Delta V calculations to moho

[Warzouz]

I had some difficulties with MechJeb in my mission around Jool. It fails to give me some good route between moons. Well it failed each time, even offer me to dive into Laythe ocean and hope to get outward...

So I tried to compare in a mission to Moho. I sent 3 identical rockets.

- One would have trajectories calculated by MJ prokshop thingy (MechJeb mission)

- One would be a pure Hohman planetary transfert (Hoffman mission)

- One would be some manual node tweak (with mid course plane change) (Manual mission)

My rocket has 3 main stages, each are asparagus. It's quite heavy (1130T on the pad). It's designed to lift 200T payload to 100km. But as the payload is only 80T, I keep part of the first stage while the engines (4 LVN) of the second stage are lit.

My goal is to check deltaV differences on each missions. There is a 4T, 2000m/s SSTO lander, but it's tank is locked.

Starting DV for each rocket : 13500m/s

In orbit at 90km : 9000m/s

I compared the DV of each ship after achieving a 40km circular, 0° inclined orbit around Moho

- Mechjeb : 8360 m/s left

- Manual : 7610 m/s left

- Hohman : 6380 m/s left

This is not possible since I burnt (from memory) 3200 using MechJeb + capture, and the others were higher. It's no way possible to have only consumed 640dV to go from Kerbin orbit to Moho orbit.

For the return :

- MechJeb calculated a 3380m/s route, which arrived with a additional 1880 to be captured (I aerobrake to avoid it)

- The manual node calculation costs me 3950, which arrived with a additional 890 to be captured.

- The Hohan transfer costs me 5430m/s, and an additional 700m/s.

On the return, calculations seems to be more coherent. But I don't understand why the DV calculations are so wrong on the first part.

I think KER and MJ calculated some false DV for my craft. It must have been higher than 13500m/s.

So to go to Moho (ignoring the landing) I started with 13500m/s, arriving in Kerbin SOI, I had

- 5000dv left with MechJeb calculations.

- 3600m/s left with a mid course plane change

- 900m/s with a pure Hohman transfer.

Edited April 18, 2015 by Warzouz

[Red Iron Crown]

It does sound like there was an error in how MJ/KER calculated the dV of your craft. Can you share a pic of it as it arrived in LKO? Preferably with the dV window and resource panel open?

[Warzouz]

The first stage is a radial asparagus

The second stage is 4 LVN and droppable tanks.

The only tweak is that the first and second stage can work together, when only the core engine of the first stage is left.

The third stage has his fuel locked (so it's a dead weight for the moment)

In the VAB

On the pad

At 20km

In orbit (part of the first stage is still attached)

I drop the lateral engine. Now The first stage core is full and the second stage can push too.

It seems that MechJeb is able to calculate the DV, but only on the pad, not in the VAB. So my DV is more near 17700m/s. (+ 2000ms in the lander). Well I like overfueled rockets...

Edited April 18, 2015 by Warzouz

[LethalDose]

Moho can be a real nightmare to reach and return from due it's inclination, orbital speed, and eccentricity. Hohmann transfers to Moho just make me sad; They're a very bad idea, and just shouldn't me used.

When you compared all the launches, did you do them all at the same time, or did perform them sequentially?

PS and it's HOHmann, not HOFFmann. It's a minor thing, but it drove me up the wall the whole time I was reading the OP.

[Warzouz]

The manual (mid course plane change) and Mechjeb where fire at 2 days interval. The Hohmann was half a year later (it would have been worse otherwise)

[LethalDose]

The manual (mid course plane change) and Mechjeb where fire at 2 days interval. The Hohmann was half a year later (it would have been worse otherwise)

That could explain why the Hohmann transfer took so much more dV to perform.

[Warzouz]

Sadly, I didn't write down the various burn I did and as I did everything in the same time, I don't recall which burn as done by which vessel.

I remember that I delayed the Hohmann ship because on of the burn was very high. By touching Moho orbit at apoaps, the cost was lower (but still higher than the other two).

But maybe what I called a "pure Hohmann transfert" is not.

- I escape Kerbin SOI, but stayed in a nearly circular orbit, close to Kerbin

- I changed plane to match Moho

- I found a gentle encounter at apoaps while burning retrograde

- The capture cost was quite low

All this was long en costly

My "Node tweak"

- I escape Kerbin SOI and burn at Moho AN/DN

- I changed plane to match Moho

- I found a encounter after a very small course correction

- Then the capture at Moho, inclination and circularize

My MechJeb plot

- MJ Burn directly to a Moho encounter

- I made a very small mid-course correction

- The encounter was steeper, then capture burn was longer.

Well, maybe I will redo it (without landing), because I don't remember the precise numbers. I've only written down the fuel I had left, but after been back, I noticed to "DV left" indicator of KER was completely wrong.

BTW, landing in Moho highlands is a pain because of slopes.Next time I'll design a much flatter lander...

[John FX]

There can be a very large difference in the Dv needed to get to Moho depending on when you left. Try making a quicksave when your craft is in orbit and leave with the same craft at the same time for each type of transfer then you will have the same base Dv needs and will get a much better comparison.

EDIT : Maybe. I just read your latest post...

Edited April 19, 2015 by John FX

[LethalDose]

Sadly, I didn't write down the various burn I did and as I did everything in the same time, I don't recall which burn as done by which vessel.

I remember that I delayed the Hohmann ship because on of the burn was very high. By touching Moho orbit at apoaps, the cost was lower (but still higher than the other two).

But maybe what I called a "pure Hohmann transfert" is not.

- I escape Kerbin SOI, but stayed in a nearly circular orbit, close to Kerbin

- I changed plane to match Moho

- I found a gentle encounter at apoaps while burning retrograde

- The capture cost was quite low

All this was long en costly

The bolded point is at least one place where you're losing a lot of dV. If at all possible, you want to combine your escape and transfer burns, otherwise you're losing huge amounts of velocity to gravity drag during your escape. This is also an application of Oberth Effect, but I think the former explanation may be easier for you to understand.

[magnemoe]

There can be a very large difference in the Dv needed to get to Moho depending on when you left. Try making a quicksave when your craft is in orbit and leave with the same craft at the same time for each type of transfer then you will have the same base Dv needs and will get a much better comparison.

EDIT : Maybe. I just read your latest post...

Cost to Moho is very dependent on plane change and if you arrive close to Moho Pe or Ap around the sun, the braking burn also depend on how well you hit moho at your own Pe around sun.

You have good TWR so the error should not be to an serious issue, as the node assume zero burn time this can cause significant error with low twr and going to Moho its expensive to correct.

However if you first burn to solar orbit and then burn for Moho you will miss the Oberth effect from burning in low orbit around Kerbin, this explains why it was so much expensive.

The MJ prokshop burn is not optimal as it will do the plane change as part of the main burn, this is usual cheaper to do later.

Note that you don't really need to match planes with Moho, this can easy cost 1500 m/s if unlucky, its enough to get an intercept and then change plane after entering an orbit with high Ap.

Recommend using mechjeb standard transfer to planet for Moho.

[Warzouz]

The bolded point is at least one place where you're losing a lot of dV. If at all possible, you want to combine your escape and transfer burns, otherwise you're losing huge amounts of velocity to gravity drag during your escape. This is also an application of Oberth Effect, but I think the former explanation may be easier for you to understand.

Yes that was the whole point of the 3 launches. I wanted a comparison.

But finally, I did'nt understant the "DV-left" I had. Something is wrong with Ker (in the VAB and flight) and MJ (only in VAB).

[TheXRuler]

KER (at least for me) gets really confused when you have stages above the root part. Could it be that?

[LethalDose]

Yes that was the whole point of the 3 launches. I wanted a comparison.

Yes, but based on your descriptions, you performed the burns in ways that aren't comparable. Since you did it this way, it seems clear that you weren't aware they aren't comparible and I was trying to explain the source of the problem to you.

Sorry?

But finally, I did'nt understant the "DV-left" I had. Something is wrong with Ker (in the VAB and flight) and MJ (only in VAB).

As TheXRuler pointed out, KER can get confused about dV calculations when the rocket isn't a simple "top down" staging.

Edited April 19, 2015 by LethalDose

[OhioBob]

But maybe what I called a "pure Hohmann transfert" is not.

- I escape Kerbin SOI, but stayed in a nearly circular orbit, close to Kerbin

- I changed plane to match Moho

- I found a gentle encounter at apoaps while burning retrograde

- The capture cost was quite low

All this was long en costly

It's not possible to have anything close to a "pure Hohmann transfer" to Moho unless you intercept Moho when it is at one of it's nodes. These ideal alignments happen only rarely. What you did was to manufacture the conditions for a Hohmann transfer by using very costly means. I don't consider what you did to be a Hohmann transfer at all.

A perfect Hohmann transfer to Moho looks like this: Apoapsis is the launch point and is located at Kerbin, periapsis is the intercept point and is located 180^o opposite the launch point. Since the launch point is Kerbin, and since Kerbin lies in the ecliptic plane, the launch point must be one of the nodes of the transfer orbit. The other node must be 180^o opposite, which is the intended intercept point. Therefore, intercept must take place when Moho is at one of its nodes. A true Hohmann transfer is possible only when Moho will be at one of its nodes at the time of intercept, and when that node is 180^o opposite of the launch point. These conditions are very rare. It's probably never possible to perform an absolutely perfect Hohmann transfer, but with the right conditions you should be able to get close. You just have to be patient and way for those right conditions to arrive.