Very high burn time

[Portmonde]

Hi guys.

I've recentrly crafted, in the hangar and in orbit around Kerbin, my biggest ship. It was intended to beat the Jool-5 challenge. I got him out of Kerbin's orbit with a mainsail, and I had already noticed it was longer than usual. Then around Kerbol's orbit, I did a maneuver node, the delta-v needed was around 350m/s, and my engine was the nuclear one, that thrusts at 60kN. Only, the burn time was over 15 minutes, and it wasn't a display bug from KSP. Is this normal? I thought in space the weight of a ship didn't matter. I precise I have no particular mod. I've already put about 200h into the game and this problem has been so frustrating I signed up here. Anyways, I really need your help, kerbonauts!

[Reactordrone]

That's normal. Thrust to weight ratio determines acceleration so a small engine will take longer to do the burn. I did a low solar rescue the other day and had 6000m/s burns that took more than half an hour.

[Gargamel]

51 minutes ago, Portmonde said:

Hi guys.

I've recentrly crafted, in the hangar and in orbit around Kerbin, my biggest ship. It was intended to beat the Jool-5 challenge. I got him out of Kerbin's orbit with a mainsail, and I had already noticed it was longer than usual. Then around Kerbol's orbit, I did a maneuver node, the delta-v needed was around 350m/s, and my engine was the nuclear one, that thrusts at 60kN. Only, the burn time was over 15 minutes, and it wasn't a display bug from KSP. Is this normal? I thought in space the weight of a ship didn't matter. I precise I have no particular mod. I've already put about 200h into the game and this problem has been so frustrating I signed up here. Anyways, I really need your help, kerbonauts!

Do the math.  What is the mass of your ship, what's your ships TWR?    So if you figure that out, you can easily see why it takes 15 minutes to execute this burn.

But simply, your ship is WAY underpowered. 

Correct?  Yes, the game is functioning correctly.  Normal? No, but that's on your end.  

The mass of a ship does matter, weight has no bearing on a ship until it is resting on the ground.   

2 minutes ago, Reactordrone said:

6000m/s burns that took more than half an hour.

His TWR would have taken ~4.2 hours to complete your burn.......   I don't call that normal, that's underpowered. 

Edited May 4, 2018 by Gargamel

[Deddly]

1 hour ago, Portmonde said:

I thought in space the weight of a ship didn't matter.

This is where the error came in. Although weightless, your ship still has mass that needs to be accelerated. If it's difficult to visualise, let's take it to the extreme: If you take a rocket and fire the engines downwards, does it send the Earth off on a new orbit? The answer is actually yes, it does - but the new orbit it virtually the same as old one because of the enormous mass of the planet.

The greater the mass, the more energy it takes to alter its course. For this reason, you will notice that the acceleration of your craft will improve as you use up fuel, because the mass is decreasing as the fuel burns. 

15 minutes for a burn isn't all that bad (you can time warp at 4x to cut that down to less than 4 minutes), but I'm more concerned about your injection burn to get into orbit at your destination... you might want to consider adding more engines or reducing mass.  

Welcome to the forum! 

[Portmonde]

1 hour ago, Gargamel said:

Do the math.  What is the mass of your ship, what's your ships TWR?    So if you figure that out, you can easily see why it takes 15 minutes to execute this burn.

Actually my TWR is about 0.06. No need to do the maths though, i have a mod for that.

 

1 hour ago, Deddly said:

15 minutes for a burn isn't all that bad (you can time warp at 4x to cut that down to less than 4 minutes), but I'm more concerned about your injection burn to get into orbit at your destination... you might want to consider adding more engines or reducing mass.  

This is right, but time isn't my problem, fuel is. Also, i can't really 4x since my ship contains a lot of docking port, that wreck it above 1x.

 

Thanks for the explainations guys!

[Laie]

1 hour ago, Portmonde said:

Only, the burn time was over 15 minutes, [...] Is this normal?

As has been pointed out a few times, it is normal that a single small engine will take a longer to do the burn.

1 hour ago, Portmonde said:

I thought in space the weight of a ship didn't matter.

And depending on how you look at it, this is at least a half truth. For many deep-space maneuvers, it makes little difference whether you can perform them quickly or will take a while.

Maneuvers in orbit around the Sun are examples where low TWR barely matters. Maneuvers near a deep gravity well, however, are somewhat time-sensitive. Quite often you can work around the issue by (e.g.) planning a series of maneuvers starting a good deal ahead of time, though this demands more dV than a single short burn performed at just the right moment.

Going from a eccentric orbit around Tylo to a low circular one can likewise be achieved over the course of several orbits, doing a 3-minute burn every time you pass periapsis. This will cost time and effort, but it will *not* demand extra dV expenditure.

If you have brought some extra dV (like, a 10% safety margin) you should still be able to execute your Jool-5 mission. If the extra effort in maneuvering (an waiting for long burns) doesn't deter you, that is. Also, the situation will improve as you burn off fuel and detach landers. I don't know your vessel but expect that about half of it's mass will be gone after the Tylo landing; this will increase TWR and reduce the need for maneuvering shenanigans.

 

[Gargamel]

36 minutes ago, Portmonde said:

Actually my TWR is about 0.06. No need to do the maths though, i have a mod for that.

Unless I'm building an Ion based probe, I really hate to have my ships have less than a .2 TWR.  The higher the TWR I can have without losing efficiency, the happier I am. 

 

The problem with low TWR craft, as you have discovered, is the burn time.   If you are in a high orbit, and doing a transfer burn, this is not a problem really.  You'll get to where  you are going, eventually.   But when you are entering the SOI of another body, and have to do the circularization burn, the amount of time you are required to do this burn may be more time than you have.  At best you'll have to do a series of burns, with your first one enough to achieve orbit, and the rest at periapsis to lower your apoapsis. 

Another issue with long burns times is your starting altitude.   If you are in a low enough orbit, and the transfer burn is long enough, you might end up burning towards the moon for too long, and come perilously close to it's surface. 

 

42 minutes ago, Portmonde said:

This is right, but time isn't my problem, fuel is. Also, i can't really 4x since my ship contains a lot of docking port, that wreck it above 1x. 

You can use physics warp while burning.  Alt-. to increase your physics warp.   This shouldn't wreck your ship, but YMMV.   If you are having kraken issues with a physics warp, try the KJR mod, redesign the ship to have less ports, or use the Konstruction mod docking ports which are weldable.   The Konstruction ports will be removed from the ship when you weld them, increasing the stiffness of the ship. 

[Deddly]

2 hours ago, Portmonde said:

This is right, but time isn't my problem, fuel is.

In that case, one engine like your setup is the most efficient for most cases. Extra engines are also extra mass, which negatively impacts efficiency

[Kerbart]

This is what engineering is all about. Designing is often balancing conflicting requirements — in this case burn time vs fuel needs.