"Direct" burn to planet?

[Carl]

1 hour ago, Nathair said:

I think if you do the math it turns out that that "modest fraction of C" is pretty darn large for "nuclear bombs" to happen. Grain of sand (which is actually pretty big) weighs, say, 50 micrograms. At .2c and applying 1/2mv^2 we get (assuming my math isn't completely wrong) a "nuclear bomb" of 0.002kT. It's a boom and you definitely want to have some plan for dealing with it but it's not a "nuclear bombs" boom.

But check my math, seriously.

I'm going of talk of the difficulties in the final chapter of colonies in space from memory, that chapter is here:

 

http://www.nss.org/settlement/ColoniesInSpace/colonies_chap16.html

[Nathair]

1 hour ago, Carl said:

I'm going of talk of the difficulties in the final chapter of colonies in space from memory, that chapter is here:

 

http://www.nss.org/settlement/ColoniesInSpace/colonies_chap16.html

1

"but when passing through a star system, the spacecraft may encounter particles the size of a grain of sand. These will strike with the energy of a ton of TNT. "

So there ya go, my math said it would hit with the energy of about two tons of TNT.  (Yay, I'm not utterly innumerate!)

[Carl]

I'm guessing i got it confused with somthing else where they talk about meteorites or somthing :p. Big book, totally worth reading cover to cover.

 

[SpacedInvader]

4 hours ago, Nathair said:

I think if you do the math it turns out that that "modest fraction of C" is pretty darn large for "nuclear bombs" to happen. Grain of sand (which is actually pretty big) weighs, say, 50 micrograms. At .2c and applying 1/2mv^2 we get (assuming my math isn't completely wrong) a "nuclear bomb" of 0.002kT. It's a boom and you definitely want to have some plan for dealing with it but it's not a "nuclear bombs" boom.

But check my math, seriously.

Considering the situation, I think you just made the argument for using the scoop (which I'm assuming is a field of charged particles of some sort) rather than traditional shielding. Even if its only the equivalent of 2 tons of TNT, you are at the very least billions of km from the nearest help and more likely light years away in a tin can probably built by the lowest bidder (profit motive isn't going away any time soon)... would you rather have a field based deflection system or armor that you have to constantly patch?

Edited January 25, 2017 by SpacedInvader

[Carl]

The field will handle cosmic rays, somthing like a micrometeorite would be a bit beyond that even if it would be affected, some won't. Star Trek actually has a proper answer to this in the Navigational deflector and on paper the basic idea, (a field to deflect charged particles and a combination of sensors and the dish's particle beam functions to zap the micrometeorites), is sound. But we don't have the sensors to really make it work. Despite what Atomic Rockets likes to claim, (amongst others), our sensor tech is not that advanced.

[Nathair]

5 hours ago, SpacedInvader said:

Considering the situation, I think you just made the argument for using the scoop (which I'm assuming is a field of charged particles of some sort) rather than traditional shielding. Even if its only the equivalent of 2 tons of TNT, you are at the very least billions of km from the nearest help and more likely light years away in a tin can probably built by the lowest bidder (profit motive isn't going away any time soon)... would you rather have a field based deflection system or armor that you have to constantly patch?

 

The scoop is actually a magnetic field which, as has been pointed out, generates drag. This necessitates running the engine as long as the field is operating to provide enough thrust to (at least) compensate for the drag. So we end up with an extremely powerful and sophisticated field generator and an engine both operating constantly for forty years. That is a high energy maintenance nightmare. A slab of plates flying along in front of the ship like a shield seems like a dependable (lower maintenance) option. Obviously this is a very dangerous aspect of the trip and there is just no way around that.

[cantab]

On 22/01/2017 at 5:21 PM, razark said:

Can someone clarify why these plans are always "thrust 1g until halfway there, then turn around and thrust 1g to slow down"?

Why is it never "thrust 1.5g halfway..." or "thrust 1g more than halfway, then turn and decelerate at 2g"?

Is there a benefit to a symmetrical acceleration/deceleration, and why limit it to 1g instead of slightly higher accelerations?

If you have engines capable of 2 g and the crew can cope with it after all, why would you run your engines at less than full thrust? Just go 2g, coast if needed, 2g, it'll be a quicker trip. The only reason I can think of to plan on doing asymmetric thrust is if the engine relies on solar or beamed power and that gives less power at one end of the trip, but I'm not sure if such an engine ould even do a 1+g brachistochrone trajectory.

(Of course you could do 1 g as standard but then halfway along you get told to hurry up so you increase the thrust if the engine can do it, but that wouldn't be planned from the start.)

There is the factor that we know basically nothing about human health in gravity other than 1 g or 0 g. A study involving a week at 1.5g is the most I've heard of and I don't know the results.

Anyway, as far as doing it in KSP goes, well of course you can use the infinite fuel cheat. Otherwise, you might be able to do a brachistochrone to the Mun at something like 0.1 g (enough to be appreciable to the crew) with stock parts, but you'll need mods to go further.

Atomic Rockets has a great page on mission profiles, relating things like travel time and thrust to delta-V needs: http://www.projectrho.com/public_html/rocket/appmissiontable.php

A 1g Earth-Mars-Earth round trip would take just 4 days at best  It also has a chart showing what reasonably-plausible engine types could manage it, and for that trip lists nuclear pulse, a couple of fusion drives, and antimatter was capable of it.

[Kosmonaut]

Tintin did it in 1954...

[DrunkenKerbalnaut]

9 hours ago, cantab said:

 A study involving a week at 1.5g is the most I've heard of and I don't know the results.

Were I to pray to google for details on this study, which key phrases might I invoke?

 

[SpacedInvader]

On 1/25/2017 at 6:35 AM, Nathair said:

The scoop is actually a magnetic field which, as has been pointed out, generates drag. This necessitates running the engine as long as the field is operating to provide enough thrust to (at least) compensate for the drag. So we end up with an extremely powerful and sophisticated field generator and an engine both operating constantly for forty years. That is a high energy maintenance nightmare. A slab of plates flying along in front of the ship like a shield seems like a dependable (lower maintenance) option. Obviously this is a very dangerous aspect of the trip and there is just no way around that.

I would say this is probably more of an issue than the form of propulsion used because in the end it almost doesn't matter how long it takes to get somewhere or how much fuel you use getting there if you arrive with a ship that looks like swiss cheese. While I can accept that the scoop is not going to be a field capable of repelling micrometeors, I don't think armor is going to be a a sufficient solution to the problem, no matter how advanced, since you wouldn't be able to carry enough patches / replacement tiles to last the full duration of the trip and you couldn't make it thick enough to not need patches without essentially making a solid block, it would just deteriorate too much over the years.

 

On 1/25/2017 at 8:55 AM, cantab said:

If you have engines capable of 2 g and the crew can cope with it after all, why would you run your engines at less than full thrust? Just go 2g, coast if needed, 2g, it'll be a quicker trip. The only reason I can think of to plan on doing asymmetric thrust is if the engine relies on solar or beamed power and that gives less power at one end of the trip, but I'm not sure if such an engine ould even do a 1+g brachistochrone trajectory.

(Of course you could do 1 g as standard but then halfway along you get told to hurry up so you increase the thrust if the engine can do it, but that wouldn't be planned from the start.)

There is the factor that we know basically nothing about human health in gravity other than 1 g or 0 g. A study involving a week at 1.5g is the most I've heard of and I don't know the results.

Anyway, as far as doing it in KSP goes, well of course you can use the infinite fuel cheat. Otherwise, you might be able to do a brachistochrone to the Mun at something like 0.1 g (enough to be appreciable to the crew) with stock parts, but you'll need mods to go further.

Atomic Rockets has a great page on mission profiles, relating things like travel time and thrust to delta-V needs: http://www.projectrho.com/public_html/rocket/appmissiontable.php

A 1g Earth-Mars-Earth round trip would take just 4 days at best  It also has a chart showing what reasonably-plausible engine types could manage it, and for that trip lists nuclear pulse, a couple of fusion drives, and antimatter was capable of it.

The entire point of 1g is human comfort, not efficiency. We evolved in 1g so if we had ships capable of it, we should use it. 

[Nathair]

4 minutes ago, SpacedInvader said:

 you wouldn't be able to carry enough patches / replacement tiles to last the full duration of the trip and you couldn't make it thick enough to not need patches without essentially making a solid block, it would just deteriorate too much over the years.

1

It really depends on the density of the matter we'd need to plow through. Is interstellar space is as empty as it seems? There's one good way to find out!

[Carl]

14 minutes ago, SpacedInvader said:

I would say this is probably more of an issue than the form of propulsion used because in the end it almost doesn't matter how long it takes to get somewhere or how much fuel you use getting there if you arrive with a ship that looks like swiss cheese. While I can accept that the scoop is not going to be a field capable of repelling micrometeors, I don't think armor is going to be a a sufficient solution to the problem, no matter how advanced, since you wouldn't be able to carry enough patches / replacement tiles to last the full duration of the trip and you couldn't make it thick enough to not need patches without essentially making a solid block, it would just deteriorate too much over the years.

 

The link i provided notes that AFAWK interstellar space isn't strewn with very many of them, it's strictly for passage through the near star volume, which is a few days total.

[HebaruSan]

Scott addresses the fusion torch drive question; apparently the biggest part of the ship will be the radiators:

 

[regex]

20 minutes ago, HebaruSan said:

Scott addresses the fusion torch drive question; apparently the biggest part of the ship will be the radiators

This is one of my biggest problems with so-called "torchships". Given what I've read it's relatively easy (in a theoretical sense) to have either high thrust or high isp, but if you want both the requirements on the ship are literally astronomical. The NASA ships depicted towards the end of the video there aren't high-thrust craft but they do boast high exhaust velocities.

[Birdco_Space]

15 hours ago, regex said:

This is one of my biggest problems with so-called "torchships". Given what I've read it's relatively easy (in a theoretical sense) to have either high thrust or high isp, but if you want both the requirements on the ship are literally astronomical. The NASA ships depicted towards the end of the video there aren't high-thrust craft but they do boast high exhaust velocities.

Its all about energy and momentum. 

The energy of propellant is a function of the energy input, divided by the mass flow rate of the propellant, and this energy goes into bulk kinetic energy and thermal energy.  The bulk velocity of the exhaust (and in turn ISP) thus increases roughly with the square root of the increase of input energy (minus Thermal losses)

The overall change in velocity is a function of the fraction of the rockets mass thrown out the back and the velocity of that stuff.  The momentum change of a rocket increases linearly with the mass flow rate of the propellant, and with the square root of the kinetic energy of the propellant.  This correlation roughly equates to thrust as well (since force = change in momentum per unit time)

When a rockets propellant is reacted with (be it combusted, heated, or fused), the resultant energy in the rocket exhaust is nearly all thermal, requiring nozzle design to convert this thermal energy into bulk kinetic energy.  A perfect nozzle would convert all the gasses added thermal energy into bulk propellant kinetic energy. However, in reality, the more the gasses interact with the nozzle to covert thermal energy into kinetic energy, the more energy is lost to the rocket nozzle in the form of waste heat.

On chemical rockets, and fusion rockets, energy imparted into a specific propellant is constant per unit mass, thus thrust increases linearly with increase with increase in mass flow rate (more fuel flow = more thrust)

Changing propellants can change the energy density of the combustion (or fusion), and lighter combustion products have greater molecular velocity for a given thermal energy, which increases the theoretical maximum exhaust velocity.  However, waste heat generated increases roughly linearly with the energy density of the reaction imparting the energy to the propellant for a given nozzle design and mass flow rate.  Fusions massive energy per unit mass also comes with a lot of its own losses, such as photons, neutrons and neutrinos.  (Bigger bang = hotter engine)

To increase exhaust velocity for a fixed propellant and mass flow rate, one needs more nozzle, which means more of the propellants energy lost to waste heat of the rocket itself, and the exhaust velocity only increasing by the square root of the increase of the kinetic energy of the exhaust gas.  In general, this means the waste heat that needs to be dealt with increases as rocket efficiency increases.  (faster gas = hotter engine)

Thus for a given waste heat rejection capacity and propellant, you can have greater exhaust velocity, but at the expense of a lower mass flow rate.  (more ISP = lower thrust)

All these factors add together to make it very difficult to raise both thrust and impulse, because it means you have a lot more waste heat to deal with.

Logic on NERVA and Ion efficiency is a little different, because of the processes involved.  NERVA have a roughly fixed thermal input to propellant, thus more mass flow means lest thermal energy imparted per unit propellant, and thus less exhaust velocity and impulse.  Therefore, more thrust for a given core thermal output means less efficiency

Ion engines require both ionization energy and acceleration energy on the propellant gas.  Ionization energy is fixed for a given mass of propellant (an atomic property of the propellant), The acceleration energy added to the propellant is extremely efficiently transferred into propellant kinetic energy, with little loss to engine heat or propellant thermal energy.  A more energetic thruster can get the same mass flow rate moving somewhat faster (slightly more thrust, and improved ISP), or more mass flow at a significantly slower velocity (much more thrust, at the expense of ISP) because of the sunk cost in ionizing the gas, and the greater impact mass flow has on thrust compared to exhaust velocity.

 

TL;DR:  You cant have your cake an eat it too, unless you want to invest in a lot of radiators (which have mass, costing total delta V)   

Edited January 27, 2017 by Birdco_Space
more ISP = less thrust

[Archgeek]

On 1/25/2017 at 9:55 AM, cantab said:

Anyway, as far as doing it in KSP goes, well of course you can use the infinite fuel cheat. Otherwise, you might be able to do a brachistochrone to the Mun at something like 0.1 g (enough to be appreciable to the crew) with stock parts, but you'll need mods to go further.

About that.  I'm more than a bit certain you can do the same with Eve, and possibly Duna.  I've got a little project (working title: Gotta Eve Fast) involving hucking a probe at Eve via brachistochrone, using ions with panels spec'ed for expected insolation as the distance to the sun dwindles for after turn-around, and an as-yet undesigned nuke stage for the first half of the trip.  I think the design TWR is closer to .02, but that's not exactly the worst when working with such crazy mass ratios.  'Point is, I think it's pretty doable in stock.

[natsirt721]

Not sure if this has been mentioned already, but in the Niven/Pournelle book The Mote in God's Eye the Moties (aliens) sent a beam rider to a nearby star system.  It was mentioned that the humans were confused that the Moties didn't use the beam to decelerate the craft, instead using a solar sail or some other method to decelerate.  In order to use the same beam to decelerate, the ship would be sent out on one leg of the path, then charge the ship up to 10kV and use the Lorenz force from the galactic magnetic fields to approach the target star from behind.  Winchell's site has more on it, but it's such a vast expanse I'm not even sure where to begin looking for it,

If the first ships deployed are terrawatt lasers, then you can remotely establish 2-way travel between stars using only laser-launch systems.

[Zeiss Ikon]

On 2/1/2017 at 2:32 AM, natsirt721 said:

If the first ships deployed are terrawatt lasers, then you can remotely establish 2-way travel between stars using only laser-launch systems.

An image from one of Niven's (without Pournelle) Man-Kzin Wars collections is of asteroids mounting lasers in this class "darting in and out on the thrust of their main weapons".  The recoil of such a laser is not inconsiderable.  And you'll need a lot more than terawatt laser power to give a light sail with an actual payload enough acceleration to bother (especially at interstellar distance; even a coherent laser beam spreads over that kind of range).  As I recall, direct sunlight at Earth's orbit is good for something like .001 g on the thinnest practical aluminized mylar without any structure at all, never mind a payload massing several times what the sail does.  You would therefore need a laser with a beam density thousands of times that of sunlight to make a light sail into a practical drive that requires less than decades to get anywhere, even within a single system.

For an interstellar probe, that kind of long-term acceleration is the best we can manage with current technology -- but if you book passage on a ship with a light sail, you're sending your long-term progeny, not going yourself.

[natsirt721]

3 minutes ago, Zeiss Ikon said:

And you'll need a lot more than terawatt laser power to give a light sail with an actual payload enough acceleration to bother (especially at interstellar distance; even a coherent laser beam spreads over that kind of range).  As I recall, direct sunlight at Earth's orbit is good for something like .001 g on the thinnest practical aluminized mylar without any structure at all, never mind a payload massing several times what the sail does.

You would need many thousands of high-power lasers to properly move a lightship, agreed.  But the nice thing about starting from home is that you can always build more!  With a battery of tens of thousands of individual lasers you can cycle them on and off (to allow for course corrections, which as you point out would be necessary).  Put them close to the sun to utilize its free energy and it is feasible to engineer a vessel with a reasonable acceleration.

A 10 meter radius terrawatt laser would have an intensity of 1x10¹²W / (10m)²π = 3.18x10⁹ W/m².  This is 2 orders of magnitude greater than the solar surface intensity of 6.33×10⁷W/m².  Now imagine thousands of these pouring their stuff into a single sail, and the newtons add up pretty quick.  Lasers can also be focused to reduce inverse square loss, to a point. 

[Zeiss Ikon]

3 hours ago, natsirt721 said:

Lasers can also be focused to reduce inverse square loss, to a point. 

Yes, I recall the zone lens used to focus the braking beam in Dragon's Egg and it's sequels.  Is there enough gallium in the solar system to make that many gallium arsenide laser junctions?  Or enough tungsten for the non-linear frequency doubling crystals you'll need when the lens construction gets put off a couple centuries?

Give me a choice, I'd rather use a laser-triggered version of an Orion drive.  Give it a mass ratio of ten or so and it'll get between nearby star systems in under a century -- and with inertial confinement's small pulses, the ship needn't be the size of a small city.

[BoPET]

On 22.01.2017 at 6:59 AM, SpacedInvader said:

Over the years I've read in several science fiction franchises about a method of interplanetary travel in which a vehicle accelerates at 1g for half the transfer then flips and reverses the process, allowing for a sort of linear gravity simulation as well as an absolute minimal transfer time. First, I'd like to know what this maneuver is called and then I'd like to know if KSP is capable of simulating it. I am getting ready to start a new career after some time away and I'm debating installing KSP Interstellar which I believe has technology capable of giving enough dV to perform such a maneuver, but I would have no idea how to go about planning or executing it.

Thanks.

I didn't had time and patience to read the whole thread, sorry if I am saying the same thing twice.

Here you are describing the concept of torchships  and torch drives - this name was given to them by Heinlein back in the 50-s and here you gave almost perfect description of them - 1g acceleration for days or delta-v of 200.000 km/s or more.
About the trajectory - it seems that there is no official name for it, as several names exists, but I prefer to call this absolute shortest transfer trajectory a Brachistochrone.

If you are really interested in the whole math behind these concepts (torchships, brachistochrones, etc), I strongly recommend you to read this guy - http://www.projectrho.com/public_html/rocket/torchships.php  That's an almost ultimate analysis of torchship concept and its performance.

As for recreating this thing in KSP - there is little to no problem in planning such trajectories - just choose any from the bottom-red part of porkchop diagram of your transfer calculator and then go burn it all the way down. The problem is - you cannot accelerate in a time warp (well, actually you can with some drives in KSPI such as magnetic nozzle, they allow it, but the produce ridicule amount of thrust and don't qualify as torchdrives whatsoever) so it will take a lot of real time to travel. But if it is ok for you to leave KSP running in the background for a night - it should be quite possible.

Actually there may be other mods that lift this limitation but I am not aware of them.

You may also be interested in Torchdrives from USI Karbonite mod. Runs on extremely rare and expensive Karborundum, these engines really pack a punch with huge twr and delta-v. They are quite well balanced (you can mine Karborundum only on Eeloo, low above the Sun, on the surface of Eve or in the atmosphere on Jool or spend billions of credits on it) so that's not cheating.

[Nathair]

On 2017-02-04 at 3:43 PM, Zeiss Ikon said:

especially at interstellar distance; even a coherent laser beam spreads over that kind of range

That's why it gets refocused by a planetary diameter fresnel lens made up of myriad chunks of plastic floating out beyond the orbit of Mars. The beauty of it is that the lens keeps getting built bigger as the flight goes on... (stolen from Forward's Flight of the Dragonfly.)