A question on the efficiency of a long burn

[king of nowhere]

9 hours ago, rynther said:

(6 day burn, I suppose that mod allows warp during burn?)

yes, exactly. unfortunately it also introduces some bugs, sometimes you can't time warp at all, sometimes your engines shut off even though they have plenty of fuel. My penchant for huge mothership doesn't help either

 

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It takes about 2.5 hours to run a test at eloo (the sailboat I just tested had something like 25 gigantors, generating 4 per sec at "peak angle", so well less than 1/ps for each fully exposed gigantor at eloo.) Still, I'm a pretty clever lab rat, there's a couple things I haven't tried yet.

For those kind of missions, i started to set up solar panes on trusses, so i can distance them more from the body of the spacecraft and they can all be exposed 100%. The Dream Big, my former mothership for that, had 64 gigantors exposed all the time, which could increase to 112 if i attached the shuttles in the right way. Of course, the Dream Big was 1500-4500 tons, i wouldn't try to use ionic propulsion for that. I needed all that energy to run greenhouses and feed my crew of 12.

For smaller ships, i use EVA construction to move around the solar panels, so whichever direction i have to point to do the burn, i can still get full power out of all of them.

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Still, I don't hold out any hope that this would scale to 1500 tons, and not in a form that would carry you beyond duna anyway. Meanwhile, a challenge is a challenge, how big can the xenon systems scale? and what does it do to counter the lack of sun at eloo in a not horrible fashion?  I don't really do kerballed missions, I fly into things too much, but it might be fun to setup a much smaller group of probes , dropping them off would still be quite a trick, even more so, because I do not fly with the least amount of elegance, the fuel would be monumental.  (my "super probe" could probably visit 4 maybe 5 planets, but not land anything, and not much science on board)

The peak power point for the xenon engines seems to be a 3 thruster system, at least for deep space. (the only configuration that can yield 1.8m/s with long range in the dark.)

Anyway, I guess I'm off to play with fuel cells for the moment, thanks for the tip on fusilages, I'll be playing with the nervs next.

 

fuel cells are a legitimate alternative. someone calculated that the fuel cell/ion engine system has Isp 1337, so still better than nuclear. the downside is that you still get the low thrust of ions.

renewable power sources can't do much at eeloo in any case. the number of rtg it would take to feed ion engines would be unacceptably heavy after a while. But as i said, at eeloo your orbits are slow, so you can afford long burns. my 6 day burn went all right. you just need a persistent thrust mod. i uninstalled it for the bugs it was causing, but i still have it stashed on my hard disk, to reinstall for such an eventuality

[rynther]

I think a "branched tree" xenon mother ship still has legs, I made a somewhat practical 30 ton model with an "any direction" solar array. (the "bad" angle drains the batteries at 10/sec, which yields a 15 minute max burn, at 1.8m/s, 7km/s on-board.) As a munar tug boat, it's not a bad solution.

The solar array is a problem for scaling, for a mothership I might go with full on fuel cells and save myself the lift nightmares of a 15m radius, haven't tried that yet. (fuel cells are also bulky)

If I did a grand tour, it would be way scaled back from 1500 tons, and no kerbals to kill, but I want a munar mining facillity. (cooling a convert-o-tron, can it be done?)

In the most recent update (1.11.x) it seems a lot of mods are broken. (they did fix a bunch of stuff in the stock game)

[rynther]

The magic number is 1 fuel cell per 2 motors, and indeed looks 1337, I tested it with a 6.3 ton model, 6 burners, 3 fuel cells and a t-200 worth of fuel for the cells, it managed to burn a bit over a third of the xenon. (8103m/s starting tank, at 1.8m/s) This should not only scale pretty well, but "tree" pretty easily,  making detachable segments that stack  and thrust together. (no idea of how to maneuver such a thing, not that far along in the process, but it's an interesting question.) I guess I'll get some practice docking stuff playing with it.

[moar ssto]

The best the way to deal with low twr burn is just to avoid using any substancial final ejection(by sending it onto a gravity assist chain, and avoid any direct transfer to non adjacent bodies), and split the orbit raising burns into many miniburns that conserves aop and pe height. This should reduce the losses to something basically negligable, reguardless you use prograde facing or node facing burn.

Prograde facing burn is not neccessarily the best, since you will rasie your periapsis, which reduces the oobert effect on later burns.  The  net cosine losses associated with not pointing prograde is only O(single burn time^3), so really small for short burns. And when trasfering to a target with relative inclination, an innaccurate ejection burn can cause significantly large correction burns.

Edited April 2, 2021 by moar ssto

[CBase]

As others mentioned already: Going prograde is most efficient as it turns all thrust into kinetic energy.

23 minutes ago, moar ssto said:

Prograde facing burn is not neccessarily the best, since you will rasie your periapsis, which reduces the oobert effect on later burns.

I do think you are messing things up: If you always do prograde burns at PE you are rising only AP, not PE.

The main problem with longer prograde burns is timing and planing: The true curve of any prograde burn is a spiral not an ellipse. The instantanoues change abstraction from in game planning will fail as well as symmetric burn timing. Fixing wrong orbital orientation at the end is way more expensive on delta v than loosing small amounts for not burning prograde, thats why following the burn indicator is a good advice for most burns.

For burns longer than a 6th of my orbital period I am using an old tool that I found here some time ago, which helps to calculate the asymmetric burn start offset, but you still want to break down very large burns into several to keep most from Oberth effect:

 

Another nice information from the tool is how much gravitional delta v loss you have, which increases when burn duration increases.

[moar ssto]

9 minutes ago, CBase said:

As others mentioned already: Going prograde is most efficient as it turns all thrust into kinetic energy.

I do think you are messing things up: If you always do prograde burns at PE you are rising only AP, not PE.

The main problem with longer prograde burns is timing and planing: The true curve of any prograde burn is a spiral not an ellipse. The instantanoues change abstraction from in game planning will fail as well as symmetric burn timing. Fixing wrong orbital orientation at the end is way more expensive on delta v than loosing small amounts for not burning prograde, thats why following the burn indicator is a good advice for most burns.

For burns longer than a 6th of my orbital period I am using an old tool that I found here some time ago, which helps to calculate the asymmetric burn start offset, but you still want to break down very large burns into several to keep most from Oberth effect:

 

Another nice information from the tool is how much gravitional delta v loss you have, which increases when burn duration increases.

But burns are not instantenaous, so pe will rise, I'v seen someone doing coulple hundred prograde burns and found himself needed to make an additional node to lower the periapsis again to the desired altitude.  

Maneuvers are not just about instantenous energy efficiency, but also about choosing a path that gives more energy efficiency. Burning in prograde, will cause you maximise instantanous energy efficiency at this given velocity and position, but if you can keep yourself on a lower, faster trajectory forehand, you may enjoy better efficiency. For the same reason, vehicles taking off from vacuum bodies don't use gravity turn, but rather picth up and fly as low and as horizontal as possible near the surface.

Edited April 2, 2021 by moar ssto

[CBase]

True a minimal raise on PE will happen, however shifting around argument of PE is probably always more expensive.

4 minutes ago, moar ssto said:

I'v seen someone doing coulple hundred prograde burns

what ? How low is the TWR if you can do a couple hunded long duration burns without escaping SOI ? Gravitional losses must be huge once you finally do the escape

[moar ssto]

1 hour ago, CBase said:

True a minimal raise on PE will happen, however shifting around argument of PE is probably always more expensive.

what ? How low is the TWR if you can do a couple hunded long duration burns without escaping SOI ? Gravitional losses must be huge once you finally do the escape

following maneuver nodes is a good way to preserve your argument of periapsis.  That small radial component tries to act againts shifting aop by the prograde component not appliead at pe, of course there are limitations, for long burn time, neither method will conserve aop. and if your pe rises, even if you can keep it at the same argument, with the same final specific orbital energy, the eccentricity, hence ejection direction ,will be different from what you have pulled with the node. That will result in change in timing of your arrival.

For that particualr burn, he didn't escape soi, just transfering to the Moon. And that is the reason you should avoid sizable final ejections. In the lower end, you need to do multiple munar/lunar assists to get to venus/eve.

Edited April 2, 2021 by moar ssto