Delta V requirements

[Thunder_86]

I was doing a mission that required me to land a station on Gilly. I've see this image:

 

 

It says you need 90 m/s of Delta V to get from an eccentric orbit to the edge of Kerbin's SOI to get to a Eve Fly-By. That was pretty much spot on (of course, I burned in one shot from LKO to an Eve encounter, but I looked at the maneuver's Delta V, and when my orbit reached the edge of Kerbin's SOI, i had pretty much exactly 90 m/s left to burn).

 

It then says you need 80 m/s of Delta V to get from a flyby to an eccentric orbit around eve. Took me over 800... What am I doing wrong here? I've been playing KSP for a while now (returning after a 6-months to 1 year break), but never really got into trying to minimize fuel requirements, I usually pack way... way... WAY more fuel than I need, to the point where I sometimes drop almost full stages before a landing or a circularization. But I'm trying to go smarter now, with the bare minimum that I need. For my first try, my plan was all prepared, I was gonna get there with a maximum of like 100 m/s of Delta V left. Let's just say that these 700 m/s missing ruined the day of a couple kerbals... Jeb was sooooo angry!! I tried to apologize, but I think he's ignoring me

 

Is that Delta V map wrong, or is it me who's doing something wrong here?

 

Thanks!

Edited June 23, 2016 by Thunder_86

[Superfluous J]

6 minutes ago, Thunder_86 said:

It then says you need 80 m/s of Delta V to get from a flyby to an eccentric orbit around eve. Took me over 800.

What was your Perapsis when you encountered Eve (before you started burning)? That chart assumes several things including a periapsis of 100km. In fact it doesn't assume it, it states it . You should do your 80m/s burn at Eve Periapsis to bring your Eve Apoapsis down. If you burn way away from Eve, you're going to miss out on a TON of Oberth-related savings.

Also, was your Eve encounter at your Sun periapsis, or was it lower? The chart also assumes you are making a pretty spot-on Hohmann Transfer from Kerbin to Eve. However, I suspect the above to be more likely a cause, as I can't imagine coming in so cockeyed as to increase your burn requirement tenfold.

I rarely do that kind of encounter. Usually when I'm going to Eve I go to Gilly, and find it's easier to just go right for a Gilly encounter.

Edited June 23, 2016 by 5thHorseman

[Thunder_86]

I'm not good enough to calculate an Gilly encounter straight from LKO :S I'd have no clue how to calculate this to get there with the correct inclinaison. and correct timing (you want to get there at precisely the right time or you might pass on the correct trajectory... but with Gilly at the other end of it's orbit lol.

 

You are absolutely right about burning at a periapsis of 100km tho... I feel a tad stupid right now... I didn't think it would make such a HUGE difference. I knew burning at periapsis is the best way to go, but I didn't think it's nooed 10 times as much... I burnt like an idiot, trying to get in orbit from way way too high I guess, and not even burning at periapsis. I have no clue at what height I was at... but it was around Gilly's orbit. And not at it's periapsis to eve XD. must have been what... 15? 20 000 000m? mabe even higher XD

 

As for getting there at eve's periapsis to the sun, just like aiming straight for Gilly from LKO, I'm not sure how to plan that... you already need to wait for a transfer window... I doubt I'd be able to aim for a time frame for that :S

 

Thanks for the reply, I'll try again tomorrow and get a 100km fly by instead of a 15 000 one and burn at Periapsis this time XD. That is definately something I'm easily able to do. We'll see what happens. Might try to use a bit of aerobraking as well (even tho my craft is really not designed for that lol). In the meantime, if you have tips to get direct encouters with natural satellites from LKO, it would be very much appreciated!

 

Thanks!

Edited June 23, 2016 by Thunder_86

[Snark]

2 hours ago, Thunder_86 said:

You are absolutely right about burning at a periapsis of 100km tho... I feel a tad stupid right now... I didn't think it would make such a HUGE difference. I knew burning at periapsis is the best way to go, but I didn't think it's nooed 10 times as much...

Yep, Oberth is your friend. 

Bear in mind that it's completely reversible.  Trying to capture into orbit at a high altitude instead of low orbit is exactly the same as trying to eject from high orbit instead of from low orbit.

When you left Kerbin, you did so with a direct burn from LKO, right?  And it only needed a smidgeon more dV than what it took to get your Ap up to the limit of the SoI, yes?

If you hadn't done a direct Eve burn-- if instead, you had done one burn that raised your Ap up to, say, 20,000 km above Kerbin, and then did your Eve burn from your Ap instead of Pe-- you would have found it a lot more expensive.

It's the same thing, it works the same way.

[Foxster]

Aerobraking is also now quite do-able at Eve, which significantly reduced the dV requirement. Just stick everything behind an inflatable heatshield. 

[Thunder_86]

Snark: yeah you're right. I am yet to understand the reasons behind the oberth effect. I understand a bunch of things about space, but the oberth effect ain't one XD. I don't fully get why burning at periapse is so more efficient. I know it but I don't understand it lol. in my mind, the slower you go (apoapse), the bigger the diffrence, percentage-wise a single m/s is. its kinda unintuitive to me to burn at a very low periapsis. but yeah I know that's what I should do. I just have a hard time to wrap my head around it.

 

sorry if this message was unclear, it's 5:30 am, I'm typing this on my phone and English isn't my mother tongue XD

[Thunder_86]

3 hours ago, Foxster said:

Aerobraking is also now quite do-able at Eve, which significantly reduced the dV requirement. Just stick everything behind an inflatable heatshield. 

yeah those are new right? I'm yet to experiment with them. but sometimes the ship design doesn't really allow for it :S

[magnemoe]

1 hour ago, Thunder_86 said:

yeah those are new right? I'm yet to experiment with them. but sometimes the ship design doesn't really allow for it :S

yes you need to design the ship for it, one tips is to put most weight close to shield and have an long tail with lots of airbrakes on back. 
This could also be the transfer stage Lots of reaction wheels also help. 

[Tatonf]

But the inflatable heatshield is probably going to weigh more than the fuel required for a 80 m/s burn. Plus, it's difficult to control the final orbit after the aerocapture. Plus, you have to do a burn at apoapsis for raising your periapsis.

[Thunder_86]

31 minutes ago, Tatonf said:

But the inflatable heatshield is probably going to weigh more than the fuel required for a 80 m/s burn. Plus, it's difficult to control the final orbit after the aerocapture. Plus, you have to do a burn at apoapsis for raising your periapsis.

There is that. I don't know about the weight, but you do need to burn to raise the periapsis... but I think you'd save more DV by aero breaking than you'll need to raise your periapsis right? I could do itwithout the heat shield too I guess, maybe just not as aggressive...

 

Anyway I'll have to experiment, but at least now I know what was my yesterday's blatent mistake. What I'm now wondering more than anything else is hot to plan a Gilly (or any other natural satellite) encounter directly from LKO... that is completely beyond me right now! It'd save a bunch of time and fuel I'm pretty sure! In-game time doesn't mean crap, but it'd save so much of MY time not having to first getting captured by a planet and then plan an encouter with one of it's moons... It would be especially useful with moons like Gilly... small moon, tiny SOI, and VERY eccentric orbit!

 

Also, when I try to encounter Gilly and I don't have the same inclinaison and orbit shape, what I often do is to plan a route that goes exactly trough Gilly's orbit (not easy as you need to eyeball it in the map...) and then maneuver by making my orbit bigger or smaller exactly at that point so that when I come back at the same spot, Gilly is right there for me. it's fiddly, but then I don't even have to match planes with it. It sometimes need more DV to capture tho as you get in the SOI in a weirder angle, making me faster relative to Gilly... Is that the best way (once you are already in the planet's orbit)?

Edited June 23, 2016 by Thunder_86

[Starhawk]

2 hours ago, Thunder_86 said:

I am yet to understand the reasons behind the oberth effect

The relationship between kinetic energy and delta v is important here.  A given prograde burn will increase the vehicle's velocity by a fixed amount no matter when the burn is done.  But kinetic energy is related to the square of velocity so you get more kinetic energy out of a burn when the burn is done at higher velocity.

Note: the effect only applies when burning prograde or retrograde.

Happy landings!

 

[Tatonf]

While saving dV, you're going to have a much more massive spacecraft with the inflatable heatshield than just adding a tiny bit of fuel. Because of that, each of your stages are going to be bigger to support the extra weigh. Don't look only at the delta-V, look also at being mass-efficient. The lighter your payload is, the smaller your rocket will be, and the easier to build. I don't know if an aerocapture is possible without a heatshield, I know that the upper atmosphere of Eve have been tweaked a bit to be more forgiving, but Eve remain a dangerous celestial body.

Planning a Gilly encounter from LKO is probably the most difficult thing you can do, due to the SOI of Gilly being super small. Plus, it's going to be very inneficient, because the Oberth effect of Gilly is almost non-existent, while the Oberth effect of Eve is huge.

I think that the most efficient way to encounter Gilly would be :

- Make a Hohmann transfer from LKO to Eve with a Pe of 100 km. Your initial orbit should be slightly inclined to account the inclination of Eve / Gilly. Your inclination while you arrive in Eve's SOI should be the same as the one of Gilly.

- Do a retro burn, so your Ap is equal to Gilly's Ap. This is a tricky part, because of the transfer window, it might not be the case. The only solution I see is waiting for a transfer window that will give you this configuration. Fortunately, there's a lot of Kerbin - Eve transfer window. If you are quite close, I think a small radial burn might be acceptable.

- At apoapsis, raise your periapsis a bit, so in a certain amount of orbit, all you have to do is raising your periapsis again to Gilly's periapsis and you'll encounter it. Gilly's SOI is so small that you can consider going to it like making a rendez-vous.

[Thunder_86]

thanks for the tips people!

[Foxster]

Don't dismiss aerobraking too quickly though, especially if planning to land at Eve. The inflatable heat shield is only an extra 1.5t and the 80dv being talked about here is only what is needed to get a very wide orbit, you'll want 1500+ to reduce your speed so your craft will have a chance at re-entry. Then the heat shield is almost essential to survive the heat of re-entry and so can be reused for that. 

Edited June 23, 2016 by Foxster

[Tatonf]

Yes but this is a mission to Gilly.

[Foxster]

14 minutes ago, Tatonf said:

Yes but this is a mission to Gilly.

Well, apart from that minor pointette, it still scary fun to aerobrake  

[Snark]

6 hours ago, Thunder_86 said:

I am yet to understand the reasons behind the oberth effect. I understand a bunch of things about space, but the oberth effect ain't one XD. I don't fully get why burning at periapse is so more efficient. I know it but I don't understand it lol.

So, a couple of explanations.  One is mathematical, the other is a bit more arm-wavy but perhaps easier to understand because it's a physical explanation.

The mathematical explanation, which @Starhawk already gave:  When you're doing a prograde burn (to eject), or a retrograde burn (to capture), why are you doing it?  Answer:  You're trying to change your kinetic energy.  For a prograde burn, you're trying to boost your energy; for a retrograde burn, you're trying to reduce it.

However, it's not linear.  Kinetic energy varies with the square of your velocity.  That means that a given amount of dV will make a bigger change in your energy when you're traveling faster.  Explanation here, with specific numeric examples:

Okay, now for the arm-wavy example.  I'll start with an analogy, then get more specific to space travel.

Let's say you're going to visit a friend.  He lives at the top of a mountain.  You're sitting in your car at the base of the mountain, with a little motor scooter strapped to the back.  Let's say that fuel is really expensive and you want to save as much as you can.  You have a couple of options:

1. Drive your car up the mountain to your friend's house.
2. Leave the car parked at the base of the mountain, unstrap the little scooter, and drive that up.

Which one uses more gas?

Clearly, you're a lot better off going with option #2.  Option #1 is going to need a lot more gas, because it involves lugging that heavy car all the way up to the top of the mountain.

Okay, now let's put it in terms of space travel:

You have a 10-ton ship in LKO, of which 7 tons are fuel and 3 tons are "dry weight".  You need to eject from Kerbin.  Let's say you're considering two options:

1. Burn all the fuel in LKO.
2. Do an initial burn in LKO that uses 4 tons of fuel to raise your Ap up near the boundary of Kerbin's SoI.  Coast up to Ap.  Then burn the remaining 3 tons of fuel there, while you're at Ap.

Think about this in terms of how much mass are you lugging to the top of the mountain (i.e. to the outer edge of Kerbin's SoI)?

In case #1, you're only carrying 3 tons to the top of the mountain (the dry mass of the ship), since you left all of that heavy fuel down in LKO.

In case #2, you're carrying 6 tons to the top of the mountain (the dry mass of the ship, plus 3 tons of fuel).  You're lifting a lot more mass to the top of the mountain than you actually need to.  This means that when you did that 4-ton burn in LKO, you actually completely wasted half of it, because half of that 4-ton burn was just to unnecessarily hoist 3 tons of fuel up to the boundary of the SoI.

 

6 hours ago, Thunder_86 said:

In my mind, the slower you go (apoapse), the bigger the diffrence, percentage-wise a single m/s is

Emphasis added for clarity.    "Percentage" is irrelevant here, it's total amount that matters.  If I'm standing on the sidewalk nearly stationary, but moving along at one million-billionth of 1 m/s ... and then I start to walk and am going at 1 m/s ... suddenly I'm going a MILLION BILLION TIMES faster!  That doesn't mean I've uncovered the secret of how to flap my arms and go to the Moon without a spaceship. 

 

4 hours ago, Tatonf said:

But the inflatable heatshield is probably going to weigh more than the fuel required for a 80 m/s burn.

^ An excellent point.  To the OP:  If you really only need an 80 m/s of dV to capture, then the heatshield is probably overkill.

In general:   if you happen to get a perfect transfer right at the optimal launch window, and if all you want to do is end up in an extremely elliptical orbit with its Ap way out near the boundary of Eve's SoI.  In this case, the OP simply wants to go to Gilly and doesn't care about Eve, so you're right-- in this particular case, the big inflatable heatshield is probably overkill.

However, in general, if the destination is Eve itself, where one typically wants to get into a reasonably low, reasonably-circular Eve orbit, so the dV in such cases is a whole lot higher than 80 m/s, and the heatshield is really worth it in such cases.

Additional discussion of the benefits of the heatshield put into a spoiler section, since it's more about going-to-Eve-in-general rather than the OP's actual use case of going only to Gilly.  If you really manage to set things up so you only need an 80 m/s burn, you really don't need to bother with a heatshield.

Spoiler

9 hours ago, Foxster said:

Aerobraking is also now quite do-able at Eve, which significantly reduced the dV requirement. Just stick everything behind an inflatable heatshield. 

^ This.  The inflatable heat shield is amazing.  Really, really makes life easier.  It only weighs 1.5 tons, and can slow you down a lot more than 1.5 tons of fuel could.

5 hours ago, Thunder_86 said:

yeah those are new right? I'm yet to experiment with them. but sometimes the ship design doesn't really allow for it

Well, sure, you need to design the ship to allow it... but that's really not hard to do, it's incredibly flexible.  It's just another 2.5m stack component.  If you were able to put a regular heat-shield on, then you can put this one on instead.  In fact, it's easier to use than a 2.5m ablator heat shield, because you have a lot more options about where to put it.  You can put it on the bottom of the ship, or on the top.  It can be on the prograde side during reentry, or on the retrograde side.  It has the ability to self-jettison when you're done with it, without needing an extra decoupler.  Great piece of hardware, I love these things. 

4 hours ago, magnemoe said:

yes you need to design the ship for it, one tips is to put most weight close to shield and have an long tail with lots of airbrakes on back.

That, or you could design the ship so that the inflatable heatshield trails on the retrograde side of the ship during aerobraking, like towing a big heat-resistant parachute behind you.  Lots of options here.

4 hours ago, Tatonf said:

Plus, it's difficult to control the final orbit after the aerocapture.

Well, yes, but that's what F5 is for. 

4 hours ago, Tatonf said:

Plus, you have to do a burn at apoapsis for raising your periapsis.

Yes, but the periapsis-raising burn is generally vanishingly tiny.  An Eve aerobraking pass with the inflatable heatshield is never going to have a Pe lower than 60 km, if you plan to stay in orbit.  The top of the atmosphere is 95 km.  Raising the Pe by a measly three dozen kilometers, if you do it while at Ap, takes practically no dV at all.

3 hours ago, Thunder_86 said:

think you'd save more DV by aero breaking than you'll need to raise your periapsis right?

^ Yes, this.  Very much this.

 

 

[Thunder_86]

actually, the mathematical explanation talked to me a lot more  lol. But thanks for the second explanation anyway! I actually though it was linear, but I now see it's exponential, so burning at periapsis makes a ton of sense when you know this!  Starhawk's explanation made me understand it, and the link you provided from you reply in another thread put actual numbers for me to easily see! Thanks a lot!

 

At first, I though the most efficient way would have been to plan your encounter super high and circularize it way up there, as I figured I'd have to kill less speed. Now that I understand that the kinetic energy isn't linear, but rather exponential based on your speed, then it makes so much more sense to burn when you are going the fastest, to reduce your kinetic energy at a very, very fast rate! That completely opened my eyes!  (I'm a very mathematical dude lol. I do lack some physics notions tho. Should have taken Physics in high school instead of Spanish XD)

Edited June 23, 2016 by Thunder_86