Nicias

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Posts posted by Nicias


  1. Part of the problem is that the inverse-square law for solar intensity is incorrectly modeled in KSP. It should be inverse square of the distance from the center of the sun, but it is inverse square from the surface. This doesn't make much of a difference away from the sun, but when you get close the intensity goes up way faster than it should, with the surface effectively having infinite temperature.


  2. I just wanted to share some off-label use I've been getting out of GT. I've been using it to implement constant vertical velocity takeoffs. (Only recommended for vacuum launches, also I've been using 10x rescale and SMURFF so I have DV costs that are about sqrt(10) times larger, but I have much better mass fractions.)

    The idea is to use just enough vertical thrust to keep from crashing into the terrain. In terms of GT settings, I set the starting velocity to 10m/s (or something small, but enough  to give the vehicle time to get truly vertical) time to AP I set to 20s for both start and end. I set sensitivity to 1. I set the desired target orbit. For the starting angle, I use the maximum angle that keeps a vertical TWR of 1. (so Cos(A) TWR = 1 or A = arcsec(TWR) )

    When I execute this the rocket goes up, turns to the angle I indicate, vertical velocity stays at about 10m/s (a little more due to the time to turn), and starts to build up horizontal speed. I think GT keeps the vehicle at the starting pitch until prograde drifts down to that pitch.Then it looks to see if the time to Ap (TTA) is under or over the desired amount. If it is over, GT follows prograde, if it is under, it pitches up to push the TTA up. This results in the craft pitching up and down (because of the PID controller I guess) around an ideal pitch angle of arccsc( Thrust/ (mass * (g - centrifugal force))) which decreases as horizontal velocity goes up and mass goes down. Eventually this angle decreases to below prograde and the rocket follows prograde to orbit. Prograde at this point is usually at most a degree or two above horizontal.

    The only way this fails is if there is higher terrain downrange. Then I just increase the time to AP setting enough to clear the terrain. Once past the terrain, I set it back down to 20s or so.

    This method is very reliable for me. It usually works on the first short and gets better DV to an actual gravity turn performed by GT. I've done some simulations in mathematica, (single stage) and get that a perfectly executed version of this (TTA fixed to 10s) slightly outperforms gravity turns for initial TWR of 1.5 to 3 on the Mun. (for example with an ISP of 345 and an initial TWR for 2 on the MUN simulation indicates 645 m/s to a 14km orbit, including circularization for a constant vertical velocity, vs 657 for best possible gravity turn)

    Just thought I would share.

     


  3. You will probably want food for the way back too. So you should pack 601 foods for the journey. You also need food for the dwell time at Duna. You can over-estimate and use the synodic period 1/(1/(Kerbin Year) -1/(Duna Year)) which gives 2.15 years (916 days). This is the time between transfer windows.  So you should pack about 1500 days of food. You could improve that guess by looking at when the next duna-kerbin departure window is after each kerbin-duna arrival window. I did that a while back and found that they are separated by 529 days. So you really only need 1129 days of food.  

    Here is the link where I work this out: 

     

     


  4. Hi, I recently had the idea to try and capture directly into low Tylo orbit from a Kerbin-Jool transfer. The thought was to add Tylo's Oberth to cut down on required dV

    What I did was make a node to set my perijool to be the same as Tylo's orbit while far away (thanks MechJeb)

    Then once I was in Jool's SOI, I made a node almost right away and put in some retrograde to get an intercept with Tylo. Fiddling with this node (adding normal and radial components) I was even able to get the correct peritylo. This cost about 50m/s. Then I coasted down into Tylo's SOI, and burned at peritylo to capture. It saved a TON of dV. I didn't get an accurate estimate because MechJeb's dv calculator had a bug in it for a couple of days. But I had a whole spare stage!

    So I decided to run the numbers, to see what the savings really was. Here are my calculations. I used vis-viva for everything.

    Jool orbital velocity:  4129 m/s. 

    Transfer orbit 2372 m/s. 

    Excess velocity at Jool: 1756s m/s.

    That velocity at the edge of Jool's SOI (2.46 E 9) with Jools mu (2.83 E 14) gives a SMA of -9.90 E 7 (hyperbolic)

    That SMA and Jools mu gives a velocity of 3332 m/s at Tylo's orbit (6.85 E 7). Tylo's orbital velocity is 2031m/s, so excess velocity relative to Tylo is 1301 m/s.

     

    1301 m/s at the edge of Tylo's SOI (1.09 E 7) with Tylo's mu (2.83 E 12) gives a SMA of -2.41 E 06 (hyperbolic)

     That SMA and Tylo's mu gives a velocity of 3141 m/s at 50km (target orbit radius 650,000 ). Circular orbit at that radius is 2049 m/s so the actual burn needs to be 1056 m/s. 

    The delta-v map gives 160+400+1100=1660.  So the direct injection method saves 36%

    That seems like way to good to be true. Is my math off?


  5. I think something is wrong with the dV calculator.

    If you just put a poodle, X200-32, and RC-001S  together, the dV calculator reports a dV of 11259 and a run time of 439.3 seconds. Both are almost exactly twice as large as they should be.

    If you just use a RC-001S, Nerv, and Mk1 LF fuesalge, MechJeb reports 3305 dv and a run time of 235.3 seconds. Both are about 90% of what they should be.

    This is with a bare 1.5.1 install with just MJ-dev (and MJ for all) installed.


  6. I find the length of the shroud on the 3.75m- 2.5m thrust plate to be too long. I was thinking of make a MM patch to clone the part and make a new "short" version, like there is for the 2.5m-1.25m thrust plate.  Is there any reason this wouldn't work? 

    Edit: I think I got it to work. This is the patch I used:

    //add a short version of the 3.5-2.5m thrust plate from SpaceY
    
    +PART[SYplate3m2mX1]:NEEDS[SpaceY-Lifters]:AFTER[SpaceY-Lifters]
    {
    @name = SYplate3m2mX1T
    
    @MODEL
    {
    	@scale = 1.0, 0.5, 1.0
    }
    
    @node_stack_bottom = 0.0, -0.1, 0.0, 0.0, -1.0, 0.0, 2
    @node_stack_bottom1 = 0.0, -2.5, 0.0, 0.0, -1.0, 0.0, 3
    @node_stack_top = 0.0, 0.1, 0.0, 0.0, 1.0, 0.0, 3
    
    @title = SpaceY A3-2 Interstage Adapter (Short)
    @description = (3.75m to 2.5m) Customize your engine clusters, in either an upper-stage (with fairing), or lower-stage configuration - Short. Suitable for Poodle, Mainsail, and Skipper.
    
    @mass = 0.25
    }
    

     


  7. 5 hours ago, dlrk said:

    Is there a way to have MJ simultaneously circularize and change inclination together?

    Sorta.

    You can combine two maneuvers if one of them can be made to happen "at a specific time". So circularization works for that. So, set up the plane inclination how you want. Then use the maneuver planner to create a new maneuver to circularize at a fixed time of t=0 seconds after the last node. Then you go to the maneuver editor, and it should have "merge with next node" button. Click that and it will combine the two nodes.


  8. 7 minutes ago, Nertea said:

    Okay, unfortunately this is probably lower priority than what I though beforet. The root cause is the same (contract system doesn't care about the actual loaded part, but only about the base version in the config) but it's only going to be hitting a much smaller set of contracts than I previously supposed, which makes it a bit lot lower on the to-address list.

    The good news is that I added some more station-building contracts so you will have some more to do at minimum next update I guess...

    Yeah, right now I just kinda ignore it.  If the contract says to add 15 seats, I add 15 seats.  The only downside is that I don't get the nice contact completion on hab inflation.  The cupola thing is more annoying.


  9. 8 minutes ago, Nertea said:

    Hang on, that's a little different than what I thought.

    I was thinking that the deployed state was never tracking for the contract correctly so "Build a station with X crew" was never completable. You're saying that for the purposes of station expansion contracts, the station simply doesn't count the deployed modules as deployed with appropriate crew capacity when considering the current station it's designing a contract. Is that correct?


    Yes, that is correct. Let's say that a station has a hitchhiker and a 8 person inflatable centrifuge (inflated).  I might get a contract that requests that I expand the station to support 9 kerbals (and indicates that it currently only supports 4). If I just switch to the station, the check box for "supports 9 kerbals" is checked. 

    I can double-check when I get home.

     


  10. 19 minutes ago, LatiMacciato said:

    @Nicias I suppose the extendables and inflatables are not suited to be crewed when packed/stowed/not extended or inflated. It makes perfect sense they have crew capacity once they're extended/inflated/unpacked.

    I agree. I just wanted the stock contract system to count them as having the crew capacity for the purpose of calculating the current capacity of a station for station expansion contracts. I suppose this was because that system was seeing the "CrewCapacity" variable and not the "DeployedCrewCapacity" variable. So I suggested switching the paradigm. 


    Presumably, the parts report their current capacity as CrewCapacity when retracted and DeployedCrewCapacity when deployed. The contract system only sees the CrewCapacity when making new contracts (It honors "Deployed CrewCapacity" when checking active contracts. It's actually neat watching the contract go green when you deploy the hab.)

    I was suggesting switching the behavior. So that the parts report their current capacity as "RetractedCrewCapacity = 0" when retracted and "CrewCapacity = 8" when deployed.

    Nertea said this was impossible for other reasons.


  11. 2 hours ago, Nertea said:

    The second thing is a good catch. The first one might not be fixable. 

    I don't understand how the inflatible/extendable parts are coded, but I see that they have in the cfg's, for example:

    CrewCapacity = 0
    ...
    DeployedCrewCapacity = 8

    Might it be possible to set them up to have:

    CrewCapacity = 8
    ...
    RetractedCrewCapacity = 0

    Of course, that would require recoding. It also might break other things and I have no idea how difficult it would be.


  12. I do basically this, but I also have a small module attached to the front of the docking port with a probe core, rcs tank, and rcs thrusters.  A couple of meters out I uncouple this module and quickly thrust it out of the way.


  13. On 4/25/2018 at 12:52 PM, Corona688 said:

    Either lash on your solids with a strut at the top, or enable autostrutting in the right click menu.  And make sure the decoupler grabs them in the middle, so they're shoved straight away | not pushed on-end / .  You can also use seprotrons to get them away from the main rocket more quickly.  And always point prograde when discarding empty-solids!

    I like to fire as many stages simultaneously as I can.  Crossfeed is enabled in decouplers and fuel priority is set so all fuel comes from bottom-most stages until depleted.  This reduces the amount of dead weight you carry in the form of noncontributing engines and empty fuel tanks.  They call this kind of asparagus twisted-candle, because it actually looks like asparagus.

    the-refresher.jpg

    Tricky part is, engines won't die when they should, you have to watch tank levels.  I wish they'd let you set a minimum fuel priority for engines.

    If you turn off crossed on the decouplers and use fuel ducts going up then you might get the behavior your want.


  14. I don't know how helpful this is, but I realized that for many trips, most of the time is coasting. So if life support supplies (and their containers) are heavy, you want to stage them. Ideally you are never burning with any empty containers.

    So I have three LS container stages. The first one gets dumped before I burn for capture. The second gets dumped before I burn to go home, the last gets dumped before capture at home. I typically also have a couple of weeks on LS in on the Kerbin capture stage, in case I need to do a two-pass capture. This is using Snacks!, which mostly just adds a mass penalty for LS.

     


  15. No, he's @JP_Magoo is correct.

    Surface gravity is proportional to M/R^2. Mass is proportional is d*R^3. So, surface gravity is proportional to  d*R. If you divide R by 10, you have to multiply d by 10 to keep the same surface gravity. 

    My favorite armchair astronomy fact is related to this. Tidal forces are proportional to M/D^3 (D is distance to body.) M is proportional to d*R^3, so  tidal forces are proportional to d * (R/D)^3.  Now R/D is (to good approximation) the angle the object makes in the sky. So since the moon and the sun have the same size in the sky, (to good approximation) , the ratio of the tidal forces from each is the same as the ratio of their densities. You can measure the tides to determine that the sun's impact is about half the moon's. So the sun is half as dense as the moon.