Delta-V questions

[Spricigo]

6 hours ago, Spricigo said:

 ..  Johannesburg Kerman.. 

I'm not editing it in the original post to share the love to autocorretor. How not love it. </irony>   JOHANNES  

[OhioBob]

20 hours ago, Kerbal101 said:

About deltaV map, @Spricigo, have you asked yourself where kerbals would get it themselves?   For example, they could do the gravity readings on planets using remove scanning (the planet info tool inside DSN) - which allows to calculate TWR manually: for example, if kerbals know that Kerbin has 1.0G and Mun ~0.2G - they can say that a rocket standing on the ground of Kerbin with 1 TWR - would have 5 TWR on Mun (since gravity is 5 times weaker). But how would they get their hands on deltaV map?.. Kraken? Kerbal god?  

They would have computed the delta-v map themselves just like the real-life guy who made the map.  You don't have to go somewhere to learn a great deal about it.  Observations from Kerbin would tell Kerbals all they need to know about the size of the solar system (assuming they have the equivalent of Newtonian physics).  Humans new the orbits, sizes and masses of the planets before we ever left earth, so I assume Kerbals would also know.  Granted, the precision of our measurements got better as we explorer and gained more information, but a first draft delta-v map is possible even with crude information.

I derived the numbers for GPP's the delta-V map, and did almost the entire thing computationally using just a few known facts about the bodies that Kerbals almost certainly could have learned for themselves through Kerbin-based observations.  The only thing I didn't do computationally was the ascent delta-v for bodies having atmospheres.  That is something that would likely take some close up investigation to figure out, but everything else could be computed without ever leaving the ground. 

[Spaceman17]

my opinion is that you just make the rocket the most similar possible to SLS block 2, then you decide what to make with this

[Kerbal101]

@OhioBob Yes, but such abstract deltaV map would be imprecise and risky to use, so this methods does not work. Historically, flight to anywhere was progressive. For example USSR and USA both managed to get into Lunar orbit with the probes after progressive launches and both had difficulties calculating the required deltaV for human landing. Just the way I do it and get reliable data for heavier, more impactive on reputation vehicles.

The ascend deltaV requirement is calculated by "before" minus "after" from "total deltaV" method, provided that each stage has enough TWR corresponding to the altitude, depending on planet gravity and atmospheric thickness. Launching one probe, ... or volunteer with parachute , suffices to get the value and round it up slightly.

[OhioBob]

3 hours ago, Kerbal101 said:

@OhioBob Yes, but such abstract deltaV map would be imprecise and risky to use... 

ALL delta-v maps are abstract, imprecise and risky to use.  I don't know if you've ever made one, but I really struggled with the one I made for GPP.  Not because I was incapable of the computations, but because there is so much variability.  I had trouble deciding what number, out of myriad possibilities, to put on the map.  Eventually I had to say that I was going to base the map on some particular set of circumstances and then compute the numbers accordingly.  So that means there is an extremely high probability that the your particular set of circumstances are going to be different than those upon which the map is based.  At best it just provides a rough idea of what it should take.  That's why I don't like delta-v maps and rarely use them.  I take every one of them all with a grain of salt.  I think pre-space age Kerbals would be able to produce a delta-v map that is nearly as good as any other because they are all pretty crappy, IMHO.
 

Edited July 28, 2017 by OhioBob

[OhioBob]

25 minutes ago, Kerbal101 said:

The ascend deltaV requirement is calculated by "before" minus "after" from "total deltaV" method, provided that each stage has enough TWR corresponding to the altitude, depending on planet gravity and atmospheric thickness. Launching one probe, ... or volunteer with parachute , suffices to get the value and round it up slightly.

The ascent dV for airless bodies I was able to determine using computer simulations.  I could have also done launch simulations for the bodies with atmospheres, but that gets a lot more complicated and I didn't want to do it that way.  Instead I just did series of test launches and measured the dV directly, i.e. before - after.  The one thing that really simplified this process is that I edited the config of the home world to give it the size, gravity and atmosphere of the body for which I wanted to determine the launch delta-v.  This way I could easily perform all my test launches from the KSC launch pad without having to transport my rocket to the actual planet.

[OhioBob]

3 hours ago, Kerbal101 said:

@OhioBob Yes, but such abstract deltaV map would be imprecise and risky to use, so this methods does not work. Historically, flight to anywhere was progressive. For example USSR and USA both managed to get into Lunar orbit with the probes after progressive launches and both had difficulties calculating the required deltaV for human landing. Just the way I do it and get reliable data for heavier, more impactive on reputation vehicles.

Your comments have gotten me thinking about something.  In GPP we made the delta-v map part of the KSPedia.  Because of its size, it's broken down into several separate pages.  I wonder if there is a way to reveal portions of it as a player works his way through the tech tree.  Perhaps better yet would be to reveal parts of it as certain goals are achieved .  I know very little about the KSPedia, so I don't know if such a thing is possible*.  If it is possible, however, it might be a good way to deal with the issues you've raised.

* I only did the math for the dV map, others get credit for the artwork and KSPedia.

[Kerbal101]

@OhioBob Oh, thats great to hear! I think the poll in GPP discussion thread is a good to find if players need it.
.. but in the end, you can't really "hide" the planets - can you? And everyone can just plan a maneuver to get the values just when level 2 DSN is unlocked.

[Spricigo]

3 hours ago, Spaceman17 said:

my opinion is that you just make the rocket the most similar possible to SLS block 2, then you decide what to make with this

So, a 'medium sized' KSP rocket?

3 hours ago, Kerbal101 said:

@OhioBob Yes, but such abstract deltaV map would be imprecise and risky to use, so this methods does not work. Historically, flight to anywhere was progressive. For example USSR and USA both managed to get into Lunar orbit with the probes after progressive launches and both had difficulties calculating the required deltaV for human landing. Just the way I do it and get reliable data for heavier, more impactive on reputation vehicles.

The difficulties for the moon landing were not about calculating the required deltaV required. When Apollo 11 touched down there was enough fuel to return to orbit form moon's surface that were a know fact, the astronauts even knew how close to being stranded they were (for the records 13s). Yes NASA sent unmanned vessel before, but only one liftoff from the moon, and a mere 4m. So Apollo 11 was not only the first manned landing on the moon but the first return to orbit from moon's surface, a unmanned craft was not necessary to prove it possible or determine how much deltav (or fuel) was needed.

What surveyor program actually was needed for is to know more about lunar soil. You don’t want to send astronauts 400.000km away just to let then glued in a ground made of cheese (well, we had a pretty good idea the ground was not made of cheese but if the moon dust was to deep that would give the term "soft-landing" a very unpleasant meaning for astronauts touching down).

For the most part century old mathematics is enough to send spacecraft  in pretty complex trajectories with not much trouble . If there is trouble, more likely someone used the wrong units .

 

[Spricigo]

2 hours ago, Kerbal101 said:

And everyone can just plan a maneuver to get the values just when level 2 DSN is unlocked.

Not even lvl2 DSN is necessary. We can calculate the things as long as we know a few bits of data (standard gravitational parameter, orbital parameters of the celestial bodies in question).

1. Launching from our Home Planet will have to deal gravity losses , drag losses and engine efficiency variation with pressure. For that one probably launching a Vessel and see how much fuel it takes is the way to go.

2.From there we start to use the vis-viva equation, but we don't need much else. We have Vessel in orbit, given it orbital height, orbital velocity and semi-major axis we determine Home's Standard Gravitational Parameter

3.For the transfer to a moon Around Home we calculate how much more velocity we need at the same orbital heigh to make our semi-major axis big enough so our orbit intercept Around's orbit.

4.Vessel's velocity relative to Around is the difference between Vessel's velocity relative to Home and Around's Velocity relative to Home. Mass of Around can be determined by Newton Kerman's Law of Universal Gravitation (I suppose the value of G was already measured by Cavendish Kerman, so we can have Around's Standard Gravitional Parameter also)

5....

Our values with this approach will be somewhat off with our assumption of instantaneous change in velocity but already usable for a rouge idea of how much it takes to travel between the celestial bodies, including landing a liftoff from airless bodies. We can increase the rigor of the mathematics involved (maybe @OhioBob can say something about that since he actually did computations for a deltaV map {while I did only educated, or so I hope, guesses}) but for a first approach the numbers are good(as in an extra 20% is enough safety margin). Celestial bodies with atmosphere brings a lot more to consider and more data is required (you can't just see how much atmospheric pressure it have with a telescope), that is where, instead of just calculate, starting our surveyor program may be a better approach.

 

[OhioBob]

58 minutes ago, Spricigo said:

The difficulties for the moon landing were not about calculating the required deltaV required. 

Yep, calculating delta-v is really not very difficult.  If the orbits and gravitational parameters of the bodies are known, then it is pretty straightforward.  And the orbits and gravitational parameters can be determined by observing the movements of the bodies, measuring distances and periods, and applying some math.  There may be some minor bodies that are difficult to observe for which good information is hard to come by, but for the moon and the major planets, we knew a great deal before ever going to space.

[Spaceman17]

anyone knows a mod for auto calculating the delta v and perform it?

[Foxster]

3 minutes ago, Spaceman17 said:

anyone knows a mod for auto calculating the delta v and perform it?

This is pretty much what MechJeb does. 

You choose the maneuver required, like circularize at Ap, and MechJeb will create a node to perform this, with the required dV for the burn being shown.

[Spaceman17]

what about delta -v +?

[Foxster]

Uh?