding the Cerebus ambush with me. I can not recall the exact numbers. However; using a plausible Specific Impulse of 3eex7 Newton-seconds per Kilogram resulted in a very reasonable mass ratio. Honorvese starships carry about 1/4 of their mass in fuel.
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Re: Bolthole location
Post by kzt   » Sun Nov 10, 2019 4:42 pm

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TFLYTSNBN wrote:The NAMELESSFLY actually shared his calculations regarding the Cerebus ambush with me. I can not recall the exact numbers. However; using a plausible Specific Impulse of 3eex7 Newton-seconds per Kilogram resulted in a very reasonable mass ratio. Honorvese starships carry about 1/4 of their mass in fuel.

Remember the density of liquid hydrogen? 1/4 mass is going to be a huge percentage of the ship volume.
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Re: Bolthole location
Post by TFLYTSNBN   » Mon Nov 11, 2019 4:38 pm

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kzt wrote:
TFLYTSNBN wrote:The NAMELESSFLY actually shared his calculations regarding the Cerebus ambush with me. I can not recall the exact numbers. However; using a plausible Specific Impulse of 3eex7 Newton-seconds per Kilogram resulted in a very reasonable mass ratio. Honorvese starships carry about 1/4 of their mass in fuel.

Remember the density of liquid hydrogen? 1/4 mass is going to be a huge percentage of the ship volume.



IIRC, Weber has mentioned​ that Honorverse ships use aneutronic fusion reactions that consume Beryllium or Boron.

Of course it would be far more plausible to burn Deuterium rathan than normal Hydrogen.
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