DELTA V NEEDED FOR INTER-PLANETARY MISSIONS

[Youngest Child of ISRO]

HELLO everyone . I am a new KSP and space science enthusiast from India!. (That's a heck of introduction LOL). 

can anyone list out HOW MUCH DELTA V IS NEEDED FOR ALL INTER-PLANETARY MISSIONS FROM KERBIN?
LIKE - for Mun, 4000m/s is a sweet spot.
 

[Gargamel]

This question about gameplay has been moved to game play questions.  
 

and I’ll point out this thread: 

 

 

[Guest]

Hi.

Yeah the dV map Gargamel is pointing you towards is a terrific resource. You might also want to look at the KSP Launch Window Planner by Alex Moon, it lets you plan and calculate all kinds of interplanetary transfers: http://alexmoon.github.io/ksp/

Keep in mind however that in many cases you can make use of gravity assists or aerocaptures to greatly reduce the dV budget. As a particularly extreme example, if your craft is designed for it, it might take as little as 100 m/s to get to Laythe, once you're on a Hohmann transfer to Jool.

[jimmymcgoochie]

The delta-V map is a good start, but fit doesn’t tell you how much delta-V you’ll need for your specific craft going from A to B at that specific time, it’s just an average. Use it as a guide when building your craft but don’t rely on it entirely.

There are a couple of options for in-game tools (mods) to help you get around:

• MechJeb’s maneuver planner can plot just about any type of maneuver you like, from a simple circularisation to an interplanetary transfer with a pork chop plot that shows you all the combinations of delta-V, time to node and time to destination so you can either pick the most efficient time, or find a faster transfer that’s more convenient. MechJeb can do a lot of other stuff too and can make your life a lot easier, but some players say automating everything is “cheating”- you can make your own mind up about that;
• Astrogator is good for getting approximate timings and delta-V requirements to most planets and any moons within the current body’s sphere of influence, though the numbers can vary a bit;
• Where Can I Go works both in the editors and in orbit, giving you a good idea of exactly where you can get to with your current delta-V based on the above delta-V map (so it has the same caveats about accuracy), plus it can calculate return trips and do the calculations including or excluding a launch from the launchpad- exclude that to plan your trips with just the payload, then include it when you add your launch rocket;
• Slingshotter shows where planets and moons will be in the future, helpful for planning gravity assists (or avoiding them when they’d throw your probe right out of the solar system- looking at you, Tylo!) which can save fuel. Gravity assists are difficult to do but allow you to get a lot of ‘free’ delta-V by exploiting a planet or moon’s gravity to change your relative velocity, and are most commonly used around Jool as it has three pretty substantial moons all orbiting at the same inclination, ideal for gravity assists.

[Guest]

Speaking of Jool, is there a tutorial for using gravity assists there somewhere?

I only recently figured out how to get a capture with any of the major Jolian moons on basically fumes (once you're on a Hohmann transfer to Jool that is).

If there isn't one, I could write it up as it's really not THAT complicated once you've got it figured out... then maybe some better orbital mechanic could make a similar explanation of how to do it in reverse, because now THAT's hard.

[king of nowhere]

14 minutes ago, Brikoleur said:

Speaking of Jool, is there a tutorial for using gravity assists there somewhere?

I only recently figured out how to get a capture with any of the major Jolian moons on basically fumes (once you're on a Hohmann transfer to Jool that is).

If there isn't one, I could write it up as it's really not THAT complicated once you've got it figured out... then maybe some better orbital mechanic could make a similar explanation of how to do it in reverse, because now THAT's hard.

I don't think so, because i don't think it is possible to write a guide. gravity assists are more art than science. there is the general rule that passing in front of a body will slow you down, passing behind the body will speed you up. and the faster you are compared to the target body, the less benefit you get from a gravity assist.

but aside from that, it's all dependant on specific situations. the more i get experience at using gravity assist, the less i think there can be a guide on them that's more than a collection of examples and a few general rules. I've used some very complex gravity assists in my latest mission, but i found them by spending several hours simulating all the possible options before picking the cheapest

[Guest]

7 minutes ago, king of nowhere said:

I don't think so, because i don't think it is possible to write a guide. gravity assists are more art than science. there is the general rule that passing in front of a body will slow you down, passing behind the body will speed you up. and the faster you are compared to the target body, the less benefit you get from a gravity assist.

Sure, but there are specific cases which are quite straightforward. Like Jool:

1. Get a Hohmann transfer to Jool however you like.
2. Set a manoeuvre node around where your orbit intersects with Dres's. 
   1. Adjust normal/anti-normal until you're pointed at the Jolian moons' orbital plane.
   2. Adjust prograde/retrograde until your trajectory is tangential to Tylo's.
   3. Make Tylo your target, and adjust radial in/out and prograde/retrograde until you encounter it near the tangent.
   4. Make fine adjustments until your Pe is tangential to your target moon's orbit.
3. Execute the burn and fine-tune as necessary. Cost is usually between 50 and 100 m/s, depending on your transfer.
4. Set another manoeuvre node just after your Jolian Pe, where your orbit is tangential to your destination moon's. Adjust retrograde until you encounter your target moon at the Pe, and then until it's as low as you dare.
5. Burn. Cost is between 0 and a few 10s of m/s.
6. Capture at the Pe. If it's Laythe, you can aerocapture. This is also a cheap burn, in the 10s of m/s.

Edited February 12, 2021 by Guest

[king of nowhere]

2 hours ago, Brikoleur said:

Sure, but there are specific cases which are quite straightforward. Like Jool:

1. Get a Hohmann transfer to Jool however you like.
2. Set a manoeuvre node around where your orbit intersects with Dres's. 
   1. Adjust normal/anti-normal until you're pointed at the Jolian moons' orbital plane.
   2. Adjust prograde/retrograde until your trajectory is tangential to Tylo's.
   3. Make Tylo your target, and adjust radial in/out and prograde/retrograde until you encounter it near the tangent.
   4. Make fine adjustments until your Pe is tangential to your target moon's orbit.
3. Execute the burn and fine-tune as necessary. Cost is usually between 50 and 100 m/s, depending on your transfer.
4. Set another manoeuvre node just after your Jolian Pe, where your orbit is tangential to your destination moon's. Adjust retrograde until you encounter your target moon at the Pe, and then until it's as low as you dare.
5. Burn. Cost is between 0 and a few 10s of m/s.
6. Capture at the Pe. If it's Laythe, you can aerocapture. This is also a cheap burn, in the 10s of m/s.

2) I find Laythe to be a more comfortable target for capture in the jool system. i only started using tylo after installing the kerbalism mod, because it includes deadly radiation belts stretching out to tylo.

4) you can often skip that part, and use another gravity assist. if you get the right jool apoapsis, you can also burn at apoapsis. for example, if you target tylo (and you generally want to visit there first, because you will then drop the really heavy lander) a good trajectory is a gravity assist at laythe setting your jool apoapsis at 60k km, intersecting tylo orbit. then from apoapsis you burn prograde to find tylo at the next apoapsis, and it's somewhat cheaper.

in theory it would also be possible to get a gravity assist at tylo to raise periapsis, and get captured by tylo at a later orbit for a much smaller cost. in practice, you come across laythe's SoI too often for comfort, and it's difficult enough to set up the whole arrangement that i never managed it without spending more fuel in course correction than i save with the assist.

 

for example, here - after getting captured by jool - i use a second laythe flyby to get a gravity assist putting me in intercept around tylo. total cost 200 m/s between correction manuevers and capture burn at tylo - without circularizing

and here i use a tylo assist to kick me directly en route to laythe. and it has a fairly low injection cost too: with just 90 m/s i could get captured by laythe in an elliptic orbit (blue trajectory)

(although, since i was using kerbalism and laythe is deep inside jool's radiation belts, i had to skip the gradual aerobraking. instead i rocket braked hard, i landed fast, i rejoined the orbiter immediately, and then i run away as fast as i could. Even then, even with heavy radiation shielding, my kerbonauts took half of a letal radiation dose)

 

 

what i'm getting at is, the straightforward part is only passing in front of tylo or laythe to get captured by jool. after that there are a lot of possibilities

[Guest]

35 minutes ago, king of nowhere said:

4) you can often skip that part, and use another gravity assist. if you get the right jool apoapsis, you can also burn at apoapsis. for example, if you target tylo (and you generally want to visit there first, because you will then drop the really heavy lander) a good trajectory is a gravity assist at laythe setting your jool apoapsis at 60k km, intersecting tylo orbit. then from apoapsis you burn prograde to find tylo at the next apoapsis, and it's somewhat cheaper.

It can't be much cheaper because it's so cheap to start with; specifically, the step 4 that you want to optimise away typically costs between 4 and 40 m/s. The most expensive one is the mid-course burn at about 50-100 m/s depending on how good my transfer is; that really can't be improved much because almost all of it is plane change; actually setting up the Tylo gravity assist is measured in cm/s. The rest of it is tens of m/s, plus the capture burn at target if you need it, which is also about as cheap as it can be. Specifically, step (4) never costs me more than a few tens of m/s – it's in a bracket of about 5 to 50 m/s.

It really does seem to me that you're making this more complicated than it is. In particular your orbits in your pictures look way sub-optimal to me, for one thing why are you lowering your Jool Pe so much? And why aren't your encounters tangential to the destination's orbit? That makes the capture burns way costlier than they ought to be.

Edited February 12, 2021 by Guest

[king of nowhere]

14 minutes ago, Brikoleur said:

It can't be much cheaper because it's so cheap to start with; specifically, the step 4 that you want to optimise away typically costs between 4 and 40 m/s. The most expensive one is the mid-course burn at about 50-100 m/s depending on how good my transfer is; that really can't be improved much because almost all of it is plane change; actually setting up the Tylo gravity assist is measured in cm/s. The rest of it is tens of m/s, plus the capture burn at target if you need it, which is also about as cheap as it can be. Specifically, step (4) never costs me more than a few tens of m/s – it's in a bracket of about 5 to 50 m/s.

It really does seem to me that you're making this more complicated than it is. In particular your orbits in your pictures look way sub-optimal to me, for one thing why are you lowering your Jool Pe so much? And why aren't your encounters tangential to the destination's orbit? That makes the capture burns way costlier than they ought to be.

so, perhaps i am misunderstanding you, because i understood a jool apoapsis change at jool periapsis, which would be in the hundreds of m/s. but perhaps you are already using the first gravity assist to enter jool with the right apoapsis.

anyway, my encounters are tangential to the destination's orbit, and the capture burns are fairly cheap as a result. the tylo insertion in the first picture was 120 m/s, and in the second the laythe insertion was 90, and there were no further burns after the plane change manuever one year before encountering jool.

perhaps you are better than me at this and you can do even better, but from the way you describe it, i got the impression you were making expensive apoapsis-changing manuevers around jool.

[Guest]

49 minutes ago, king of nowhere said:

so, perhaps i am misunderstanding you, because i understood a jool apoapsis change at jool periapsis, which would be in the hundreds of m/s. but perhaps you are already using the first gravity assist to enter jool with the right apoapsis.

I'm using the first (and only) gravity assist to get a Pe that's tangential with Laythe's orbit. The Ap will be somewhere between Tylo and Bop, but that doesn't really matter. Then at the Pe, when my and Laythe's orbits touch, I make an adjustment burn to get the encounter. As it's tangential, the capture burn will be close to optimal too. I just tried this: the mid-course burn was just under 50 m/s, and the Laythe encounter burn at step 4 was 2.2 m/s (but I got lucky with that). The capture burn for Laythe would have been slightly sub-optimal at about 257 m/s. So the only savings to be had really are for the Laythe capture, and because you can aerocapture there, there's usually no point. (There might be if I was aiming for a Tylo or Vall capture of course -- in that case, a second grav assist or tuning the orbit better would knock maybe 100-150 m/s off that, but of course some of that might be lost in the manoeuvring needed to get there.)

The point is really that this is a straightforward, simple, and repeatable technique that might not be the absolutely most efficient, but still very efficient, and yes it's simple enough to be tutorialised.

The mid-course burn -- the crucial thing is to set up the Tylo encounter more or less at that point in its orbit:

The Laythe capture:

 

Edited February 12, 2021 by Guest

[king of nowhere]

1 hour ago, Brikoleur said:

The capture burn for Laythe would have been slightly sub-optimal at about 257 m/s. So the only savings to be had really are for the Laythe capture, and because you can aerocapture there, there's usually no point. (There might be if I was aiming for a Tylo or Vall capture of course -- in that case, a second grav assist or tuning the orbit better would knock maybe 100-150 m/s off that, but of course some of that might be lost in the manoeuvring needed to get there.)

 

ah, ok, then it's a matter of different ship designs. you clearly are equipped for aerobraking at laythe, and your manuever is indeed the most efficient if you aim for laythe capture and you can aerobrake there - only spending a few tens of m/s in course corrections.

for various reasons, i never sent to jool a mothership capable of aerobraking. also, i never entered aiming for laythe; the first time i went first for tylo because i had a really heavy tylo lander and dropping it would save tons of fuel afterwards. the second time i aimed for vall because my isru-equipped ship could only land there - and i had the additional constraint of making a pass inside jool's atmosphere (which is why i had such low jool periapsis). the third time i had the kerbalism radiation problem and it was paramount to get out of the death zone as soon as possible.

which highlights an additional problem with trying to standardize gravity assists: the best trajectory depends not only on the alignment of planets, but also on your specific ship and your specific mission objectives.

[Guest]

1 hour ago, king of nowhere said:

ah, ok, then it's a matter of different ship designs. you clearly are equipped for aerobraking at laythe, and your manuever is indeed the most efficient if you aim for laythe capture and you can aerobrake there - only spending a few tens of m/s in course corrections.

Even if I wasn't aerobraking, it would be pretty efficient: a total of about 300 m/s to capture, including the mid-course plane change. No doubt it's possible to cut another 150 m/s from the Laythe capture if you're not aerobraking; however, that manoeuvring doesn't come for free either, so I'd expect the best possible net savings from my procedure would be on the order of 100-150 m/s. As you said yourself, at that point it's more of an art than a science -- if you enjoy that kind of orbital mechanics it is no doubt really satisfying; if you're only concerned about getting there, designing your craft with 150 m/s more dV is probably easier.

You can use the same technique to get to Vall: just tune your mid-course burn so that you end up with a Pe tangential to Vall's instead of Laythe's. If you're aiming for Tylo, then you'd want to do a second grav brake, either off Tylo itself or one of the other moons, or see if you can get there by grav braking off Vall or Laythe. Either way, the crucial bit is the mid-course correction, which you tune with radial in/out and prograde/retrograde so you can encounter the moon you want to grav brake off at exactly the right phase angle. And that's virtually free: on top of the plane change that you need anyway, it's literally cm/s -- I usually have to set my thrust limiter to 1% to get it right, or use RCS if I have it. 

Once more: the point is that this is a simple, repeatable grav braking technique which gives you predictable results, which you can then factor into your ship designs. I know I can get to Laythe from LKO with about 2500 m/s, and still have a bit left over for contingencies. That's immensely useful!

Edited February 12, 2021 by Guest

[Anonymous49]

3600km/s of delta v

[Spricigo]

18 hours ago, Brikoleur said:

Once more: the point is that this is a simple, repeatable grav braking technique which gives you predictable results, which you can then factor into your ship designs.

 

Personally, I don't think I will be using it much but seems like a good guide to have around when a new player get interested in the subject or a more experienced player is looking for a quick refresh.