Estimating atmospheric dV and where to place staging?

[ghpstage]

In my latest career i've been playing with 20% rewards for everything, while also banning myself from using the outsourced R&D and avoiding contract grinding as much as possible.

Early in the career I started with the usual, the altitude and orbit missions, and followed by high space science goo, materials and pod launches, and then manned flybys to the Mun, carrying science equipment, and Minmus without. Then I sent a one way, battery powered probe to the Mun to land and send mat lab results to clear the contract.

At this point however I hit a serious bottleneck. With Electrics I culd send a probe to Minmus and do a few satellite missions around Kerbin, the Mun and Minmus to get some money for upgrades without feeling guilty, but I needed nearly 90 science for that. I realised how much better it would have been if I could have taken science parts on the Minmus flyby, or if I could have returned that material lab probe I landed on the Mun to make the 4 science I gained into 20, which would have made return trips to other biomes potentially well worth it!

For the first time I started using the dV equation and found that the dual liquid stages, and the solid rocket stages (I did expect SRBs would be poor here, just cheap) I had used on previous probe designs appeared to be seriously cutting in to the dV I could achieve on paper. In practice the difference wasn't as big as it appeared due to atmospheric effects, take off TWR and not having a system for estimating ISP, but it was certainly there.

It also nicely cut down on parts counts and surprisingly didn't drive costs up vs SRBs as much as I had expected.

On that note, I have some questions on improving efficiency in design and flight,

1. First is how to estimate the ISP of single stage of rocket lifting from sea level to sub orbital space. With values for sea level and vacuum, the majority of the burn occurs in the lower regions of the atmosphere, so how does 0.2 or 0.3 times the difference between ASL and vacuum values sound?
   
2. With dV calculations is a total dV calculated using Kermin's gravity valid for a trip back from the Mun or Minmus? From other bodies? Or do you need to use the local gravity to compare with values on the dV map?
   
3. Are there any rules of thumb, or quick calculations regarding the when's and where's of staging?
   
4. TWR. I think I have seen a TWR of around 1.3 being said to be good around here somewhere, but I don't remember where nor the context. Would 1.3 be a good launch TWR, and if so how much can you deviate from it without serious losses?
   
5. For landing on a body with no atmosphere does it really matter where you kill the orbital velocity, aside from the size of the suicide burn required?
   
6. For a return from the Mun what's the most fuel efficient method. Burn straight up from the retrograde side, or go into orbit and burn to raise the retrograde apoapsis from there?
   

Edited December 31, 2014 by ghpstage

[UmbralRaptor]

1. My rule of thumb is 0.2 atm for an SSTO, 0.5 atm and then vacuum for a 2STO.

2. 9.82 is a unit conversion factor, and should be used in all cases.

3. Ignoring TWR considerations, ÃŽâ€V for a stage in a multistage rocket should be vaguely proportional to Isp. In practice you can't ignore TWR, though.

4. A 1.3 TWR is adequate, and deviations should probably be upwards if anything (to 1.8ish)

5. As low as possible. An ideal suicide burn is entirely horizontal. In practice, it needs a small vertical component due to finite TWR and planets/moons having mountains.

6. Reverse the horizontal suicide burn to exit the Mun's SOI on the trailing side. Losses from entering a low (say 7-15 km) parking orbit or suborbital trajectory to actually line up for the exit burn are ignorable.

[LethalDose]

Yikes, theres a lot here.

First, I'd recommend getting Kerbal Engineer Redux ("KER") to get stage dV and TWR.

Instead of trying to calculate average ISPs for engines, I'd just recommend building lifting stages with about 4.5 km/s of atmospheric dV (~ 3.5 km/s for FAR). These are just good and well accepted values from tons of players' experiences. YOu can stage them however you want, pretty much. Experiment and see what happens. It's a good way to get some practical knowledge instead of just being told the "right" way, because there are tons of "right ways".

Lots of people have lots of opinions on what the "right" TWR is for lift off. I use FAR and usually aim for 1.1 - 1.25. In stock, it's typically a bit higher (I think usually 1.3-1.7), but never over 2.

For landing on airless bodies, it's typically more efficient to kill your velocity at lower altitudes, because there's less time for gravity to speed you back up again.

The most efficient way to exit the Mun is to take off going east, and then burn to exit velocity a little bit before your vessel is right between the Mun and Kerbin. BUT If you're already on the backside of the moon (near the twin crater) and you have a good TWR (> 3 on the Mun), then burning straight up costs just a bit more dV than the "perfect" return (it's so little, you wont notice). It's best to burn to get your targeted Kerbin Periapsis while you're still around the Mun.

Hope that helps.

[Anquietas314]

TWR. I think I have seen a TWR of around 1.3 being said to be good around here somewhere, but I don't remember where nor the context. Would 1.3 be a good launch TWR, and if so how much can you deviate from it without serious losses?

You want TWR to be around 1.6-2.0 at launch (much higher means you'll either be fighting drag or have to throttle back). If you have FAR/NAR installed 1.3 is closer to the mark from what I hear. Deviating much in either direction means a lot of deltaV lost to drag or gravity. Apparently with FAR as long as your rocket's aerodynamic there isn't really an upper speed limit though.

For a return from the Mun what's the most fuel efficient method. Burn straight up from the retrograde side, or go into orbit and burn to raise the retrograde apoapsis from there?

I'm not sure if it's really the most efficient method, but what I do is launch towards a retrograde (west) orbit and then burn for an escape that drops me outside of Mun's SoI further out than Mun is. That way you don't need as much deltaV to return to Kerbin (still best to do the deorbit burn inside Mun's SoI though!).

EDIT: Nope. Definitely not the most efficient. You're better to use a prograde orbit as others have suggested (see this post for data)

Edited December 31, 2014 by armagheddonsgw

[OhioBob]

1. First is how to estimate the ISP of single stage of rocket lifting from sea level to sub orbital space. With values for sea level and vacuum, the majority of the burn occurs in the lower regions of the atmosphere, so how does 0.2 or 0.3 times the difference between ASL and vacuum values sound?

I think most people just use the vacuum ISP in their calculations. For instance, it is typically stated that it takes 4550 m/s to reach Kerbin orbit, which is based on vacuum ISP. The loss resulting from the reduction of ISP due to atmospheric pressure is about 150 m/s. In other words, it takes about 4400 m/s to reach orbit if you perform the computation using the actual instantaneous ISP integrated over the ascent. I find that if you use the average of sea level and vacuum ISP for the lower stage, and vacuum ISP for the upper stage, you'll get a pretty good estimate of what the actual ÃŽâ€V will be. It easier just to use vacuum ISP for all calculations.

With dV calculations is a total dV calculated using Kermin's gravity valid for a trip back from the Mun or Minmus? From other bodies? Or do you need to use the local gravity to compare with values on the dV map?

ÃŽâ€V caluculations are performed using "standard gravity", which is 9.81 m/s² (though I've read that it's actually closer to 9.82 m/s² within the game). Local gravity has nothing to do with ÃŽâ€V calculations.

Are there any rules of thumb, or quick calculations regarding the when's and where's of staging?

There are some general staging rules of thumb that go like this:

1. Stages with higher ISP should be above stages with lower ISP.

2. More ÃŽâ€V should be provided by the stages with the higher ISP.

3. Each succeeding stage should be smaller than its predecessor.

4. Similar stages should provide the same ÃŽâ€V.

I have my own rules of thumb for designing a good launch vehicle. See below.

TWR. I think I have seen a TWR of around 1.3 being said to be good around here somewhere, but I don't remember where nor the context. Would 1.3 be a good launch TWR, and if so how much can you deviate from it without serious losses?

It depends on the stage. I've done quite a bit of research into TWR in an effort to optimize my payload fraction. Here's are OhioBob's rules of thumb:

1. Stage 1 TWR = about 1.65

2. Stage 2 TWR = about 1.30

3. Ratio of Stage 2 thrust to Stage 1 thrust = 0.35

Stage 1 refers to the lower stage that ignites at liftoff (KSP numbers the stages from the top down, which is the reverse of how it's done in the real world). If you are using SRBs or other strap-on stages, then the TWR refers to the combined liftoff TWR. I figure it only takes two stages to reach Kerbin orbit. If you have any upper stages they are most likely used for orbital maneuvering or ejection from orbit. These upper stages can be of much lower TWR, generally <1.

These numbers apply to stock aero only. If using NEAR or FAR, then different rules apply.

For landing on a body with no atmosphere does it really matter where you kill the orbital velocity, aside from the size of the suicide burn required?

I like to kill my horizontal velocity at as low an altitude as possible. The lower you are, the less you have to fall, thus the less vertical velocity you have to kill. I just make sure I'm high enough to give me the time necessary to get reoriented and make a good landing without crashing. I don't like to be rushed.

For a return from the Mun what's the most fuel efficient method. Burn straight up from the retrograde side, or go into orbit and burn to raise the retrograde apoapsis from there?

Personally, I like to start my ascent vertical and then transition into a horizontal burn, accelerating up to about 250-300 m/s. This will usually put me into a sub-orbital trajectory with an apoapsis of about 10-15 km. I then circularize the orbit at apoapsis. Once in orbit I preform my "transKerbin injection" burn at an ejection angle of about 135-140 degrees. In other words, if Mun is moving toward the 3 o'clock position, and Kerbin is in the 12 o'clock direction, then I start my burn at about the 1:30 position. The burn will send me off into the 9 o'clock direction.

The above is my personal preference, I don't know if it's the most efficient.

Edited January 5, 2015 by OhioBob

[MarvinKitFox]

Are there any rules of thumb, or quick calculations regarding the when's and where's of staging?

Simple rule of thumb:

Your engine's ISP, * (7 to 9), is a good balanced delta-v for the stage.

Tack on enough/right sized engine to get the TWR to where you want it.

This results in a stage that is worth dropping when empty, but doesn't suffer too much penalty while in use not stages so early that you waste engines.

[Empiro]

My recommendation is to not worry about atmospheric ISP versus vacuum ISP too much if you're trying to get into orbit. The reasoning is that most of the engines you'd use on the launch pad (SRBs / engines with good TWR) all share similar atmosphere - vacuum curves. If your launch TWR and flight paths are all similar, then the "atmospheric ISP loss" will be a similar percentage across all the rockets.

Instead, work exclusively with vacuum ISP. The rule of thumb of 4550 m/s in stock, and 3300 - 3500 m/s in FAR (depending on the mass and shape of your rocket) of vacuum delta-V will serve you very well. Things like better piloting, and picking the optimal TWR at launch will have a far greater impact on the net cost of your ship and total delta-V expended.

[Red Iron Crown]

Lots of good advice in this thread.

I'll add that a simple rule of thumb for staging is to have each stage mass about twice as much as the the stage above it. It's a gross simplification and not always ideal, but it is easy to remember and can be done simply in stock by counting tanks.

[ghpstage]

Thanks all!

You've given me a lot to think about here.

Should have mentioned I was playing stock, sorry.

OhioBob[/url'>]ÃŽâ€V caluculations are performed using "standard gravity", which is 9.81 m/s² (though I've read that it's actually closer to 9.82 m/s² within the game). Local gravity has nothing to do with ÃŽâ€V calculations

. I was pretty sure this was the case, the equation wouldn't really make any sense without strange modifications otherwise. I just wanted to be sure.

Incidentally, I have looked up where ISP comes from now.

OhioBob]I think most people just use the vacuum ISP in their calculations. For instance, it is typically stated that it takes 4550 m/s to reach Kerbin orbit, which is based on vacuum ISP. The loss resulting from the reduction of ISP due to atmospheric pressure is about 150 m/s. In other words, it takes about 4400 m/s to reach orbit if you perform the computation using the actual instantaneous ISP integrated over the ascent. I find that if you use the average of sea level and vacuum ISP for the lower stage, and vacuum ISP for the upper stage, you'll get a pretty good estimate of what the actual ÃŽâ€V will be. It easier just to use vacuum ISP for all calculations.

4550, So the dV map i've been looking at already factors in the losses. That's great news!

LethalDose]First, I'd recommend getting Kerbal Engineer Redux ("KER") to get stage dV and TWR.

Just got it, looks like it should be part of stock!

[UmbralRaptor]

Instead of trying to calculate average ISPs for engines, I'd just recommend building lifting stages with about 4.5 km/s of atmospheric dV (~ 3.5 km/s for FAR). These are just good and well accepted values from tons of players' experiences. YOu can stage them however you want, pretty much. Experiment and see what happens. It's a good way to get some practical knowledge instead of just being told the "right" way, because there are tons of "right ways".

Disagree -- the 4.5 km/s comes out of the pre-0.16 days when engines had constant Isp. Looking at actual expended ÃŽâ€V also seems reasonable given that varying target orbits (eg 70 vs 100 vs 600 km), on-pad TWRs, and craft layouts (boosters are draggy in stock, some cockpits much less) will affect required ÃŽâ€V by several hundred m/s.

...what I do is launch towards a retrograde (west) orbit...

This costs ~18 m/s compared with launching east, at least at the equator. Not very significant, but unless you're doing a rendezvous with an existing craft, you may as well launch east.

[Anquietas314]

This costs ~18 m/s compared with launching east, at least at the equator. Not very significant, but unless you're doing a rendezvous with an existing craft, you may as well launch east.

Okay, now what about the part about returning to Kerbin? The whole point is to leave Mun's SoI as high as possible over Kerbin, with the idea being that it'll save fuel for returning to Kerbin. Obviously you want to do that part in the same burn as your Mun escape though.

EDIT: Like this (apologies for crappy paint skills):

Edited December 31, 2014 by armagheddonsgw

[UmbralRaptor]

Okay, now what about the part about returning to Kerbin? The whole point is to leave Mun's SoI as high as possible over Kerbin, with the idea being that it'll save fuel for returning to Kerbin. Obviously you want to do that part in the same burn as your Mun escape though.

Well, I'm fairly certain that exiting the Mun's SOI against its orbital motion, rather than radially is the lowest ÃŽâ€V way of getting back to Kerbin.

[Anquietas314]

Well, I'm fairly certain that exiting the Mun's SOI against its orbital motion, rather than radially is the lowest ÃŽâ€V way of getting back to Kerbin.

Which is exactly what is illustrated?

[UmbralRaptor]

Which is exactly what is illustrated?

Looks to be, but the part about "as high as possible" confused me and made it seem like you were going radially out from Kerbin. I'm not sure if the extra altitude is worthwhile, depending on your velocity vector on SOI exit.

[Anquietas314]

Looks to be, but the part about "as high as possible" confused me and made it seem like you were going radially out from Kerbin. I'm not sure if the extra altitude is worthwhile, depending on your velocity vector on SOI exit.

Hmm. Well, I installed the precise nodes mod a couple hours ago and am in the middle of a few simultaneous Mun satellite contracts. I can plot a few dummy maneuver nodes to test each case

EDIT: The results for a 8x8km orbit using a direct return to Kerbin from LMO with 32km periapsis at Kerbin:

273.8m/s required in a prograde orbit

279.1m/s required in a retrograde orbit using my method.

Those are both the minimum needed, determined by tweaking the node time and prograde vector.

Not a lot in it to be honest, but I guess my method's slightly less efficient. Oh well

There is also the small caveat of the plotted orbits not being perfectly equatorial, so I guess that might be a factor.

Edited December 31, 2014 by armagheddonsgw