How to use the Oberth to do wierd things.

[PB666]

Everyone remembers the

https://www.xkcd.com/1244/

But just what can you do in Kerbal space program?

So I have a ship in LKO with 30,200 dV of fuel, and I could pitch out 1500 dV leaving 28700 fuel and burn at the angle to prograde around 180 and then arriving in front of kerbin burn on the 10000 m/s cricumkerbol velocity for a total of 38700 plus whatever I had when I left kerbin.

Jeb however left me with one astronaut, bob, needs or two planets and a deep space mission. I could plan my mission to fly by duna on the way out, and try to get a fly-by of Ike, but Jeb claims that there are secret resources on Moho. As everyone know Moho is a dV hog.

So lets do some Oberth. First I head to Duna, approach is from kerbol side Remaining dV is 27400,

As I pass Duna I use a reversal of the oberth affect around Duna to slow me down relative to Kerbol
This also gives me a chance to change the course so that I get a close flyby of Ike, Ike has no atmosphere to speak, I can get very close and use the oberth effect to create an ellipse that crosses but does not intercept moho's orbit.
By this time the ship is on the space side of Duna just beyond the pe, heading to Ike. Two close flybys done, now for the third, its a ball-buster.
Once I get to Ike once again I use the oberth effect to create an intercept course with moho - dV is now 25900. Unfortunately the plane change points are close to latus rectum points, so I can't burn normal/antinormal for the 7' I need

I then make my plane changes and im down to about 24900 dV
I am not lucky enough to intercept Moho at periapsis, but once I reach the Kerbol periapsis, I can add just a few prograde dV (which I will get back a later as we will see) to create an orbit that intercepts Moho.
Moho does not have an atmosphere and turns relatively slowly, so you can get close enough to fry dust bunnies.
At Moho intercept about 5.5 GM im traveling at 18 km/sec versus Mohos 13.5, dV remaining is 24,812.
After entering Moho's SOI I burn just about as much fuel as I can and by the time I exit my circumsolar velocity is now 26,900 m/s and I still have >15,000 dV left
Since I am basically traveling perpindicular to Kerbol I can burn out the rest of my fuel, the final velocity is 42400.

Now you might think aha but by the time you get back to kerbin orbit you will have lost alot of velocity. Actually because I accelerated so much in the moho flyby, getting back to Kerbin only robs me of 1800 dV so that by the time I reach Kerbol Im going 40000 meter/sec and by the time I leave the system (200GM) im going 39,500 m/s.

Logic should have it that if I did a retrograde burn at Duna, an inclination burn near Kerbin orbit that I should not have lost much of the dV for my fast exit, but because I save dV on two retro oberth effects, and saved dV on one positive at Moho, and then burn heavily at the at the low point of my circumkerbol orbit, I manage to obtain very high kinetic energy, only a small part I needed to give to thermodynamics, the rest was kept of my kerbol system exit.

This is another peculiarity of the oberth effect, depending how much you burn just before, at and just after you can use the effect to adjust course to flyby objects, and in the end you can get energy back to do high velocity things like leave the system or search for kuiper belt objects, whatever.

 

[fredinno]

2 hours ago, PB666 said:

This also gives me a chance to change the course so that I get a close flyby of Ike, Ike has no atmosphere to speak, I can get very close and use the oberth effect to create an ellipse that crosses but does not intercept moho's orbit.

Wouldn't this be worse since Ike is so small and can't offer very much in the Oberth effect?

Also, this sounds like a https://en.wikipedia.org/wiki/Bi-elliptic_transfer

[PB666]

5 hours ago, fredinno said:

Wouldn't this be worse since Ike is so small and can't offer very much in the Oberth effect?

Also, this sounds like a https://en.wikipedia.org/wiki/Bi-elliptic_transfer

Yes indeed, Ike is an unplanned rendevous, the problem in ksp is the patched conics only allows two layers, so that once acceleration inserted into ike you could only guess about the moho overlay, i provided as much dv as i could imagine for moho and then had to complete using ike. The odd thing is i did this manuever twice and ike got in the way both times. The satellites relatively large size and close orbit and the fact you have to cross through it twice to oberth ike increases the probability greatly the it will interfere with the patched conics. 

the other problem with ksp is that if you target while still inside another system you cannot see the  nodes, and the graphics just arent good enough, even on a high resolution 32" monitor to see the two orbital planes clearly. You can do some, in one try i got 1.5 degrees, in another try 0.

I could have actually gotten more efftect here, the problem was i was trying to keep the dV of the craft as high as possible, i have gotten up to 40k withou using ion drive systems, which would be totally reasonable with a moho flyby, but would be feckless with a moho oberth effect. If you had ion drive in ths case you would want to have a much more robust engine for the ike power burn and get as low in the kerbol orbit as the heat radiators can take, about 1 GM, once below 2 GM you full throttle the ion droves and leave them. 

Both duna, ike and moho are non ideal. with jool you can get some really good retro dV for doing a kerbol flyby, but for moho, duna and ike will suffice. If you can manage to get a jool flyby on the escape trajectory this is also good. The problem is that the ion drives will be useless. 

ION drives are actually the reason  i gave this example, provided something like VASMIR becomes a reality, i could invision a sun tolerant system traveling down to mercury, using some sort of weight efficient thruster and ion drives, which wont work behind mercury, unless you carry fairly large batteries or nuclear, blaster a booster at perigee. Then get an oberth that exits traveling at a slight negative radial, with the ions on full throttle for the entire time it takes to get to venus. At this point it would release vasmir and argon tanks, and use the remaing metal ion system and solar panels to push itsel out of our system. 

[YNM]

Basically what you've done is something like... Rosetta. You can do any weird things you want with them as long as you have the time, or the moment. Lately I've tried to make something like Cassini where it flyby almost every moon of Saturn with little to no energy requirement, and it kinda worked - a Laythe encounter gives a Vall encounter which gives back a Laythe encounter. All while going retrograde wrt the moons.

Edited April 8, 2016 by YNM

[PB666]

3 minutes ago, YNM said:

Basically what you've done is something like... Rosetta. You can do any weird things you want with them as long as you have the time, or the moment. Lately I've tried to make something like Cassini where it flyby almost every moon of Saturn with little to no energy requirement, and it kinda worked - a Laythe encounter gives a Vall encounter which gives back a Laythe encounter. All while going retrograde wrt the moons.

Rosetta had something like seven encounters, its problem is it has to work with planets that have atmosperes. But not like ksp, much bigger wells and atmospheres are relatively thin. 

[magnemoe]

My favorite KSP tricks is to save 1 km/s by going to Minmus orbit first then to top the tanks then get to high Kerbin orbit and drop down to LKO to do the escape burn, going to Eve or Duna orbit is pretty free but the real saving is going to Moho or Eeloo.