Tanker orbits

[DarkGravity]

I recently placed my first tanker in orbit around Kerbin. I started at the launch pad, headed East and never looked back. Now I just got a contract to rescue a dude from high Kerbin orbit, out by Minimus. First step, get my rescue ship into orbit. Second step, visit the tanker to fuel up. Third step: realize the dude in distress is travelling backwards in his orbit. So I revert to launch and head West this time. First stop: the tanker. Oh wait, the tanker is now heading in the wrong direction. I can't get to it! (you know what I mean)

Are there enough times you need a tanker orbiting in the anhedrical direction to warrant putting two tankers in orbit? If not, is there any way to tell before you accept a contract what direction the dude in distress is heading? What's the best direction for stations orbiting the Mun ? Thanks in advance.

p.s. I just made up the word "anhedrical." What is the language to specify the direction of the orbit? Sure there is a word to mean "clockwise when viewed from looking down onto the North pole."

Edited May 27, 2015 by DarkGravity

[n0xiety]

I should say you hardly need a tanker for a rescue mission over minmus. Other than that you can meet with your tanker and reach minmus as you always do then change your minmus orbit inclanation right when you enter its soi. It should't cost you more than 150dV if you do it right when you enter its soi and don't wait to get into an orbit. It can even be done before you enter minmus soi while you are 3/4 on your way and be more dV cost effective.

[ElWanderer]

To answer your PS, I think you mean a retrograde orbit. Prograde orbits are rotating the same way as the parent, retrograde orbits are in the opposite direction.

If you intend to accept a lot of rescue contracts and your rescue craft need top-ups, then it sounds like a retrograde tanker could work for you. I usually aim to have enough delta-v off the launch pad so don't use tankers in Kerbin orbit (yet), though I might be cutting it pretty fine for the example you've given.

[DarkGravity]

I'm sure some people could do it without stopping at a tanker. Maybe we don't all have the same stuff unlocked.

This particular mission has nothing to do with any SoI other than Kerbin's. It is just high enough, that it happens to be sort of near Minimus' orbit. As far as changing direction when I am far away from Kerbin being more efficient, are you sure about that? I thought the Oberth effect suggests the exact opposite? This game makes my head spin sometimes!

Edited May 23, 2015 by DarkGravity

[n0xiety]

I'm sure some people could do it without stopping at a tanker. Maybe we don't all have the same stuff unlocked.

This particular mission has nothing to do with any SoI other than Kerbin's. It is just high enough, that it happens to be sort of near Minimus' orbit. As far as changing direction when I am far away from Kerbin being more efficient, are you sure about that? I thought the Oberth effect suggests the exact opposite? This game makes my head spin sometimes!

Oh sorry i tought the guy you were trying to rescue was in minmus soi. Well think it this way then the higher your orbit gets your orbital speed will get slower. Minmus is moving at 270m/s velocity which means to get into a retrograde orbit at that hight you would need to spend 270 + 270= 540m/s. You are allready going up there to meet with the kerbal anyways. Well still 540m/s could be a big number indeed and launching into the right inclanation to meet would be more effective for sure. If you think you can spare that much dV for the inclanation change then go with your usual launch, if not your only solution is to have another fueling tanker at retrograde orbit.

Now that i think about it again here is another thing you can do. After you meet with your tanker get your Apoapsis up to the point where you need to meet with the kerbal that needs to be rescued. When you reach your apoapsis your speed will probably be really slow since you will start falling after you pass that point. Right at that time do a retrograde burn while you are still moving horizontal to Kerbin. You will probably need only a bit more than half the dV to change your inclanation then changing it at a circle orbit at that height. After that you will need to burn a bit more and create a circle orbit.

Edited May 23, 2015 by n0xiety

[Goomblah]

I'm sure some people could do it without stopping at a tanker. Maybe we don't all have the same stuff unlocked.

This particular mission has nothing to do with any SoI other than Kerbin's. It is just high enough, that it happens to be sort of near Minimus' orbit. As far as changing direction when I am far away from Kerbin being more efficient, are you sure about that? I thought the Oberth effect suggests the exact opposite? This game makes my head spin sometimes!

Even better than switching your orbital direction once you reach Minmus is performing a mid-course adjustment to make sure you cross the correct side of Minmus on your way there; if you need a prograde orbit, stay to the right of Minmus (or whatever side is turning away from you) and for a retrograde orbit stay to the left. You can change your direction, inclination, and altitude of your orbit by making a tiny burn (as in 10m/s tiny) a little while after you finish your transit burn; I usually do it once I'm about a quarter of the way there. Burning normal or anti-normal will put you in a polar orbit or change your inclination up or down; radial and anti-radial will determine which side of the object you cross.

[ElWanderer]

As far as changing direction when I am far away from Kerbin being more efficient, are you sure about that? I thought the Oberth effect suggests the exact opposite? This game makes my head spin sometimes!

Changing inclination is cheaper the slower you are going.

To do a complete u-turn you need to burn twice your current velocity - better to do that when you're travelling at 200m/s far from Kerbin than at 2200m/s when in Low Kerbin Orbit.

[KerikBalm]

I'm sure some people could do it without stopping at a tanker. Maybe we don't all have the same stuff unlocked.

This particular mission has nothing to do with any SoI other than Kerbin's. It is just high enough, that it happens to be sort of near Minimus' orbit. As far as changing direction when I am far away from Kerbin being more efficient, are you sure about that? I thought the Oberth effect suggests the exact opposite? This game makes my head spin sometimes!

I don't think its a matter of what is unlocked, if you have the ability to stage and use LV-909s, you can get to him without a tanker.... and you certainly do if you have the docking ports needed to use a tanker.

Next: After the first burn of a standard Hohman... if your apoapsis is out near minmus, reversing direction will be very cheap there. So cheap that I doubt it would be any advantage to launch retrograde at Kerbin (since then it takes more dV to get into orbit, you fight kerbin's rotation).

Oberth effect is about the change in energy for a given burn.

Plane changes should always be done as slowly as possible.

Consider a highly eliptical orbit, at perapsis, you're doing ~3,100 m/s

At apoapsis, you are doing ~150 m/s

To reverese direction at perapsis, you have to cancel 3,100 m/s of forward velocity, and accelerate 3,100 m/s in the other direction... a dV of 6,200 m/s to accomplish a 180 degree plane change.

At apoapsis... its 150 + 150... a mere 300 m/s to do a 180 degree plane change.

Don't fight your KE (kinetic energy), store it up as PE (potential energy) as your craft heads towards apoapsis... then when your velocity is really low, you can change your direction easily, and the velocity comes back as your craft falls back down from apoapsis, and PE goes to KE again.

Of course, in a Hohmann transfer, you match orbits at PE, rather than dropping back down to PE... but you end in a higher energy orbit.

[DarkGravity]

Thanks!

[match]

If your tanker is situated to accommodate vessels leaving the launch pad, then leave it there. Vehicles can be engineered to meet the needs of specific missions. You can calculate what will be needed for any mission using your tanker as your baseline. Plot a course from the tanker itself and you will find how much delta V is required.

Even if you have a rather eccentric orbit to hit, starting from the tanker will work. The simplest way is to plot your maneuver to intersect your Apoapsis with an ascending or descending node of the intended orbit and another maneuver at that point to match the orbit (this takes a bit of "eyeballing" your maneuver to get an intersection of your Apo and the intended orbit track).

This may not be the most efficient as it often will include radial burns to match the orbit. If you are attempting a rendezvous (like OP's example), you will want to plot the course at a time when the target will also be at the AN or DN (when you arrive!). Since your target is in a high orbit you may have to wait a bit to gain the proper intersection, but also will save delta V as you are performing the maneuver at the slowest point of your orbit. (For more efficiency, rendezvous at the node which is farther from Kerbin)

Like others have offered, you may decide to bypass your tanker depending on the pros and cons of the mission, but I don't think having a tanker in a retrograde orbit will add much future benefit for your program.

[DarkGravity]

Vehicles can be engineered to meet the needs of specific missions. You can calculate what will be needed for any mission using your tanker as your baseline. Plot a course from the tanker itself and you will find how much delta V is required.

I appreciate the helpful suggestions. What is the best way to go about plotting a course before launch?

[match]

If you already have a tanker in orbit, then take control of your tanker and plot the maneuvers from it. Once you find your solution to the desired orbit, add up the delta V of each maneuver node (mouse-over a closed node and it will display the delta v needed). Since your vessel will be docking with the tanker once in orbit, it will be starting from the same orbit and use the same amount of delta V. Now you know how much the mission will require and you can build your mission vessel accordingly.

I suggest adding 10-20% into your delta V budget, or more depending on how efficient you feel you are.

Don't forget to plot a return maneuver node so you know how much you will need to get home!

[cybersol]

One thing to note is that if your apoapsis is already as far out as far as minmus, then switching from prograde to retrograde orbit is easy and cheap. Just burn retrograde at apoapsis until your going in a similar retrograde orbit. Out there the cost will be low (<600 m/s) because you are not going that fast to begin with.

EDIT: Looks like noxiety already mention this... but its good advice

Edited May 23, 2015 by cybersol

[Laie]

I appreciate the helpful suggestions. What is the best way to go about plotting a course before launch?

Just use your tanker to lay down a series of maneuvers that will take you where you need to go, and don't forget that you need to get back as well. The dV requirement is the same for every vessel (different vessels will need different amounts of fuel for the same delta-v... but the dV you need to perform any given maneuver is the same for everyone). For the second and later maneuvers, you can see the total delta-V by hovering the mouse over the maneuver node. Add up all numbers and build a vessel that has at least as much.

[DarkGravity]

So, if I want to do something that is going to require x dV, I should design a ship with 1.2x dV. Is there any way to know how much dV my ship actually has in the stock game?

[match]

So, if I want to do something that is going to require x dV, I should design a ship with 1.2x dV. Is there any way to know how much dV my ship actually has in the stock game?

I figured you'd get to this eventually!

You can calculate delta-V using Tsiolkovsky's Rocket equation which goes a bit like this:

<shamelessly copied from wikipedia>

If you feel up to the math, there is a wealth of info on the internets that can do far better than I explaining it.

Otherwise, the stock game is a bit lacking in regard to delta V info. The Isp listed for each engine gives a fair approximation as far as efficiency, but you will have to do your own work to figure out delta V on your vessel.

The mod community steps in where Squad leaves off, and there are a few mods that will calculate for you. Personally, I plugged in the numbers for a little while but eventually let Kerbal Engineer do the heavy lifting while I pondered the mysteries of Eve.