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Burn duration limitation and dV loss


Warzouz
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Sending ships to interplanetary efficiently is the heart of a successful mission to the Kerbol system.

With low TWR, I know you can do multiple burns to escape LKO. But they are delicate outside Mun, Minmus, Duna or Eve (maybe Jool). Either way, I'm much mod comfortable with a reasonably high TWR for escaping Kerbin SOI.

I know that the issue is not the really the TWR, but the time you spend burning at LKO.

2 questions

1- I read somewhere that dV loss drastically increase after a certain burn duration : what was it ? What would be a reasonable burn duration at LKO (I usually try to avoid burning more than 3 minutes)

2- As higher orbits are slower, I should be able to burn longer with messing too much my trajectory. Is there a way to have a simple rule for that ?

With those data, I could calculate the TWR I need for a specific burn (dV, orbit) without messing everything...

Thx

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My habit now is to extend the launch stage to cover LKO + 700-800 m/s more. The last launch stage destroys itself by reserving a smallest fuel tank and deorbit using those fuel at apoapsis. Since I use this design I have never worried about low TWR, even with Ions.

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Well, I successfully sent a station with low TWR (0.3 iirc) to Dres with 2 pre-burns, but I'm not too comfortable. I tried for Moho and failed miserably, ended on non recoverable trajectory. For Eeloo, Moho and Jool, I prefer packing a TWR and fuel and do a one burn go.

My issue is not overdoing the interplanetary stage.

For example : I've designed space stations self-powered of pushed by a interplanetary stage. Return vehicles and landers have to depart on their own. As TWR is already low (but still for a 1 burn go) I can't add the landers and return at LKO for a single flight. Each vehicle I attache bring it's own fuel, so dV doesn't drop significantly. But TWR do, hence my question about burn duration.

A 5 minute burn seems quite long, You do nearly 1/4 of an orbit.

BTW is there a quick table to get speed and orbital period on a specific orbit ? (80 ,100 , 120km ....)

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To get an orbital period, you can make a maneuver node just behind your ship and check the time to burn. Poodles are my interplanetary engine of choice, because while they are not as efficient as nukes, they are pretty decent, and they also have a decent TWR. I've sent several missions that using a poodle had maybe 500 m/s less dV than a nuke, and the burn was waaay faster.

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I tried for Moho and failed miserably, ended on non recoverable trajectory.

Remember that when you use maneuver nodes, the longer the burn, the less accurate the node's trajectory prediction will be. This is fine for most planets, but Moho is the exception. It orbits quickly, so unless your burn is short, you'll be far enough off the node's prediction that you'll miss your encounter, and have to spend a lot of dV to adjust (and it can be a lot!). The inclination of Moho's orbit just makes this worse.

So, Moho is one where I'll usually spend the extra funds/effort to get a beefier transfer stage for the Kerbin->Moho burn. (I like to just refuel the core boost stage, since it usually already has good TWR)

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I whipped up a quick table for you:

The Vis-viva Equation and Kepler's Second Law are the math behind calculating these yourself, if you are interested.

Thx

So at LKO, there is not much difference between a 80km and a 150km orbit. Your rule of thumb for a long burn gives 1/6 of an orbit max

For example, if I have a 2500m/s burn to do. If I want to burn 1/12 of an orbit max (half your rule of thumb), it needs a 1.64 TWR at LKO, but only a 0.45 at 1000km.

- - - Updated - - -

To get an orbital period, you can make a maneuver node just behind your ship and check the time to burn. Poodles are my interplanetary engine of choice, because while they are not as efficient as nukes, they are pretty decent, and they also have a decent TWR. I've sent several missions that using a poodle had maybe 500 m/s less dV than a nuke, and the burn was waaay faster.

Well, I like to send big stuff into space. I usually go directly to Rhino :D . Or 2 of them + 8 LVN for my successful Moho trip.

http://tof.canardpc.com/view/017d2bde-716d-4ca8-a3b2-050147fd4dff.jpg

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Thx

So at LKO, there is not much difference between a 80km and a 150km orbit. Your rule of thumb for a long burn gives 1/6 of an orbit max

For example, if I have a 2500m/s burn to do. If I want to burn 1/12 of an orbit max (half your rule of thumb), it needs a 1.64 TWR at LKO, but only a 0.45 at 1000km.

Well, I like to send big stuff into space. I usually go directly to Rhino :D . Or 2 of them + 8 LVN for my successful Moho trip.

http://tof.canardpc.com/view/017d2bde-716d-4ca8-a3b2-050147fd4dff.jpg

The problem with higher orbits is that it takes more dV to get up there, so it is usually better to transfer from LKO in my experience. Not that I will in any way claim to be an expert.

I sent a few ships the size of your upper stage to Duna with only a single poodle each. I'm not sure having all those LV-N's is actually that efficient, because they're really heavy compared to the poodle and other engines. That being said, it was a successful trip, so who am I to judge? But it may be worth using KER to find the most efficient transfer vehicles for your purposes.

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The problem with higher orbits is that it takes more dV to get up there, so it is usually better to transfer from LKO in my experience. Not that I will in any way claim to be an expert.

The tradeoff being that the transfer burn will cost less dV from a higher orbit. You're right in thinking it's a net loss, though.

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The following is something I posted before that I think helps to answer this question. This test was performed prior to v1.0, though that really shouldn't matter. I just added the "burn time" column. As you can see, the losses are pretty small for burns lasting just a few minutes.

I just performed an experiment in which I simulated an interplanetary injection from low Kerbin orbit using different thrust-to-weight ratios. I assumed starting from a 70 km circular orbit and maintained the thrust vector pointed in the prograde direction throughout the duration of the burn. I stopped the burn when the hyperbolic excess velocity reached 1,000 m/s, which is about the median for a trip to Duna or Eve (a little less for Duna and a little more for Eve). I assumed a specific impulse of 390 seconds, though Isp really doesn't make that much difference. Below are the results:

[TABLE=width: 300]

[TR]

[TD]TWR[/TD]

[TD]ÃŽâ€v

(m/s)[/TD]

[TD]Burn

Time

(s)[/TD]

[/TR]

[TR]

[TD]∞[/TD]

[TD]1101.5[/TD]

[TD]0[/TD]

[/TR]

[TR]

[TD]2.00[/TD]

[TD]1102.6[/TD]

[TD]49[/TD]

[/TR]

[TR]

[TD]1.50[/TD]

[TD]1103.2[/TD]

[TD]65[/TD]

[/TR]

[TR]

[TD]1.00[/TD]

[TD]1104.9[/TD]

[TD]98[/TD]

[/TR]

[TR]

[TD]0.75[/TD]

[TD]1107.3[/TD]

[TD]131[/TD]

[/TR]

[TR]

[TD]0.50[/TD]

[TD]1113.8[/TD]

[TD]197[/TD]

[/TR]

[TR]

[TD]0.30[/TD]

[TD]1133.6[/TD]

[TD]333[/TD]

[/TR]

[TR]

[TD]0.20[/TD]

[TD]1167.9[/TD]

[TD]513[/TD]

[/TR]

[TR]

[TD]0.15[/TD]

[TD]1209.2[/TD]

[TD]705[/TD]

[/TR]

[TR]

[TD]0.10[/TD]

[TD]1301.7[/TD]

[TD]1125[/TD]

[/TR]

[/TABLE]

A TWR of ∞ is, of course, a theoretical instantaneous burn. As you can see, for TWRs greater than about 0.50, the losses are very small. However, for extremely small TWRs, the losses start to become significant.

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The tradeoff being that the transfer burn will cost less dV from a higher orbit. You're right in thinking it's a net loss, though.

Very true, but my usual issue is not dV, it's TWR. I can easily pack more fuel on a interplanetary stage, but increasing TWR is harder : you have to add more engines, thus even more fuel. It usually wrecks your design...

So I don't bother with altitude. All my interplanetary stage are overfueled.

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Remember that when you use maneuver nodes, the longer the burn, the less accurate the node's trajectory prediction will be.

Honestly ... I think that is complete ........, although people keep repeating it. The only thing that is really inaccurate here is the estimated burn time. Stock KSP seems to calculate that wrong for long burns. So it might be that you do the burn at the wrong time, which would in fact butcher both your departure time and angle. But in general, it should not matter to your trajectory if your burn lasts 1min or 5min, if you compare that to the time that it takes to actually reach the SoI edge.

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Honestly ... I think that is complete ........, although people keep repeating it. The only thing that is really inaccurate here is the estimated burn time. Stock KSP seems to calculate that wrong for long burns. So it might be that you do the burn at the wrong time, which would in fact butcher both your departure time and angle. But in general, it should not matter to your trajectory if your burn lasts 1min or 5min, if you compare that to the time that it takes to actually reach the SoI edge.

For my understanding, nodes dV and estimated trajectories are calculated on a instant burn (infinite TWR). Then the burn duration is simply calculated from dV and you engine configuration. Even staging is not well integrated into the stock calculation (KER and MJ have better calculation though).

But the burn durations don't change de needed dV. So if you burn slowly (for example, during a full orbit, which is stupid), your trajectory will be a mess.

Someone did calculation of dV loss for extended burns. I can't find it though.

Edit : Stupid me : OhioBob did it few post before ...

Edited by Warzouz
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