For Questions That Don't Merit Their Own Thread

[p1t1o]

1 hour ago, ARS said:

Does a laser gun (which purely shoots energy, either continuous stream or pew pew pew variety) is supposed to have recoil? Does a laser gun even need barrel?

Well the term "barrel" is not relevant because you ar not using bullets, so no theres no barrel on lasers, though you might use an optical mechanism (mirrors, diffraction gratings, optic fibres) to transport the beam from the place it is generated to a pointing mechanism, which could be termed a "barrel".

It seems counter intuitive, but lasers actually do have recoil, its just very small. A laser with an output of 300MW will have a 1Newton recoil force. And since a 300MW laser is usually quite massy, its not really something you will often notice. It scales exactly with power so your 1mW laser pointer has a recoil of....<counting on fingers>...3 trillionths of a Newton (3e-12N).

If you're thinking "can that be used to propel spaceships", you'd be right, it is what gets you the theoretical maximum possible Isp (exhaust velocity is c). But it takese a LOT of power to get any appreciable thrust (300MW/N as above).

Edited June 1, 2018 by p1t1o

[NSEP]

How do fusion reactos turn off? Where is all the extremely hot matter discarded from a fusion reactor?

[p1t1o]

13 minutes ago, NSEP said:

How do fusion reactos turn off? Where is all the extremely hot matter discarded from a fusion reactor?

They dont vent live, fusing plasma. The matter is allowed to cool, then its just rarefied gas (eg: ITER only uses a few grams at a time).

I believe in current test reactors it is pumped out and analysed to study the reaction.

Edited June 6, 2018 by p1t1o

[ARS]

Is it possible to make an orbit where an object appear "stationary" relative to earth's rotation from third person view? For example, (seen from above) the object and the groundstation is on 12 o'clock position, then the earth rotates, and the groundstation changed position by 1 hour (15 degrees), but the object is still on 12 o'clock position. As the earth rotates even further, the groundstation changed position by 15 degrees for each hour, but the object is still in 12 o'clock position. It orbit earth, yet it's "stationary", fixed on a single point relative to earth and moon movement. Is this possible?

[p1t1o]

11 hours ago, ARS said:

Is it possible to make an orbit where an object appear "stationary" relative to earth's rotation from third person view? For example, (seen from above) the object and the groundstation is on 12 o'clock position, then the earth rotates, and the groundstation changed position by 1 hour (15 degrees), but the object is still on 12 o'clock position. As the earth rotates even further, the groundstation changed position by 15 degrees for each hour, but the object is still in 12 o'clock position. It orbit earth, yet it's "stationary", fixed on a single point relative to earth and moon movement. Is this possible?

If Im understanding you correctly, it sounds like you are describing an object which is not rotating around the Earth, in which case, no this is not possible.

 

However, there is a name for this because it is possible to use them around stars, they are called Statites. They consist of the payload and a solar sail tuned to support the weight of the mass against solar gravity without being in orbit - the solar wind provides the "lift".

A curious property of these is that, because both gravity and the solar wind tail off in proportion to the inverse square law, the same area of sail can support the statite at any altitude, so it can be moved around the solar system at will and it will remain in place where you leave it (subject to the gravity of other astronomical bodies of course).

[wumpus]

On 6/1/2018 at 9:56 AM, p1t1o said:

If you're thinking "can that be used to propel spaceships", you'd be right, it is what gets you the theoretical maximum possible Isp (exhaust velocity is c). But it takese a LOT of power to get any appreciable thrust (300MW/N as above).

You hardly need a laser, an LED (or even some more efficient light) would work just as well (and the lower the wavelength the better).  In the end you have the same basic claim of the em drive (electricity in, momentum out) but with wildly lower efficiency.  Also, for any ultra-high Isp drive, don't be surprised if your heat sink produces significant thrust (possibly as much as your "main" thrust).  Maximize the albedo of the thing, except for low albedo bits on the engine side and only for high frequencies (how you want all your heat emitted as energy).

[p1t1o]

36 minutes ago, wumpus said:

You hardly need a laser, an LED (or even some more efficient light) would work just as well (and the lower the wavelength the better).  In the end you have the same basic claim of the em drive (electricity in, momentum out) but with wildly lower efficiency.  Also, for any ultra-high Isp drive, don't be surprised if your heat sink produces significant thrust (possibly as much as your "main" thrust).  Maximize the albedo of the thing, except for low albedo bits on the engine side and only for high frequencies (how you want all your heat emitted as energy).

Advantage of laser is that 100% (or near enough) goes in the correct direction. Thrust is woeful though, I doubt it would ever be practical, mainly for the reason you point out - thermal inefficiency.

And we cant forget the Kzinti lesson...

Spoiler

a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive.

 

[wumpus]

3 minutes ago, p1t1o said:

Advantage of laser is that 100% (or near enough) goes in the correct direction. Thrust is woeful though, I doubt it would ever be practical, mainly for the reason you point out - thermal inefficiency.

And we cant forget the Kzinti lesson...

  Hide contents

a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive.

 

Spoiler

This only applies for missiles being fired with the reaction drive.  See the "lasers as space weapons" thread to see how badly lasers spread out (at least some of this has to do with the limits of optics).  Also note that an LED that has been focused such that light is emitted that only focuses light to within 9 degrees off center is still 99% efficient as a thruster: I doubt the laser can make that up on the LED (although you might pull off some sort of UV laser that a LED can't and get higher "momentum efficiency").

I must have missed that Kzin book, but this is a common trope by now.  But really it should only work for "point blank" range for direct damage of the energy output.

 

[p1t1o]

24 minutes ago, wumpus said:

  Hide contents

This only applies for missiles being fired with the reaction drive.  See the "lasers as space weapons" thread to see how badly lasers spread out (at least some of this has to do with the limits of optics).  Also note that an LED that has been focused such that light is emitted that only focuses light to within 9 degrees off center is still 99% efficient as a thruster: I doubt the laser can make that up on the LED (although you might pull off some sort of UV laser that a LED can't and get higher "momentum efficiency").

I must have missed that Kzin book, but this is a common trope by now.  But really it should only work for "point blank" range for direct damage of the energy output.

 

Consider putting a 300GW laser on your car and see if its attenuation at range makes anyone feel better

 

 

 

 

 

[kerbiloid]

Also lasercrafts need no fuel. So, they need only an external source of energy, with constant mass of the projectile.

Spoiler

Laser-propelled osmium penetrators with osmium-covered laser sails.

 

Edited June 7, 2018 by kerbiloid

[Mad Rocket Scientist]

On 5/28/2018 at 11:48 AM, winged said:

I mean the real VAB located in Florida

It is limited by door size. There are four, two each on side. The doors are 456 ft or 141.7 m tall, and for the lower 113 ft or 34.4 m is 149 ft or 45.4 m, and the remaining height is 76 ft or 23.2 m. I've seen references to the lower part of one of the doors being widened by 40 ft or 12.2 m for the space shuttle, and a cutout for the tail being added in the upper part, so it can stay closed while a shuttle goes through.

 

[Diche Bach]

I'm curious about the most promising propulsion technologies for the first 50 years of interstellar probing (whenever that might initiate). Stuff that is speculative is certainly interesting, but for my purposes, I am more interested in the actual feasible stuff.

If several billionaires (lets say Bezos, Persson, Gates and Buffet and Zuckerberg) all promised (in a legally binding contract) to provide up to 50% of their net worth to fund the contractor who could come up with a prototype 2000kg interstellar probe that could make it to say Alpha Centauri by say 2038 (50% c = ~8 year journey which would leave two years for proposal development, 10 years for prototype development, and ~8 years for journey): what would be the top 5 systems of propulsion that competitors for the contract would propose?

I say 2000kg cause that is the mass of the planned Mars 2022 orbiter and I figure, something in that ballpark might be sufficient to actually survive to orbit the Alpha system, make close observations and transmit back data?

 

Edited June 10, 2018 by Diche Bach
typo

[Diche Bach]

On 6/10/2018 at 3:42 PM, Diche Bach said:

I'm curious about the most promising propulsion technologies for the first 50 years of interstellar probing (whenever that might initiate). Stuff that is speculative is certainly interesting, but for my purposes, I am more interested in the actual feasible stuff.

If several billionaires (lets say Bezos, Persson, Gates and Buffet and Zuckerberg) all promised (in a legally binding contract) to provide up to 50% of their net worth to fund the contractor who could come up with a prototype 2000kg interstellar probe that could make it to say Alpha Centauri by say 2038 (50% c = ~8 year journey which would leave two years for proposal development, 10 years for prototype development, and ~8 years for journey): what would be the top 5 systems of propulsion that competitors for the contract would propose?

I say 2000kg cause that is the mass of the planned Mars 2022 orbiter and I figure, something in that ballpark might be sufficient to actually survive to orbit the Alpha system, make close observations and transmit back data?

 

*Crickets Chirping . . . for days . . .*

RIGHT! Nuclear pulse propulsion it is then! 

[kerbiloid]

2 hours ago, Diche Bach said:

RIGHT! Nuclear pulse propulsion it is then! 

OK, let's do it this way.
(Though, it's a little unclear can the nuke energy really be focused in a particular direction like in Orion project.
I've read somewhere that just 6% of its energy was focused in practical tests.)

P.S.
Also Orion was more about steam, oil, and steel pistons, than about rockets.
So, maybe Siemens ship would be the first, they can into trains and boilers.

Edited June 13, 2018 by kerbiloid

[p1t1o]

4 minutes ago, kerbiloid said:

OK, let's do it this way.
(Though, it's a little unclear can the nuke energy really be focused in a particular direction like in Orion project.
I've read somewhere that just 6% of its energy was focused in practical tests.)

You could Daedalus it if you're willing to spend a handful of decades in R&D.

[kerbiloid]

1 minute ago, p1t1o said:

You could Daedalus it if you're willing to spend a handful of decades in R&D.

Daedalus wants ~50000 t of deuterium and helium-3 mined in Jupiter.

A rather realistic project.

[p1t1o]

25 minutes ago, kerbiloid said:

A rather realistic project.

Did you notice the part about travelling to the nearest star at 0.5c?

Besides, you dont have to follow the design proposal to the letter.

Edited June 13, 2018 by p1t1o

[kerbiloid]

3 minutes ago, p1t1o said:

Did you notice the part about travelling to the nearest star at 0.5c?

Daedalus was by design 4 times slower, 0.12 c. Too weak, too slow.

[p1t1o]

18 minutes ago, kerbiloid said:

Daedalus was by design 4 times slower, 0.12 c. Too weak, too slow.

And the fastest versions of Orion were rated at 0.05-0.08c

[kerbiloid]

Just now, p1t1o said:

And the fastest versions of Orion were rated at 0.05-0.08c

But at least it didn't require Jupiter mining.

[p1t1o]

3 minutes ago, kerbiloid said:

But at least it didn't require Jupiter mining.

Meh, its just another challenge in getting around space quickly-ish.

Lets face it, the OPs specifications of travelling to the nearest star at 0.5c (also with implied stopping at said target) by 2038 (including travel time) is not possible.

All that is left is the currently impossible.

[kerbiloid]

14 minutes ago, p1t1o said:

(also with implied stopping at said target)

We can aerobrake.

Spoiler

2000*(0.5 * 3*10⁸)²/2/4.2*10¹⁵ = 5400 Mt of released energy.

It's nice to realize that somebody's 0.5 c probes are passing by the Earth from time to time, and we even don't know about them.

 

 

[p1t1o]

12 minutes ago, kerbiloid said:

We can aerobrake.

Now that would be something to see! 

Briefly, before your eyeballs melt.

[Diche Bach]

6 hours ago, p1t1o said:

And the fastest versions of Orion were rated at 0.05-0.08c

Are you sure about that!? I seem to recall stumbling onto some web page or another some months/years back and the value 0.1 c was definitely in print, possibly more.

 

6 hours ago, p1t1o said:

Meh, its just another challenge in getting around space quickly-ish.

Lets face it, the OPs specifications of travelling to the nearest star at 0.5c (also with implied stopping at said target) by 2038 (including travel time) is not possible.

All that is left is the currently impossible.

Well then how fast IS possible by 2038!? 

If by some marvel of philanthropy, a willing national space agency could liberate immense sums from the assets of every billionaire on Earth, how fast do current physics and tech suggest we humans could be propelling our hunks of metal through space by say 2038? 0.1 c? 0.237c? What?

[magnemoe]

8 hours ago, p1t1o said:

Did you notice the part about travelling to the nearest star at 0.5c?

Besides, you dont have to follow the design proposal to the letter.

.5c forget anything outside of antimatter and laser pumped sails and that is for an flyby. For slowing down you need something reactionless or better as in warp or other stuff who breaks physic more than an reactionless drive. 
Yes you might manage to do this with an probe with an dyson swarm, but that is not near future  
Its not about cost or simply scale up technology to an insane level. Easy stuff like building orbital cities or teraforming Mars. 
.1 to .2 will work well enough for near stars.