0 Isp No thrust Engine?

[scuderiacab]

Simply, I am working on my physics thesis and I need to observe the effects of gradual change in mass of a rocket in orbit without any force being applied, namely thrust. For this I need fuel consumption, the rate of which I can modify but no thrust. The only thing that comes to my mind is using a 0 Isp engine but I don't know how to change the files. Do you have any suggestions other than a modified engine? If not, how do I change the Isp and thrust values of engines in the game? Is there a mod I can use?

[ARS]

Well, you can use any mod that provides rocket engine, open up the parts folder, search for engine's .cfg files and mess with it as you like, including the rate of fuel consumption

[Snark]

43 minutes ago, scuderiacab said:

Simply, I am working on my physics thesis and I need to observe the effects of gradual change in mass of a rocket in orbit without any force being applied

I'm kinda curious... what "effects" are you talking about?  In KSP, I can go ahead and tell you what the "effects" will be:  None.  Zero.  If you don't apply any force to the rocket, then the rocket's movement doesn't change.

So other than getting less massive, the answer to "what does a rocket do if you don't thrust" is going to be "it just sits there".

43 minutes ago, scuderiacab said:

For this I need fuel consumption, the rate of which I can modify but no thrust. The only thing that comes to my mind is using a 0 Isp engine but I don't know how to change the files. Do you have any suggestions other than a modified engine? If not, how do I change the Isp and thrust values of engines in the game? Is there a mod I can use?

If you want to modify a KSP engine to have zero Isp, the config you need to modify is atmosphereCurve.  For example, if you look at the config for the Swivel engine, you'll note that it contains the following section:

atmosphereCurve
{
    key = 0 320
    key = 1 270
    key = 6 0.001
}

...that's the part that says "Isp at 0 atmospheres is 320, Isp at 1 atmosphere is 270, Isp at 6 atmospheres is 0.001".

So, I assume you could change that to Isp 0 thus:

atmosphereCurve
{
    key = 0 0
    key = 1 0
}

...and that ought to do the trick.  (Haven't actually tested it myself, but I think it ought to work.)

Though, again, I can't imagine why you would want to do that, because I'm having trouble conceiving of any interesting "study" that could be done from this.  "Study" implies "discover something that we don't already know", and in this case, we do already know what will happen, which is "nothing".  It'll get less massive, and its CoM will shift if the fuel tanks aren't centered, but that's it.

[DoctorDavinci]

33 minutes ago, scuderiacab said:

Simply, I am working on my physics thesis and I need to observe the effects of gradual change in mass of a rocket in orbit without any force being applied, namely thrust. For this I need fuel consumption, the rate of which I can modify but no thrust. The only thing that comes to my mind is using a 0 Isp engine but I don't know how to change the files. Do you have any suggestions other than a modified engine? If not, how do I change the Isp and thrust values of engines in the game? Is there a mod I can use?

Easy solution without having to change anything in the game ........

Take a fuel tank that has lfo and lf, attach a bunch of the largest batterys to it and remove all the ec from them then attach a probe core and a large fuel cell or two 

Launch the vessel and use the alt f12 cheat menu to put the assembly into orbit and then do your thing ... start the fuel cells which will use lf and lfo to generate ec for the empty batteries you attached (ec is massless) thereby reducing the total vessel mass as ec is generated

One thing to note is that KSP does not simulate n-body physics so it may not work out as you intend in a stock game ... although there is a mod for that, I forget the name though (google n body physics ksp)

Good luck with your thesis 

[StrandedonEarth]

Another simple solution is to have two engines thrusting in opposite directions, so they cancel out...

[scuderiacab]

46 minutes ago, Snark said:

I'm kinda curious... what "effects" are you talking about?  In KSP, I can go ahead and tell you what the "effects" will be:  None.  Zero.  If you don't apply any force to the rocket, then the rocket's movement doesn't change.

So other than getting less massive, the answer to "what does a rocket do if you don't thrust" is going to be "it just sits there".

If you want to modify a KSP engine to have zero Isp, the config you need to modify is atmosphereCurve.  For example, if you look at the config for the Swivel engine, you'll note that it contains the following section:

atmosphereCurve
{
    key = 0 320
    key = 1 270
    key = 6 0.001
}

...that's the part that says "Isp at 0 atmospheres is 320, Isp at 1 atmosphere is 270, Isp at 6 atmospheres is 0.001".

So, I assume you could change that to Isp 0 thus:

atmosphereCurve
{
    key = 0 0
    key = 1 0
}

...and that ought to do the trick.  (Haven't actually tested it myself, but I think it ought to work.)

Though, again, I can't imagine why you would want to do that, because I'm having trouble conceiving of any interesting "study" that could be done from this.  "Study" implies "discover something that we don't already know", and in this case, we do already know what will happen, which is "nothing".  It'll get less massive, and its CoM will shift if the fuel tanks aren't centered, but that's it.

I am exploring the relation between liquid fuel engine's Isp and the magnitude of work it does in the time of the burn. I calculate the work not via the common equation which involves total distance covered due to the force (thrust), but by looking at the change in mechanical energy by calculating the gravitational potential and kinetic energies of the vessel before and after the burn. And the thing is, "something" happens. Because the mass changes the gravitational potential energy and kinetic energy are affected causing change in the total mechanical energy of the vessel, even changing the the orbit in the process (relatively insignificant but it does—30 meters increase in apoapsis of a uniform 2,000,000 meter altitude orbit). This is not the main part of the experiment anyway, I am trying to come up with a mathematical relation between work and Isp, which I did with the data I gathered from normal rockets. I need a 0 Isp rocket to be able to verify the y-intersect of my graph. I tried releasing mass by using docking ports but it obviously did not give the desired value because it is instantaneous and does not take the same amount of burn time as my other data, consequently doing more work, so I need a way of gradually decreasing mass. The relation I am working on may be useful in calculations or design procedures for orbital transfer missions around the same celestial body, like Hohmann transfer. I need this to know I am not drawing the graph wrong, that's all. Thank you very much, btw.

[steuben]

The mod is Principia.

You might get better results using it than stock KSP. Stock makes a fair number of approximations and shortcuts with the calculations. 

But, I too am curious about what result you're aiming for with your experiment. My high-school level physics half predicts that your orbit will get wider, since you have reduced the mass but kept your velocity the same. I think the effects might be most noticeable in the steeper sections of the gravity wells rather than flatter space.

You might want to use the chartam-calamum calculation methods to confirm any results you get. Though I've heard good things about the tabella-album approach as well.

As for the opposing engines... you might get into weird spaces of floating point errors. So hacking up an engine to zero thrust will be the better approach. Hacking up a battery to have a larger capacity and running a fuel cell, to completely remove the possibility of thrust.

 

oops ninja'd

 

Edited July 30, 2018 by steuben

[scuderiacab]

1 minute ago, scuderiacab said:

I am exploring the relation between liquid fuel engine's Isp and the magnitude of work it does in the time of the burn. I calculate the work not via the common equation which involves total distance covered due to the force (thrust), but by looking at the change in mechanical energy by calculating the gravitational potential and kinetic energies of the vessel before and after the burn. And the thing is, "something" happens. Because the mass changes the gravitational potential energy and kinetic energy are affected causing change in the total mechanical energy of the vessel, even changing the the orbit in the process (relatively insignificant but it does—30 meters increase in apoapsis of a uniform 2,000,000 meter altitude orbit). This is not the main part of the experiment anyway, I am trying to come up with a mathematical relation between work and Isp, which I did with the data I gathered from normal rockets. I need a 0 Isp rocket to be able to verify the y-intersect of my graph. I tried releasing mass by using docking ports but it obviously did not give the desired value because it is instantaneous and does not take the same amount of burn time as my other data, consequently doing more work, so I need a way of gradually decreasing mass. The relation I am working on may be useful in calculations or design procedures for orbital transfer missions around the same celestial body, like Hohmann transfer. I need this to know I am not drawing the graph wrong, that's all. Thank you very much, btw.

I explained it briefly to Snark and here it is. You are also correct about the orbit as I've mentioned it above. I will also take a look at the calculation methods you mentioned, I either don't know them or in my language they have a different name. Thank you a lot.

[steuben]

They are older methodologies, remaining dominant up until the mid-eighties,. They were supplanted a bit earlier in some fields. But used very extensively in the early space programs.

[Snark]

1 hour ago, steuben said:

My high-school level physics half predicts that your orbit will get wider, since you have reduced the mass but kept your velocity the same.

No, high school physics says that the orbit will stay exactly the same.  Because your orbit only depends on your velocity and position, not your mass.

IRL physics digression in spoiler section, and I'll shut up about this now because this is getting off-topic from the OP question, which was simply "how do I do this thing in KSP".

Spoiler

The only way to change a rocket's orbit is to change its velocity.  That's why we care about dV. 

"Investigating" what happens to the energy when mass is lost without thrust isn't going to give any interesting answers, because we already know exactly what happens, using only basic algebra.  Both kinetic and potential energy are directly proportional to mass.  If you have a 2-ton ship, and it loses half its mass (without thrusting) so that now it's a 1-ton ship... guess what?  Its total energy will now be exactly half what it was before.  Not because any work was done (there wasn't any), but simply because we stopped tracking the energy of half of the mass.

Here's a thought experiment for you:

You're orbiting the Sun, just wearing a space suit.  You're carrying a massive bowling ball.  You have a certain orbit.  You release the bowling ball.  What happens?

Well, that depends what you do with the bowling ball when you "release" it.  For example, suppose you give it a strong shove, and throw it away from you as hard as you can.  Well, that's action/reaction.  For example, if you throw the bowling ball in the direction relative to your orbit, then you're propelling the bowling ball backwards and propelling yourself forwards.  The result?  Your orbit will get slightly bigger than the original orbit (because you boosted a little bit), and the bowling ball's orbit will get a little smaller than the original orbit (because it boosted a little bit).

But suppose you don't do that.  Suppose you very carefully just "let go" of the bowling ball, without pushing it at all.  You don't give it any velocity at all, relative to yourself.  It's just floating there right next to you, at zero relative velocity.

Now what happens?

Answer:  Nothing.  Nada.  You will still have exactly the same orbit as before.  And so will the bowling ball.  The two of you will continue completely uninterrupted as you were doing before, floating side by side.  (Well, not quite, because you will mutually gravitate so that within a few hours it will come back and settle against you, but that's a microscopic effect.)

You're not doing any "work' by simply releasing (rather than pushing) the bowling ball, and you're not changing your energy per kilogram.  To take an analogy:  You've done the equivalent of taking a gallon jug of water and pouring it into two half-gallon containers:  you haven't changed how much water there is just by dividing it into two pieces.

Anyway, this is all way off topic from the original question of "how do I set the Isp of an engine to zero", so I'll stop here.  If someone wants to post a topic in Science & Spaceflight to talk about the physics and energy of rockets, I'm happy to go on at length. 

 

1 hour ago, scuderiacab said:

I tried releasing mass by using docking ports but it obviously did not give the desired value because it is instantaneous and does not take the same amount of burn time as my other data, consequently doing more work, so I need a way of gradually decreasing mass.

You won't get anything interesting out of this.  The orbit will not be affected, at all.  KSP models physics using a simple Newtonian model with planets treated as immobile point masses, and a ship's orbit won't change at all unless you change its velocity.  Doesn't matter whether it's instantaneous or gradual.  There will be zero effect on the orbit.  If you want to go ahead and verify this by running the "experiment" in KSP, then by all means go ahead, if that makes you happy.     But it won't change the orbit... and in any case, you won't be verifying anything "physical" per se, you'll just be verifying the way KSP is coded.

[scuderiacab]

23 minutes ago, Snark said:

KSP models physics using a simple Newtonian model with planets treated as immobile point masses, and a ship's orbit won't change at all unless you change its velocity.  Doesn't matter whether it's instantaneous or gradual.  There will be zero effect on the orbit.  If you want to go ahead and verify this by running the "experiment" in KSP, then by all means go ahead, if that makes you happy.     But it won't change the orbit... and in any case, you won't be verifying anything "physical" per se, you'll just be verifying the way KSP is coded.

Again, any change in orbit does not mean anything even if it happens because I am observing the mechanical energy. Orbit doesn't change, it's good, it does, still good. I can just add what you said in my thesis and it would be correct but I am obliged to provide data either from real or simulated experiments. I can't just reach orbit, so... yeah. Also, again, this is not the actual focus of my thesis, just trying to see if data is consistent with what mathematics say.

[Snark]

1 minute ago, scuderiacab said:

Again, any change in orbit does not mean anything even if it happens because I am observing the mechanical energy. Orbit doesn't change, it's good, it does, still good. I can just add what you said in my thesis and it would be correct but I am obliged to provide data either from real or simulated experiments. I can't just reach orbit, so... yeah. Also, again, this is not the actual focus of my thesis, just trying to see if data is consistent with what mathematics say.

Fair enough, and if you want a software experiment to demonstrate that "multiplying by a number half as big will give an answer half as big", by all means.    Apologies for the digression,  I'm sure you know your course requirements best.  Good luck with your project! 

[MechBFP]

Let us know your results, I am curious. 

[scuderiacab]

Just now, MechBFP said:

Let us know your results, I am curious. 

Right away.

As Snark pointed out and mathematically expected, the only change in mechanical energy was due to the loss of mass. The orbit was modified but the change was so insignificant that I rule it out as random error, probably caused by a difference in engine's firing times. This meant that my relation graph, which seemed to be a straight line, had a false y-intercept, a.k.a. inaccurate work in joules to 0 Isp value. I then realized that after reaching the escape velocity, no matter how much the vessel exceeded this speed, the final mechanical energy was going to be same, 0. That's because at infinity both kinetic and gravitational potential energies are nullified. The graph of Isp (x) vs. Work (y) resembles a Gaussian graph, but a little different.

Now I will try to pinpoint the factors affecting the graph to come up with a general equation. I assume the total mass, mass flow rate and propellant mass have an effect on the graph, but we will see. They affect mainly through time I suspect, as more burn duration means more impulse delivered.

[Pand5461]

45 minutes ago, scuderiacab said:

I then realized that after reaching the escape velocity, no matter how much the vessel exceeded this speed, the final mechanical energy was going to be same, 0. That's because at infinity both kinetic and gravitational potential energies are nullified.

That's incorrect. Past the escape velocity, the object has nonzero velocity at infinity (called the hyperbolic excess velocity), hence nonzero kinetic energy. And if you add some speed at periapsis, that hyperbolic excess velocity increases as well.

Another thing that bothers me, how do you justify the calculation of work from the change in the total mechanical energy of a ship when the ship at the end is not the same as the ship you started with (because some fuel was burned)?

Edited July 30, 2018 by Pand5461

[Gargamel]

14 hours ago, Snark said:

  KSP models physics using a simple Newtonian model with planets treated as immobile point masses, and a ship's orbit won't change at all unless you change its velocity.

The fact that we have kraken drives in the game may prove using KSP as a data source moot.  If we can create acceleration from nothing, then that negates any findings from any zero thrust experiments in the game for real world applications.   Scott Manley did a video a while back where he had two orange tanks out on long booms of struts.   He set the structure spinning slowly, and by shifting the fuel back and forth between the tanks, he could move the COM up in altitude, and created a 'spinning ladder'.  I forget the details, but since KSP uses CoM point physics, it's just another example of how not useful KSP is for this experiment. 

[theJesuit]

Rather than an engine use a fuel cell.

Peace.

[p1t1o]

The mod "Ship Manifest" allows you to dump stored resources without thrust, and there are a few other mods which add a fuel dump valve part (essentially exactly what you are asking for 0Isp, 0thrust engine) I think one is called "SmartParts".

 I have so many mods installed I am no longer entirely sure what things come from what or even what is stock anymore...

[scuderiacab]

16 hours ago, Pand5461 said:

That's incorrect. Past the escape velocity, the object has nonzero velocity at infinity (called the hyperbolic excess velocity), hence nonzero kinetic energy. And if you add some speed at periapsis, that hyperbolic excess velocity increases as well.

Another thing that bothers me, how do you justify the calculation of work from the change in the total mechanical energy of a ship when the ship at the end is not the same as the ship you started with (because some fuel was burned)?

You basically saved me from a slap from my physics professor with hyperbolic excess velocity . About the question, I am rather comparing the change of energy induced by the different engines with different Isp values using the same mass of fuel. Technically all the initial and final masses are the same, allowing me to draw a comparison between rockets. View it as intentional systematic error, if you will.

[Blaarkies]

If you just need data points for 0thrust/0isp, just use the vis-viva equation to find your velocity at different altitudes. This will yield the correct results, since burning with a 0isp engine, or not burning, or not having the engine at all will give these results. 

If you need the change in mass from burning the 0isp engine, it is better to factor that alongside the equation to find your [velocity, mass] at any point in your orbit. You can make your graph as fine grained and accurate as you want with this

[Pand5461]

3 hours ago, scuderiacab said:

You basically saved me from a slap from my physics professor with hyperbolic excess velocity . About the question, I am rather comparing the change of energy induced by the different engines with different Isp values using the same mass of fuel. Technically all the initial and final masses are the same, allowing me to draw a comparison between rockets. View it as intentional systematic error, if you will.

OK, but be prepared: said change of energy depends on the initial orbital state (that's what Oberth effect is about). So, in order to make a comparison that's at least sane, the initial orbital states must be all the same as well.

[scuderiacab]

1 hour ago, Pand5461 said:

OK, but be prepared: said change of energy depends on the initial orbital state (that's what Oberth effect is about). So, in order to make a comparison that's at least sane, the initial orbital states must be all the same as well.

Thanks to the wonderful artpiece called Hyperedit, all the initial orbits are perfect circular orbits with the identical orbital radii.

[Gargamel]

You're really not intending on applying data gathered from KSP to real word physics are you?