Predicting What Δv is Enough

[Kerbal2023]

I noticed wild fluctuations in how much Δv i need to get to orbit. Sometimes 2900 will do it and sometimes 3800 won't. The rockets are all similarly aerodynamic and I fly pretty well now. The difference seems to be engine Isp at different heights. So I guess what I'm asking how to predict this. Should I take the atmospheic Δv summary from the base game. It seems to assume all engine work at sea lebel, and hence underpredict higher liquid fuel stages. So then should I sum up the values of lower stages at atmosphere and higher stages at vacuum to get an estimate? Tjese values are very high (4000+) and I still run out of fuel when circularizing.

[Bej Kerman]

2 hours ago, Kerbal2023 said:

I noticed wild fluctuations in how much Δv i need to get to orbit. Sometimes 2900 will do it and sometimes 3800 won't. The rockets are all similarly aerodynamic and I fly pretty well now. The difference seems to be engine Isp at different heights. So I guess what I'm asking how to predict this. Should I take the atmospheic Δv summary from the base game. It seems to assume all engine work at sea lebel, and hence underpredict higher liquid fuel stages. So then should I sum up the values of lower stages at atmosphere and higher stages at vacuum to get an estimate? Tjese values are very high (4000+) and I still run out of fuel when circularizing.

There's a lot of factors at play here, the biggest of which is probably your gravity turn profile. The time you spend in the lowest parts of the atmosphere should be short either way, and the amount of time you wait till beginning the turn can make a big impact on dV margins.

I think either Matt Lowne or Stratzenblitz came up with this technique for getting consistent turns, but a good way to repeat the same gravity turn profile consistently is to turn to five degrees on your navball right after launch then set SAS to prograde at a certain altitude (or speed, I cant remember but the latter may be better for low TWR launchers). Experiment with when you begin the turn and see what works most efficiently.

Addendum: It was Stratzenblitz (3 parts to Duna, Ike and Minmus) and they began the gravity turn at 80m/s. That was for their particular vessel though and it resulted in the vessel intentionally overheating so you may benefit from experimenting with the technique.

[N_Danger]

The D/V calculator in the VAB can be set to different altitudes.

Click on the button that says Altitude and below that there will be a slider. You can move it to adjust the altitude.

So if you always stage at certain altitude you can set the slider to that height (say 15 KM) to see the effects on DV at that moment on the current stage. 

[Gargamel]

Moved to gameplay questions 

[king of nowhere]

On 11/24/2023 at 10:56 AM, Kerbal2023 said:

I noticed wild fluctuations in how much Δv i need to get to orbit. Sometimes 2900 will do it and sometimes 3800 won't. The rockets are all similarly aerodynamic and I fly pretty well now. The difference seems to be engine Isp at different heights. So I guess what I'm asking how to predict this. Should I take the atmospheic Δv summary from the base game. It seems to assume all engine work at sea lebel, and hence underpredict higher liquid fuel stages. So then should I sum up the values of lower stages at atmosphere and higher stages at vacuum to get an estimate? Tjese values are very high (4000+) and I still run out of fuel when circularizing.

atmospheric and vacuum deltaV can be very different. using a vacuum engine for the first stage, or an atmospheric engine for the second stage, can make a huge difference.

if you use a reasonably aerodinamic rocket with a good atmospheric engine for the first stage and a vacuum engine for the second stage, then 3400 m/s (vacuum) is a reasonable amount to orbit kerbin. if it takes 4000+ m/s, you are doing something very wrong.

if you want a more detailed answer, post some pictures of your rockets and your ascent profile.

[Kerbart]

On 11/24/2023 at 4:56 AM, Kerbal2023 said:

I noticed wild fluctuations in how much Δv i need to get to orbit. Sometimes 2900 will do it and sometimes 3800 won't. The rockets are all similarly aerodynamic and I fly pretty well now. The difference seems to be engine Isp at different heights. So I guess what I'm asking how to predict this.

It's not just I_sp; it's thrust as well. Your goal is orbital (tangential) velocity and any power perpendicular tothat (vertical) is basically wasted; this is what's called gravity loss. Intuitively you'd think this is a function of altitude (Work = Force × Distance) but as gravity is acceleration that factor effectively becomes time. The more time you spend burning vertical, the more fuel you waste. And a big factor in that is your TWR, which should be roughly in the 1.3-1.7 range. Anything less (you'll notice by how long it takes to clear the tower) and you'll need lots and lots of fuel. Anything more and once Science arrives you'll have fantastic pyrotechnic effects during launch, caused by extreme drag (a function of velocity). As you get higher and your trajectory is more horizontal you can do with less thrust—usually engines with better I_sp—which is one of the reasons why multiple stages are so effective.

Once you have that under control your DV for orbit should consistently be closer to 3000 than to 4000.

[Superfluous J]

TWR at launch is big. Too low, and you're burning fuel just hovering near the launchpad. Too high, and your rockets will flip out of control or experience excessive wobbliness. I find 1.2-1.6 to be a good range of starting TWR to both reach orbit with 3-4k m/s of dV and also actually reach orbit

Edited November 28, 2023 by Superfluous J

[Kerbal2023]

21 hours ago, king of nowhere said:

if you want a more detailed answer, post some pictures of your rockets and your ascent profile.

This is pretty standard for me. A large booster at the bottom, and two liquid stages. The top liquid stage usually has a Terrier, the lower one something heftier like a Swivel. At sea level the game predicts 3200 Δv, but if I check the liquid stages in vacuum it's a lot more.

Edited November 28, 2023 by Kerbal2023

[king of nowhere]

3 hours ago, Kerbal2023 said:

 At sea level the game predicts 3200 Δv, but if I check the liquid stages in vacuum it's a lot more.

 

yes, of course it is. the terrier quadruple its deltaV in vacuum - though i don't understand how you can have 2 stages both based on terrier engines with similar twr there, unless it's some mod to rescale (i checked, only the terrier has that atmosphere Isp).

anyway, this leaves you with 2300 + 2300 + 1200 = 5800 m/s. you should actually go to orbit and have half fuel left in the second stage. if you don't, then there are two possibilities. maybe the rocket is a lot more draggy than you think - just because it looks aerodinamic, it doesn't mean it is. aerodinamic is glitchy. or maybe you are not doing your gravity turn correctly

[Kerbal2023]

So I can use the vaccum numbers for the upper stages?

[Superfluous J]

5 hours ago, Kerbal2023 said:

So I can use the vaccum numbers for the upper stages?

Yes. Vacuum numbers are actually pretty accurate even at 10km up.

[Spricigo]

6 minutes ago, Superfluous J said:

Yes. Vacuum numbers are actually pretty accurate even at 10km up.

I was about to say almost the same, just 20km instead of 10km

 

On 11/24/2023 at 9:34 AM, Bej Kerman said:

I think either Matt Lowne or Stratzenblitz came up with this technique for getting consistent turns, but a good way to repeat the same gravity turn profile consistently is to turn to five degrees on your navball right after launch then set SAS to prograde at a certain altitude (or speed, I cant remember but the latter may be better for low TWR launchers).

You mean Spricigo's Technique*? Just that turning after launch still require a bit of skill/practice to be consistent,  turn it before leaving the VAB and hold it with launch clamps.  How much you turn  varies with the rocket (More TWR, shallower angle)  If you are precise enough with the design/setup you don't even need SAS or launch clamps e. g.  Still, is a tradeoff, you get consistent turns at the cost of more time spent at the design phase.

 

*Certainly others came up with the same idea independently. Still, I'm using it for almost every single rocket except   launch since 2016 and sharing crafts designed to do it  since 2017,  I guess my case is pretty good.

 

[Poppa Wheelie]

21 hours ago, Kerbal2023 said:

So I can use the vaccum numbers for the upper stages?

Use vacuum numbers only, for the whole thing.  Your 3400m/s target for achieving orbit is the vacuum dV total of all stages used from Kerbin surface until orbit is achieved.

You should use the sea level setting of the dV calculator only to make sure TWR is within range.

[Spricigo]

44 minutes ago, Poppa Wheelie said:

Use vacuum numbers only, for the whole thing.  Your 3400m/s target for achieving orbit is the vacuum dV total of all stages used from Kerbin surface until orbit is achieved.

3.4km/s with twr~1.5 is a convenient rule of thumbs. However keep in mind: those are "rookie numbers".  I'd say an experienced player can consistently archive orbit with less than 3.2km/s and do it with  less  than 3km/s if really trying. 

Ironically, with enough experience we stop to follow our own advice.

 

[Kryxal]

If you want to work with a lower TWR and higher delta-v, that's fine.  Fuel is cheap, boosters are cheap, decouplers are somewhat expensive, engines are expensive.  I routinely launch below 1.3 TWR, aiming to hit 4-5 degrees by maybe 100 m/s.  Not the fastest launch, not the smallest launch for the payload, but it works.  If the first stage is boosters, I tend to use a higher TWR to start but not for long.