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How does one read the m/s levels on spaceplanes?


miklkit
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I've been building spaceplanes and have a problem figuring out how to get their range in m/s to show correctly.  The numbers are just all over the place! 

I know about putting each type of engine in its own stage, but that is no help.   This is one in the hangar and it is showing 1356 m/s.

JGXFEja.jpg

Here it is taking off and showing 11474 m/s.  The numbers are different again when in space, even after refueling on Minmus so the tanks are almost full.  Is there any way to get consistent readings so I know what kind of range they have?

4f5ie06.jpg

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I see several issues in your screenshots which can be corrected.

- In your first photo, you only have 4 engines in the first stage.  It looks like there should be 8 engines in the first stage. 
- The second photo also shows incorrect staging.  The 8 active engines are in the second stage, and 4 inactive engines in the first stage.  You should correct the staging discrepancy if you want accurate dv info. 
- Your Rapier engines have two modes, air-breathing and closed-cycle. Although your screenshots do not provide enough information, I suspect the engines are defaulting to closed cycle in the hangar.  
- If you want air-breathing dv in the hangar, you need to set all the engines to air-breathing.  

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Put all your engines on the same stage and set the thrust limiters of the ones you want to ignore to zero.

This works with KER and MJ, not sure about the stock DV meter (I disabled mine because it’s a performance hog)

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Ack!  I was not notified that there were replies.  Apologies.

@18watt -

1)  All my spaceplanes are staged like that, and there is a reason.  The Rapiers and NERVs must be separated or the burn times given will be waaay off.  I place the NERVs in the 1st stage to get accurate burn time readings in space, and the Rapiers are in the 2nd stage because they have to go somewhere. 

2)  The Rapiers are toggled on and off with action group 1, mode is toggled in AG 2, and the NERVs are toggled on and off with AG 3.  The Rapiers are set to air breathing by default. 

I should not have posted that 2nd screenie as it shows the m/s of the air breathing Rapiers.  Perhaps this one illustrates my issue better.  It is the same spaceplane heading out under NERV power.

MYrmV7j.jpg

@splashboom - I have never built a spaceplane with just one type of engine.  What a novel idea.

@FleshJeb - Adjusting 8 engines sounds clumsy.  

What I am trying to find out is the actual m/s of these spaceplanes to see if they have the range to do a round trip to Eve and Duna.  They should be able to get there, but I don't know if they can make it back.  The m/s readings vary from around 1400m/s to almost 6000 m/s with full tanks.

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You’re seeing different numbers because air-breathing engines are much more efficient than pure rockets in terms of ISP, so their delta-V is much greater. The Rapier in particular has variable efficiency with speed so the numbers will change in flight, then switching over to rocket mode you’ll get a totally different number again. And of course, atmospheric versus vacuum ISP for something like the NERV makes a HUGE difference.

Spaceplanes are tricky, since unless you’re using rockets all the way to space (bad idea), using only NERV rockets (difficult due to weight and low thrust) or else using propellers for atmospheric flight, you’ll have two different fuel mixes to contend with: liquid fuel and oxidiser, and liquid fuel and air. Getting the ratios right can be difficult since in both cases liquid fuel is required- pack too much oxidiser and you’re carrying around dead weight, pack too little and you could run out. All this assumes you’re in an atmosphere with oxygen in it, which Eve and Duna don’t have; propellers are useful here, but their top speed is limited.

One reason for using NERVs on spaceplanes is that they simplify the fuel arrangements significantly by removing the oxidiser requirement. You’ll still need some “proper” rockets to boost out of the atmosphere due to the NERV’s low thrust, but if you’re already using Rapiers then that’s taken care off.

The real question is, what is this spaceplane for? Carrying all that plane-related gubbins about with you will eat into your delta-V margins and unless you’re going somewhere with an atmosphere they’re just dead weight.

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7 hours ago, miklkit said:

Ack!  I was not notified that there were replies.  Apologies.

@18watt -

1)  All my spaceplanes are staged like that, and there is a reason.  The Rapiers and NERVs must be separated or the burn times given will be waaay off.  I place the NERVs in the 1st stage to get accurate burn time readings in space, and the Rapiers are in the 2nd stage because they have to go somewhere. 

2)  The Rapiers are toggled on and off with action group 1, mode is toggled in AG 2, and the NERVs are toggled on and off with AG 3.  The Rapiers are set to air breathing by default. 

I should not have posted that 2nd screenie as it shows the m/s of the air breathing Rapiers.  Perhaps this one illustrates my issue better.  It is the same spaceplane heading out under NERV power.

MYrmV7j.jpg

 

What I am trying to find out is the actual m/s of these spaceplanes to see if they have the range to do a round trip to Eve and Duna.  They should be able to get there, but I don't know if they can make it back.  The m/s readings vary from around 1400m/s to almost 6000 m/s with full tanks.

the readings seem very correct. you are using nervs, so you hare making full use of your liquid fuel and high isp. the staging is set to use the rapiers after the nervs are exhausted; but when the nervs have run out of liquid fuel, then you have no more liquid fuel for the rapiers either. 4000 m/s looks right for a spacecraft of that kind. which is just right for a round trip orbiting eve and then duna, and then returning to kerbin.

anyway, as a general rule of thumb, when you have different engines with different fuel requirements and different performances, then the stock deltaV becomes very unreliable. there's nothing to be done about it, it's just too complex a system.  personally, i do the manual calculations with the rocket equation whenever i have something more complex than a single rocket.

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9 hours ago, miklkit said:

@18watt -

Yes, unfortunately KSP can (and does) get confused with multiple engine types, even in flight.  I have not found a way to make the dv value work 100% of the time for vessels like that.

If you really want to be sure how much dv you have, you may have to break out your calculator.  You’ll need to know the following values:

  • The starting mass of your ship
  • The ending mass of your ship.  Take the starting mass, and subtract the mass of the LF you have onboard.
  • The specific impulse of the engines you are using.  For Nukes that is 800 s.

DV = ln(start mass - end mass) * Isp * 9.81 

I don’t use mods, but I suspect some of the common mods may give you a more accurate dv reading than what KSP does.

Here’s a link to a page which has some common equations you might find handy..

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Thanks for the replies.  Since I have a gas station on Minmus methinks the best way to find the true m/s is to go there, refuel, and then check it after takeoff.   I have already done that and get readings close to 6000 m/s.  Since I am a lousy pilot it will take more fuel than that for a round trip with any safety margins, and I don't want to strand any Kerbals.  I am finding that Delta V chart to be wildly optimistic.

Why spaceplanes?  Rockets offend my sensibilities.  I send this big giant machine out there and only a tiny capsule comes back.  With spaceplanes what I send out comes back.  This one is my attempt to break out of the Kerbin system and go interplanetary.  Hopefully.

I need to find out what KER and MJ are.

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@miklkit You should only have to do it once in the editor. I would empty all your oxidizer tanks, and thrust limit the Rapiers. That will give you your max DV in space.

Kerbal Engineer Redux is the most universal and accepted mod for DV calcs in the editor and informational readouts in flight.

This is the latest version: https://github.com/jrbudda/KerbalEngineer/releases

It still won’t necessarily solve your problem the way you want it solved because it doesn’t understand intent.

If you want to figure out range AFTER using the Rapiers in Closed-Cycle, assume that all the oxidizer is gone, and (9/11) * Oxidizer of your liquid fuel is gone. Set the tanks to the appropriate levels in the editor, set the Rapiers to 0 thrust, and any DV calculator will spit out the right answer. (Be sure to lock any tanks on your payload, if any.)

Yes, it’s a bit of work, but you only have to do it one time. Be sure to note the tonnage of liquid fuel from the above if you want to do mid-mission calcs by hand. None of the calculators do well with switching engines on and off via action groups in flight.

For the DV maps, I add a safety factor of 20%.

EDIT: Spaceplanes are just harder operationally...That's why I like them. This is a really good thread:

In addition some common rules of thumb are to aim for 1 Rapier per 15-40 tons of plane (15 is reasonably easy, over 30 is very hard), and 1 Nerv per 2 Rapiers, or 2 Nervs/3 Rapiers.

 

Edited by FleshJeb
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Yeah, the biggest problem I have figuring Delta V now is how it changes as fuel burns off.  In my heavier MK3s it can actually go up as fuel burns off. 

That rule of thumb on engines is situational.  I'm running the Rapiers in the 20-35 ton range.  Just had a scare as I landed one of the heaviest ones on the Mun and found that 4 NERVs were just barely stronger than the Mun's gravity and the Rapiers are what slowed it down and landed it.  So now one has 5 NERVs and another has 6. 

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24 minutes ago, miklkit said:

Yeah, the biggest problem I have figuring Delta V now is how it changes as fuel burns off.  In my heavier MK3s it can actually go up as fuel burns off. 

I've taken to minimizing the amount of oxidizer capacity, or just going pure-LF only. I enjoy the long, slow ride to orbit though.

You'll probably be interested in Editor Extensions, RCS Build Aid, and CorrectCoL. Really useful for us spaceplane zealots. Links are in my sig.

31 minutes ago, miklkit said:

That rule of thumb on engines is situational.  I'm running the Rapiers in the 20-35 ton range.  Just had a scare as I landed one of the heaviest ones on the Mun and found that 4 NERVs were just barely stronger than the Mun's gravity and the Rapiers are what slowed it down and landed it.  So now one has 5 NERVs and another has 6. 

Have you tried a https://wiki.kerbalspaceprogram.com/wiki/Constant_altitude_descent? If I did my math right (arccos 305/800), you can point up to 67deg above retrograde with the Nervs before you have the same effective ISP as the Rapiers thrusting pure retro. By coincidence, that's about the starting angle you need to land with a local TWR of 1.1 (arcsin 1.0/1.1 = 65 deg).

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I carry enough oxidizer to get to orbit and have some left over for landing assists if needed.   My spaceplanes can't get to orbit on NERVs alone. 

What I meant by saying the NERVs were just barely stronger than the Mun's gravity is that when coming down vertically at full throttle it was slowing down by a tenth every once in a while.   The Rapiers prevented a crash.  It does fine at Minmus though.

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15 hours ago, miklkit said:

 Just had a scare as I landed one of the heaviest ones on the Mun and found that 4 NERVs were just barely stronger than the Mun's gravity and the Rapiers are what slowed it down and landed it.  So now one has 5 NERVs and another has 6. 

Be careful: NERVs are very heavy, and their own mass will quickly eat into your deltaV to the point that they won't be convenient anymore. I once made a Vall lander with NERVs, and 70% of its dry mass were the engines. I later run some calculations, if I had used chemical engines I'd have saved so much mass, I'd have gained deltaV even with the lower Isp. In my experience, for a large ship it's better to keep the NERVs TWR around 0.1-0.2 (kerbin), and use some chemical engines to have higher thrust for landings.

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That particular craft weighed 221 tons and 4 NERVs just aren't enough.  It now weighs 241 tons and flies much better.  It was actually slowing down when heading out.

I guess the only way I can find out if it has the range is to just send it out when a window to Duna opens.  There is a relay satellite in orbit there now and it took 2300 m/s to get it there.  That is the M/s goal i need to achieve and methinks this one can do it.

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6 hours ago, miklkit said:

There is a relay satellite in orbit there now and it took 2300 m/s to get it there. 

Those are Jool-numbers. Duna should be MUCH less from low-Kerbin orbit. I will respectfully suggest that you have a planning/timing issue.

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7 hours ago, miklkit said:

That particular craft weighed 221 tons and 4 NERVs just aren't enough.  It now weighs 241 tons and flies much better.  It was actually slowing down when heading out

If your craft is properly drag optimised, 55 tons per Nerv should not be too much to get into orbit, even with zero Rapier closed cycle. I've gotten the best results with 3 Rapiers and 2 Nervs for a 110 ton craft, so doubling it would give 6 Rapiers and 4 Nervs for a 220 ton craft.

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1 hour ago, FleshJeb said:

Those are Jool-numbers. Duna should be MUCH less from low-Kerbin orbit. I will respectfully suggest that you have a planning/timing issue.

The built in planner and MechJeb agreed it was the way to go.  I expected to have a lot of fuel left over but instead it had just enough.  I would be thrilled if the next flight does better.

Oh it gets into orbit just fine.  It is sitting on Minmus now waiting for a window to Duna to open.

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