Unusual Problem with planetary transit

[Rocket Farmer]

Hi. I am totally stock with no mods at all.

So I have launched probably about 2 dozen items at various planets and am very comfortable using the mapping system but I just sent my first item in .21 and I had a strange problem.

I lined up on the planet of choice (jool). The one difference being this is the largest load I've ever launched and it was glitchy coming off the pad. Transit weight started at about 300 tonne.

I burped the nuclear engines at full throttle to get an estimate of my required burn time. It indicated 23.5 minutes. So I set my burn to start at 11:20 before the maneuver point (you always burn shorter than the estimate as you lose weight). I aimed at the blue target and fired all engines and was watching but it hasn't worked.

This time however I have now burned to +16 minutes (5 minutes passed my expectation) and I still haven't finished off the DV countdown nor the timer yet. It jumps around going up or down by 5 seconds and it seems to have stalled with 3 minutes left even though I am at full burn. Also I still haven't got out to the orbital curve I need yet for a encounter with jool.

Lastly and most strangely on my velocity counter is increasing by 1m/s while my dv countdown from the maneavour node only drops by .2m/s while I am aimed directly on the blue target?

Is it due to the large load, is it something different with .21 or am I just glitched?

[Guest]

Make sure you're controlling your vessel from the correct part, one that lies along the axis the engines are on. It sounds like your control reference is the wrong pod or probe.

[Rocket Farmer]

That's a pretty reasonable guess but I checked and I am controlling from the right part. I have used the same part since takeoff. Per the math I had already done one burn and had 1400m/s left to push. I avg. push about 1m/s so 23.5 minutes seemed right. I have now pushed 33 minutes and it still says I am 180m/s short and the orbital graph agrees as I'm nowhere near a jool intercept yet. But I am also now at 3400m/s just off of Kerbal which seems very quick.

So by the velocity meter and my math calculations I have pushed up 2000m/s but per the maneavour node it figures I've only done 1200m/s?

Any other ideas?

[Guest]

vOv You dip into the atmosphere or something? What altitude did your burn start at?

[Vanamonde]

The larger mass you're hauling on this trip is requiring a longer burn. The longer the burn is, the more your current path deviates from the point in space where you placed the original maneuver node. But the burn direction and duration were calculated on the ideal case of an instantaneous acceleration at that point. And so you are finding that your actual burn is deviating from the estimate. Don't worry. It will still get you pretty close. Just plot a mid-course correction to bring yourself back spot-on for the intercept.

[Captain Sierra]

This seems most likely. With the long burns of nukes, it's easy to cut into the atmosphere, catching drag, when starting your burn to early. I trick I use is hold prograde rather than at the blue marker. the curve of your orbit negates pretty much all of the angular deviation and no atmosphere skimming.

[Rocket Farmer]

This seems most likely. With the long burns of nukes, it's easy to cut into the atmosphere, catching drag, when starting your burn to early. I trick I use is hold prograde rather than at the blue marker. the curve of your orbit negates pretty much all of the angular deviation and no atmosphere skimming.

Again a really good thought but no.

I always keep a close eye on my perapsis and in this case it didn't drop below 95K (this was my second burn as in my first I had pushed the first 800m/s).

[Temstar]

What the blue node is telling you is you need to instantly add (correct me if I'm wrong) 2300m/s to your velocity for Kerbin-Jool transfer PROGRADE at that exact spot on your orbit. Both the "prograde" and "at that exact spot instantly" are important. The orbital projection after the burn assumes your burn is impulsive, meaning the delta-V is added nearly instantaneously. For a interplanetary transfer a burn of maybe a few minutes is close enough to be called "instantaneously", but as you already know if your craft has low TWR and it takes 30 minutes to perform the burn then we must modify the process, meaning burning earlier as you already know.

But once we've modified how we perform our burn the blue marker for prograde direction is no longer useful for steering. Remember the velocity change you need to do your interplanetary transfer is calculated "prograde" and not just "in that direction". The blue marker just shows what that direction is if you are able to complete your burn instantly. If your burn is stretched out in an arc on your orbit then you need to burn prograde and not just follow the blue marker.

When you perform a burn for hohmann transfer you really only care about the velocity added to either prograde or retrograde direction. Because over a period of 30 minutes your blue marker will drift significantly as your orbit curves a lot of your thrust at the start and end of your burn goes into changing your radial velocity (negative radial at first, then positive radial, so they cancel out and is a waste of delta-V). That's why dv counter down is moving so slowly despite your absolute velocity increasing at normal rate - your thrust is not in the correct direction to add 100% of the vector in prograde direction.

Imagine in the extreme case your perform your burn on the wrong side of the planet so that the blue marker is exactly on the retrograde marker. You do a 2300m/s burn in the blue marker's direction and instead of heading for Jool you find yourself falling back down to Kerbin and now your delta-V required to reach Jool is 4600m/s. Even though you've spent 2300m/s you've actually increased your delta-V required to reach Jool because you've pointed your engines in exactly the opposite direction of where you need to add velocity, despite following the blue marker.

Edited September 11, 2013 by Temstar

[Diazo]

As Vanamode said, maneuver nodes assume instantaneous acceleration.

The longer a burn gets, the less precise the maneuver node gets due to this effect. On a 24 minute burn, the margin of error within a single pixel on the navball will stack and can cause a large deviation at the burn's end.

I find I have to "stage" my long burn maneuver nodes. Once the burn is mostly complete and the DV is not dropping as fast as I want, I delete my current maneuver node and make a new one a little bit in the future to finish the burn.

Essentially it is the mid-course correction, but I do it right away. That way my mid-course correction burn can move closer to my target and I can use it to control the entry angle, useful for getting scanning probes into polar orbit on arrival rather then having to make an inclination change burn after establishing orbit.

D.

edit: As Temstar said, if your orbit is fast enough that your prograde heading changes significantly during your burn, you have to account for that also. (I tend to give ions a boost from a regular engine to get things started, I'm only so patient.)

Edited September 11, 2013 by Diazo

[arq]

As the above post suggests, manuever nodes assume instantaneous course changes.

An LKO orbit takes around 31 minutes. For burns more than 4-6 minutes, you can't do it cleanly in one burn.

Splitting a 23 minute burn into maybe 13 before/10 after would mean you start out burning when you are nearly on the opposite side of the planet. Anything you do there will be a complete waste in terms of an efficient transfer. You really shouldn't burn more than 20-30deg from a maneuver node (the further off you are, the worse the difference).

What you should do is make one burn from 20-30deg before to 20-30deg after the node, then coast the remaining part of your orbit. This is called a 'periapsis kick' (once your orbit becomes elliptical, you will always be burning near your periapsis). After repeating this process a few times, you will break into an escape trajectory, at which point you should (in Scott Manley's words) burn "hell for leather" until you get the transfer to the other body lined up.

[Rocket Farmer]

As the above post suggests, manuever nodes assume instantaneous course changes.

An LKO orbit takes around 31 minutes. For burns more than 4-6 minutes, you can't do it cleanly in one burn.

Splitting a 23 minute burn into maybe 13 before/10 after would mean you start out burning when you are nearly on the opposite side of the planet. Anything you do there will be a complete waste in terms of an efficient transfer. You really shouldn't burn more than 20-30deg from a maneuver node (the further off you are, the worse the difference).

What you should do is make one burn from 20-30deg before to 20-30deg after the node, then coast the remaining part of your orbit. This is called a 'periapsis kick' (once your orbit becomes elliptical, you will always be burning near your periapsis). After repeating this process a few times, you will break into an escape trajectory, at which point you should (in Scott Manley's words) burn "hell for leather" until you get the transfer to the other body lined up.

Well that might be the issue but I did break this burn into a couple of parts. This last burn is after I have already burned 800m/s on a previous pass but you still could be right. On this burn point I had a 2.5 hour orbit so I was quite near my prograde the entire time (maybe off 20 degrees to begin with tops). It still doesn't seem right that I am only getting .2m/s credit for every 1m/s I burn.

This time around I have lowered my burn down to 13.5 minutes (I kicked extra engines into gear just to test) and I am seeing the same thing. I started at 6:40 minutes before and at my maneuver node it says I have 8.5 minutes left???

I appreciate all the help.

[Temstar]

PE kick is probably not necessary for a 24 minute burn. Remember once you actually start burning you are increasing your orbital altitude and thus your period. As long as you burn prograde and eject in the correct direction you are still nearly 100% efficient in converting your delta-V budget into actual delta-V.

For a long burn this is what I do to ensure correct ejection angle:

1. Make a quick save

2. Take the length of the burn, start the burn at half of the burn time before the ejection point, so for a 24 minute burn I will start 12 minutes before the ejection point

3. Very import: follow your prograde marker at all time!

4. Once the burn is complete look at the ejection orbit

• if ejection path shows I will be dipping down inside Kerbin orbit it meant I started the burn too late, load the save and start the burn earlier
  
• if ejection path shows I will be leaving Kerbin at some positive radial velocity relative to Kerbol it means the burn was started too early, load the save and start the burn later
  
• if ejection path shows I will eject almost exactly on parallel with Kerbin's orbit around Kerbol it means the ejection angle is just right. When this happens your PE should be exactly on Kerbin's orbit for going to superior planet, or alternatively your AP should be exactly on Kerbin's orbit if going to inferior planet
  

When you eject from Kerbin, once you leave the near vicinity of LKO your orbit will be nearly a straight line due to your enormous velocity. For the "just right" ejection angle compare your orbit to Kerbin's terminator, if they are exactly parallel then you've nailed your ejection. As long as your burn is all done prograde (instead of following the unmoving blue marker) and your ejection angle is correct then you're about 100% efficient, even if your burn is done over a wide arc.

It's only when your ejection burn is measured on the order of month that you start to lose out big on not taking advantage of oberth effect. But even then the advantage of an extremely efficient engine might offset that loss. Von Braun once designed a Mars mission called Stuhlinger Mars 1957 which used crafts using ion engine powered by onboard nuclear reactor resulting in extremely poor TWR but extremely high Isp. The spacecraft will spend 4 month! burning prograde in a orbit that looks like a gradually wider and wider spiral until it's finally built up enough velocity to achieve Earth escape velocity.

[wilt57]

It's only when your ejection burn is measured on the order of month that you start to lose out big on not taking advantage of oberth effect. But even then the advantage of an extremely efficient engine might offset that loss. Von Braun once designed a Mars mission called Stuhlinger Mars 1957 which used crafts using ion engine powered by onboard nuclear reactor resulting in extremely poor TWR but extremely high Isp. The spacecraft will spend 4 month! burning prograde in a orbit that looks like a gradually wider and wider spiral until it's finally built up enough velocity to achieve Earth escape velocity.

Very interesting! I once had the same problem that OP is experiencing in one of my longer burns to Duna. I assumed that since my burn vector was pointed towards Kerbin, I'd add some velocity to my orbit by virtue of lowering my orbit closer to the gravity well and when the blue marker raised above the horizon, all that velocity I gained from that radial vector would cancel out the velocity loss to the outward radial vector. Guess I was wrong! Wouldn't be the first time. So it begs to ask, if pointing prograde versus node is better for long ejection burns, wouldn't it be better for all ejection burns?

[M4ck]

Another thing which might cause strange burn/acceleration mismatch: if something lies not too far behind one of your engines, directly behind one, the engine can actually be pushing against it when throttled. It can reduce the effective thrust per unit of fuel by a LOT.

Edited September 11, 2013 by M4ck

[Temstar]

Yes if you want efficiency you should always burn exactly prograde or retrograde in any sort of burn, unless you're doing an inclination change.