Help! :) Is this gonna make into orbit on eve?

[David104]

Hi everyone!

I´m planing a eve mission at the moment and trying to design a ascent vehicle that brings my kerbals back into orbit. There they gonna dock with the mothership and fly back to kerbin. So thats the plan..problem is, this is the first time I try this and I´m really unsure about how much delta v I need. So I just wanted to here your opinions...you think this is gonna make it? Cheers!

[Streetwind]

Budget at least 7,500m/s (vacuum), better 8,000m/s, to launch from sea level into low Eve orbit. However, if you plan to touch down on a mountain, what you have here might be barely adequate.

Also, you can set Kerbal Engineer to use Eve as the reference body to calculate TWR. That will let you plan with more precision. Finally, make sure you plan your TWR while in atmospheric mode, as your engines are liable to lose most of their thrust. Especially if you happen to land at sea level.

Edited August 24, 2016 by Streetwind

[David104]

23 minutes ago, Streetwind said:

Budget at least 7,500m/s (vacuum), better 8,000m/s, to launch from sea level into low Eve orbit. However, if you plan to touch down on a mountain, what you have here might be barely adequate.

Also, you can set Kerbal Engineer to use Eve as the reference body to calculate TWR. That will let you plan with more precision. Finally, make sure you plan your TWR while in atmospheric mode, as your engines are liable to lose most of their thrust. Especially if you happen to land at sea level.

cheers for the answer! KER is set to Eve...atmospheric TWR is around 1,20 at sea level. Gonna try to add a bit more delta v...gonna be tricky..:)

[OhioBob]

On one of my launches from Eve I was able to make it to orbit from sea level for just under 7000 m/s, but I would never recommend trying that.  I think that was a perfect storm of everything happening just right.  I agree with Streetwind that 7500 m/s is a more reasonable budget for an efficient design.  I also see that you are using Twin-Boars and Thuds.  While those engines aren't bad for use on Eve, they're not the best.  I tend to group them in tiers, with Aerospike, Vector and Mammoth being in the first tier, Twin-Boar and Mainsail in the second tier, and Thud third tier (OK ISP, poor TWR).  With those engines you are going to lose quite a bit of efficiency at low altitude.  Streetwind's recommendation of 8000 m/s sounds like a reasonable precaution considering the engines you're using.  Always better to have too much than too little.

If you have no objection to using modded parts, these engines will provide improved Eve sea level performance:  Eve Optimized Engines.

[James Kerman]

I copied this from another thread - apologies to the creator as I cannot give them the credit (cant remember the user) but this might help with stock engine selection.

[fourfa]

8 hours ago, James Kerman said:

I copied this from another thread - apologies to the creator as I cannot give them the credit (cant remember the user) but this might help with stock engine selection.

That's me - no apology needed.  I'll stick my name in the image somewhere

[OhioBob]

That's an interesting chart, very useful for comparing specific impulse.

Some time ago I tried a different method of comparison in which I wanted to see how much Δv each particular engine could produce.  Instead of just looking at Isp, I tried to take into account each engine's mass and thrust.  What I did was to take each engine and pile on fuel tanks until it's Eve sea level TWR ratio was 1.2.  I then simply computed the Δv.  The results showed that the Aerospike, Vector and Mammoth were all about the same and had a clear advantage over all the other engines.  After the top three engines there was a pretty big gap back to the Twin-Boar and Mainsail, with the Twin-Boar having a small advantage over the Mainsail*.  There was then another big gap back to the Thud.  Bringing up the rear was the Reliant, with all other engines being so bad that they were even worth considering.  The two surprises were the Aerospike and Thud, with both performing well below what one might except based on their Isp curves.

* I actually think the Twin-Boar is underweight given its size and power.  If you plot a graph of engine mass vs. propellant flow rate, the Twin-Boar lies off the curve and is underweight by about 1.5 tons.  This (unfair) mass advantage is why it outperforms the Mainsail.

 

Edited August 25, 2016 by OhioBob

[foamyesque]

The Twin-Boar has, I believe, the best TWR of any LFO rocket engine. It carries the same tankage as a -64 tank, so if you subtract that mass, the actual engine produces 2000kN vacuum on 6.5t. That's a better TWR than even the Mammoth, and by a large enough margin that it applies even at Kerbin sea level despite the Mammoth's higher sea-level Isp.

[OhioBob]

2 hours ago, foamyesque said:

The Twin-Boar has, I believe, the best TWR of any LFO rocket engine. It carries the same tankage as a -64 tank, so if you subtract that mass, the actual engine produces 2000kN vacuum on 6.5t. That's a better TWR than even the Mammoth, and by a large enough margin that it applies even at Kerbin sea level despite the Mammoth's higher sea-level Isp.

Yeah, the Twin-Boar is the best in the game.  And the worst for its size is the Rhino.

Rather than looking at TWR, I tend to look at engine mass versus propellant flow rate.  Looking at TWR makes it difficult to compare engines of different classes, i.e. booster engines vs. orbital engines.  Comparing the engines based on how much mass they pump out provides more consistent results.  When plotted on a graph we see that most of the engines lie on a nice straight line, with the Twin-Boar and Rhino being the obvious outliers.  I've actually added a little ModuleManager code to my installation to bring the masses of the Twin-Boar (+1.5 tons) and Rhino (-2 tons) into line with the other engines so that there is no advantage or disadvantage to using them.

[Laie]

TwinBoars used to be an excellent choice for Eve landers. I think of them as resilient self-propelled fuel tanks that can serve as a landing gear; as the outmost layer in an asparagus scheme, only a relatively small fraction of their fuel is actually burned in their (inefficient) engines, and on my landers they were gone in twenty seconds. Their raw TWR is still enough to make them worthwhile.

However, the last time I tried that (1.0.5 perhaps) I found them to be extremely... well, brittle? While the Boars themselves can survive a 20m/s impact, they would not stay attached and come off at the slightest provocation. Six struts per boar and it couldn't hold it's own weight standing on a gentle slope, much less any kind of touchdown.

The testing regime for Eve landers should include a drop on the launchpad (turn destructible facilities off).

[foamyesque]

I dunno why people use vertical-launch/land systems on Eve. Eve is made to screw around with wings.

[David104]

WOW! Couldnt check the forums for a while! You guys are awesome! Thanks for the infos!

I came up with a different design in the end which is already in Eve orbit...gonna upload some pictures once its landed.

@foamyesqueI thought about that as well but as I´m playing in career mode and also have life support installed, for a manned mission, I need to plan pretty excact so my kerbals survive. If to screw around with wings I would need to do a lot of testing which is not really possible. With the vertical-launch I at least more or less know that the rocket is gonna make it because I have the delta-v readouts..

[David104]

Heyho!

Busy times...sorry it took so long. So here is the (most likely the ugliest thing I ever made..haha) beast:

 

Now it sits there waiting for the base (still working on how to get it to Eve :D) for supporting my kerbals until the next window back to Kerbin opens..

[nascarlaser1]

18 minutes ago, David104 said:

Heyho!

Busy times...sorry it took so long. So here is the (most likely the ugliest thing I ever made..haha) beast:

 

Now it sits there waiting for the base (still working on how to get it to Eve :D) for supporting my kerbals until the next window back to Kerbin opens..

I think it looks quite nice .

[DerekL1963]

On 9/10/2016 at 0:20 AM, David104 said:

I thought about that as well but as I´m playing in career mode and also have life support installed, for a manned mission, I need to plan pretty excact so my kerbals survive. If to screw around with wings I would need to do a lot of testing which is not really possible.

That's what the sandbox is for...  Build your vehicle in your career save, then transfer it over to a sandbox for testing.  Modify it, and test again.  Lather, rinse, repeat.    When you've got the design and your mission techniques dialed in, go over to your career save and fly it for reals. 

[David104]

5 hours ago, DerekL1963 said:

That's what the sandbox is for...  Build your vehicle in your career save, then transfer it over to a sandbox for testing.  Modify it, and test again.  Lather, rinse, repeat.    When you've got the design and your mission techniques dialed in, go over to your career save and fly it for reals. 

yeah you´re right...I could do that, but somehow it feels a bit like cheating. Got enough cheating going on with the revert-options..haha

[Foxster]

Just to throw this in for anyone else planning an Eve return mission...

Only part of the problem is the required dV. You can lose a shed-load of dV getting to Eve orbit by using fat parts because of the huge drag in Eve's atmosphere. You can actually get a craft with much less dV into orbit if you stick with mk1-sized parts and the minimum number of stacks you can get away with.  

[foamyesque]

19 hours ago, Foxster said:

Just to throw this in for anyone else planning an Eve return mission...

Only part of the problem is the required dV. You can lose a shed-load of dV getting to Eve orbit by using fat parts because of the huge drag in Eve's atmosphere. You can actually get a craft with much less dV into orbit if you stick with mk1-sized parts and the minimum number of stacks you can get away with.  

 

For drag minimization purposes you want long and skinny, but that doesn't necessarily mean 1.25m stacks. If you're driving something substantial you need to move to 2.5m or 3.75m parts to avoid something that's too long to land or fly well.

[David104]

On 19.9.2016 at 6:02 PM, Foxster said:

Just to throw this in for anyone else planning an Eve return mission...

Only part of the problem is the required dV. You can lose a shed-load of dV getting to Eve orbit by using fat parts because of the huge drag in Eve's atmosphere. You can actually get a craft with much less dV into orbit if you stick with mk1-sized parts and the minimum number of stacks you can get away with.  

Thanks for the tip! How much dV are we talking about? Do you have maybe an example-craft?

Its the first time that I try an Eve return so any tip for the next mission is very welcome.

[Foxster]

1 hour ago, David104 said:

Thanks for the tip! How much dV are we talking about? Do you have maybe an example-craft?

Its the first time that I try an Eve return so any tip for the next mission is very welcome.

Something like this will get you down to sea level and back to orbit with a bit over 7000dV at the surface...

 

Edited September 22, 2016 by Foxster

[David104]

7 hours ago, Foxster said:

Something like this will get you down to sea level and back to orbit with a bit over 7000dV at the surface...

 

thank you! Why air-intakes?

[Foxster]

1 hour ago, David104 said:

thank you! Why air-intakes?

Lowest weight and lowest drag of mk1-size "nosecones".