UnHohmann transfers

[steuben]

I'm looking for the max delta-v required to go from orbit to orbit, assuming a Hohmann style transfer. So one burn at each end, and maybe one in the middle for plane change.

The math is looking pretty dense for me to take a run at it myself. And a lot of what I'm seeming to find just calculates the Hohmann Δv. Or I'm not reading it very well... also probably true.

Anybody got some pointers? Absolute preference is a nice precalculated table. But, I'm kind of prepared to do the lifting needed to build it... I just can't seem to find a decent place to start on the calcs.

[Not Sure]

It kinda just depends on the planet and you learn things from experience. I know Duna takes about 400dV to capture and circulrise, but I wouldn’t just put 400 “extra” dV. I really don’t know how I go about my budgeting, but adding 1000dV when you estimate you only need 600 is never a bad thing

[HebaruSan]

2 hours ago, steuben said:

I'm looking for the max delta-v required to go from orbit to orbit, assuming a Hohmann style transfer. So one burn at each end, and maybe one in the middle for plane change.

Do you want to include the effects of starting and ending in planetary spheres of influence, or are you thinking about just going from one simple solar orbit to another? If the latter, the math is pretty easy. You would apply the vis viva equation four times:

1. Calculate your velocity in your starting orbit
2. Calculate your velocity in your ending orbit
3. Calculate your periapsis velocity in your transfer orbit
4. Calculate your apoapsis velocity in your transfer orbit

The first burn is #3 minus #1, the second is #4 minus #2. Factoring in planetary SOIs introduces some complications.

And do you need something that isn't provided by a standard subway map?

Spoiler

 

[steuben]

The subway map is good for low orbit to SOI edge. But it only gives averages for SOI to SOI. 

 

[GoSlash27]

Steuben,
Are you talking about interplanetary burns? If so, I can explain the math. It's not actually that hard.

Best,
-Slashy

12 hours ago, steuben said:

The subway map is good for low orbit to SOI edge. But it only gives averages for SOI to SOI. 

steuben,
 Not actually. Those values are calculated based on perfectly circular planetary orbits (thus giving a mean excess velocity) and then rounding to the nearest 10 m/sec. They also assume that the ship is in low orbit for the burn.
Nevermind and apologies. I think I just misunderstood what you were trying to say.

Best,
-Slashy

Edited January 20, 2018 by GoSlash27

[Spricigo]

1 hour ago, steuben said:

The subway map is good for low orbit to SOI edge. But it only gives averages for SOI to SOI. 

 

How about https://alexmoon.github.io/ksp/  then?

As @HebaruSan pointed is just a matter of applying the vis viva equation. (A SoI change will require to take in acount the change of reference frame, but otherwise is  still the same idea)

Btw, the "max deltaV required" is just the min required plus "how much we waste" because we chose to do it in a inefficient way(or fail to do efficiently). It can take ridiculous amount of deltaV more if we simple decide to ignore things like launch window, Oberth effect and cosine losses. 

[GoSlash27]

I never heard back, so I'll assume that we're talking about "how to calculate an interplanetary burn".

The DV at each end accounts for two necessities: 1) Escaping Planet A's sphere of influence and 2) Transferring from Planet A's orbit about the sun to an elliptical orbit that intersects Planet B's orbital radius.

Step 1) is easy, assuming you are in a circular orbit to begin with. Multiplying your starting orbital velocity by "sqrt(2)" yields the velocity necessary to reach SoI edge. We'll call this value Escape V.

Step 2) is simply a vis-viva. The first burn of a Hohmann; Excess DV.

Your two values (Excess DV and escape V) are vector added Pythagorean- style sqrt(escape²+excess²) and then you get to subtract the orbital velocity you started with. Yay Oberth! That's pretty much it. You can account for eccentricity of either part by adding or subtracting DV to compensate for the difference between that and a perfectly circular orbit, but I don't do that because I'm lazy

The capture burn at the other end is calculated in exactly the same way as you would calculate a transfer burn coming home. The only additional step is calculating what your orbital velocity would be at your desired capture orbit.

HTHs,
-Slashy

 

 

Edited January 20, 2018 by GoSlash27

[Pand5461]

14 hours ago, steuben said:

The subway map is good for low orbit to SOI edge. But it only gives averages for SOI to SOI. 

 

The problem when you want some deltaV for guaranteed transfer from A to B is there's no upper bound, really.

Subway map gives you dV requirements that guarantee that you can in principle travel from A to B with them. However, you may want to have a ridiculously fast travel (say, 1 hour from Kerbin to Duna) and there will be trajectories with that time but they have ridiculous deltaV requirements.

You may want to check Transfer Window Planner mod or web-based Launch Window Planner http://alexmoon.github.io/ksp/ to see how timing the launch and time-of-flight affect the mission deltaV requirements.

 

[Kerbal101]

@steuben The game literally gives us the required dV in plain value.

If one opens a map view (M) and makes a maneuver node from one point to another, like from 80/80 Kerbin to 300k Mun intersect - the "m/s" is exactly the dV requried. Ofc, I also write down the direction I use for both points to account for planetary rotation speed (clockwise or CCW when looking from above).

Mods such as KER and Dmagic's Basic DeltaV/Basic Orbit, give the total and stage dV stat in engineer and fly areas, allowing to build vehicles that exactly match the measured dV, the dV that is required using YOUR piloting skills.
The subway map shows ideal dV, thus you may end up missing fuel if you use that values.

This is how extremely lazy people like me "calculate" it.

[steuben]

Playing the alexmoon site I'm getting ranges from 3 to 13 kps Δv for lKo to lEo, depending on date. So a less rough gestimate, based on there, is that worst case I'm looking at approx. 15kps Δv to get from Kerbin to Eeloo. Though, if I'm reading it right an extra 5 kps will third the travel time, with a corresponding increase in capture Δv .

Looking it through the vis-viva equations... and ignoring for the moment the capture Δv. My worst case Δv will be when Eeloo will be at periapsis on arrival and I'm leaving Kerbin when it's velocity is anti-parallel?  In the critical case, where the trajectory just kisses the SOI, the apoapsis velocity and periapsis velocity can be considered the same as the ending and starting speeds respectively?

 

 

 

[Snark]

2 hours ago, steuben said:

In the critical case, where the trajectory just kisses the SOI, the apoapsis velocity and periapsis velocity can be considered the same as the ending and starting speeds respectively?

Note that you really don't want the "just kisses the SOI" case, because that completely throws away all Oberth benefit and will waste a bunch of dV.

Your ideal case, in terms of conserving dV, is when you do your departure burn from low orbit (the lower the better) around your origin, and you do your capture burn directly into low orbit (the lower the better) around your destination.

2 hours ago, steuben said:

ignoring for the moment the capture Δv.

Just want to make sure that you're aware that capture dV is just departure dV in reverse, which is a very convenient and useful fact.  @GoSlash27 already pointed this out above, but it was in the middle of a fair amount of other excellent information and I wanted to make sure you didn't miss it. 

For example, suppose I'm trying to solve the following problem:

"I'm in circular orbit over Kerbin at <some altitude>, and I want to go to circular orbit around Duna at <some other altitude>, using the least dV possible.  I'm willing to wait until the planets are lined up right so that I can get a good transfer window.  How much dV will I need, total?"

1. First, find the departure burn.
   • Go to your preferred transfer-burn calculator.  Personally, my favorite is http://ksp.olex.biz, since it's so simple and easy to use, with a nice graphical display that's easy to read.
   • Enter in your origin, destination, and height of parking orbit around the origin.
   • Note down how many m/s of dV your burn needs to be.
2. Next, find the capture burn.
   • Just use the same tool, but in reverse:  that is, you put in your destination planet as "origin", and your origin planet as "destination"; and you enter the altitude of your intended parking orbit around the destination.
   • Note down how many m/s of dV your burn needs to be.
3. Add together the departure burn and capture burn, and there you go! 

 

Of course, this assumes that you're not taking advantage of any aerobraking at the destination.  If you are aerobraking there, then your capture burn goes down to basically zero, if you shave it right-- basically, a single aerobraking pass to lower your Ap to the desired altitude, then a very small burn at Ap to raise your Pe out of atmosphere.  (That burn should be very small, i.e. "too small to be worth the bother of calculating it", if your Ap after aerobraking is nice and low.)

[steuben]

Actually chucking Oberth and Δv conservation is what I'm after.  The capture and ejection Δv are trivial and well solved problems, which is why I'm ignoring them.

 

[Spricigo]

2 hours ago, steuben said:

Playing the alexmoon site I'm getting ranges from 3 to 13 kps Δv for lKo to lEo, depending on date. So a less rough gestimate, based on there, is that worst case I'm looking at approx. 15kps Δv to get from Kerbin to Eeloo.

The point is: we don't plan a mission to maximize the deltaV it requires. Our effort is to do more with the deltaV budget we have. IOW for practical purposes, we are not interested in the 'upper limit'. In fact we only look at higher deltaV requirements because of something else being a limiting factor (e.g, mission deadline or mission time) and even then we only consider it if it is within our deltaV budget for the mission.

Also, you are still considering the lower end of the 'required deltaV'. We can trade a lot more for flight/mission time if our budget is high enough. If we could had an infinite deltaV budget we could follow the fastest trajectory, accelerating towards our target half the way than turning around and  decelerating to not overshot.  In this video Scott Manley demonstrate that kind of trajectory from Kerbin to Eve (using infinity fuel). With the info given (the trip took 3h, the vessel was capable of accelerating 'just shy of' 20g), we can estimate it took about 2.000Km/s.

 

[steuben]

You may not be interested in the upper bound. But I am. And running it as a torchship was ruled out in the OP.

[HebaruSan]

33 minutes ago, steuben said:

You may not be interested in the upper bound. But I am. And running it as a torchship was ruled out in the OP.

Hmm. You said Hohmann in the OP, which actually does not include the sub-optimal transfers that seem to interest you. Hohmann is about using a transfer orbit that has its apoapsis/periapsis at your start/destination radii. If you do a 13 km/s burn, that's not the kind of transfer orbit you'll end up in.

What's interesting is that there's no clear distinction between a sub-optimal transfer and a near-brachistochrone. In order to encounter a destination at the "wrong" phase of its orbit, (assuming a transfer from inner planet to outer) you have to raise your solar apoapsis above that planet's orbit, so you are whizzing past it at the right time. In order to accomplish that, you'd have to burn more than would be required for a Hohmann transfer, which is exactly what you do in a brachistochrone.

To sum up, the lower bound is given by a typical Hohmann transfer, but there really is no upper bound. You can always burn a little bit more to get there a little bit quicker.

[GoSlash27]

I'm with Hebaru on this one. I'm having trouble understanding exactly what it is you're after. The absolute maximum DV you could expend during a Hohmann transfer?

[Spricigo]

1 hour ago, steuben said:

You may not be interested in the upper bound. But I am.

As @HebaruSan pointed out, required deltaV can always increase.

You need to pick something else to be the limiting factor if you want an upper bound.

 

 

[steuben]

There exists a Δv such that when the planets are aligned is the global minimum Δv to go from edge of SOI A to  edge of SOI B. Let us call this point of global minimum Δv Able. Let us call this Δv Harry. For Harry there exists a Δv Harold such that Harold is greater than Harry. Harold allows two things. The first is to shorten the transit time, the other is to allow the transit from points away from point Camp.

There also exists a Δv such that when the planets misaligned is the local maximum Δv to go from from edge of SOI A to  edge of SOI B. Let us call the point of maximum Δv Charlie. Let us call this Δv Tom. For Tom there exists a Δv Thomas such that Thomas is greater than Tom. Thomas allows one thing, to shorten the transit time.

Between Harry and Tom there is a Δv Dick.  Dick is greater than Harry, but less than Tom. As Dick gets closer in value to Tom the point in the orbits that Dick will just get from edge of SOI A to  edge of SOI B gets closer to Charlie.

I'm looking for Tom.

 

[Snark]

20 minutes ago, steuben said:

There exists a Δv such that when the planets are aligned is the global minimum Δv to go from edge of SOI A to  edge of SOI B. Let us call this point of global minimum Δv Able. Let us call this Δv Harry. For Harry there exists a Δv Harold such that Harold is greater than Harry. Harold allows two things. The first is to shorten the transit time, the other is to allow the transit from points away from point Camp.

There also exists a Δv such that when the planets misaligned is the local maximum Δv to go from from edge of SOI A to  edge of SOI B. Let us call the point of maximum Δv Charlie. Let us call this Δv Tom. For Tom there exists a Δv Thomas such that Thomas is greater than Tom. Thomas allows one thing, to shorten the transit time.

Between Harry and Tom there is a Δv Dick.  Dick is greater than Harry, but less than Tom. As Dick gets closer in value to Tom the point in the orbits that Dick will just get from edge of SOI A to  edge of SOI B gets closer to Charlie.

I'm looking for Tom.

I'm seriously confused.  If you want to know the maximum dV to go from one planet to another, it's infinite.  If you want to know the minimum dV to go from one planet to another, you want to maximize Oberth benefit.  You appear not to want either one of those two things, but darned if I can figure out what it is.

Could you describe the actual practical problem that you're trying to solve?  i.e. not math, but in terms of "I want to build a ship that can do X" or whatever.

[Kerbart]

It's always risky to answer for someone else, but I sense the OP is after this:

“Consider the case that I want to perform a two-burn transfer between two planets (burn "A" to initiate the transfer and burn "B" to finalize the transfer, i.e. orbiting the target planet.)
There is no wasteful burning (overshooting and correcting, etc, but there is no particular attempt to optimize the transfer either (oberth effect). Assuming a perfect Hohmann transfer (the transfer orbit touches origin and destination orbit, doesn't cross it), what's the worst-case scenario DV I need to design my spacecraft for?"

[GoSlash27]

3 minutes ago, Kerbart said:

It's always risky to answer for someone else, but I sense the OP is after this:

“Consider the case that I want to perform a two-burn transfer between two planets (burn "A" to initiate the transfer and burn "B" to finalize the transfer, i.e. orbiting the target planet.)
There is no wasteful burning (overshooting and correcting, etc, but there is no particular attempt to optimize the transfer either (oberth effect). Assuming a perfect Hohmann transfer (the transfer orbit touches origin and destination orbit, doesn't cross it), what's the worst-case scenario DV I need to design my spacecraft for?"

Kerbart,

 I hope this is indeed what the OP is after. I can answer that

Best,
-Slashy

[GoSlash27]

1 hour ago, steuben said:

There exists a Δv such that when the planets are aligned is the global minimum Δv to go from edge of SOI A to  edge of SOI B. Let us call this point of global minimum Δv Able. Let us call this Δv Harry. For Harry there exists a Δv Harold such that Harold is greater than Harry. Harold allows two things. The first is to shorten the transit time, the other is to allow the transit from points away from point Camp.

There also exists a Δv such that when the planets misaligned is the local maximum Δv to go from from edge of SOI A to  edge of SOI B. Let us call the point of maximum Δv Charlie. Let us call this Δv Tom. For Tom there exists a Δv Thomas such that Thomas is greater than Tom. Thomas allows one thing, to shorten the transit time.

Between Harry and Tom there is a Δv Dick.  Dick is greater than Harry, but less than Tom. As Dick gets closer in value to Tom the point in the orbits that Dick will just get from edge of SOI A to  edge of SOI B gets closer to Charlie.

I'm looking for Tom.

steuben,
 There is no "Tom".  Tom is undefined. You can't execute a Hohmann transfer between planets that are misaligned. If you wish to forego a Hohmann transfer to enable an immediate transfer between planets that are "perfectly" misaligned, it will take an infinite change in velocity, but it is no longer a Hohmann transfer. If, however, you insist on a true Hohmann transfer then the DV remains as the minimum. You just have to wait for an infinite period for your transfer orbit to wander into the SoI of the target planet.

Best,
-Slashy

[Spricigo]

38 minutes ago, Kerbart said:

It's always risky to answer for someone else, but I sense the OP is after this:

“Consider the case that I want to perform a two-burn transfer between two planets (burn "A" to initiate the transfer and burn "B" to finalize the transfer, i.e. orbiting the target planet.)
There is no wasteful burning (overshooting and correcting, etc, but there is no particular attempt to optimize the transfer either (oberth effect). Assuming a perfect Hohmann transfer (the transfer orbit touches origin and destination orbit, doesn't cross it), what's the worst-case scenario DV I need to design my spacecraft for?"

In that case, cost is exactly as much as a perfect Hohmann transfer. 

But if done at the wrong moment, the planet will no be there when the craft reach the apoapsis.

 

 

Edited January 24, 2018 by Spricigo

[steuben]

Quote

it will take an infinite change in velocity,

citation or proof required.

[Spricigo]

56 minutes ago, steuben said:

citation or proof required

 

Veloccity=distante/time

For distance≠0, as time approaches 0 velocity become infinity.