Question about Delta-V

[Rylant]

Hey all. I am playing the latest release on PC through steam. I just started career mode. I was trying to achieve my first orbit of Kerbin and was using that Delta-V sheet that most tends to use. It looks like it says 3400 DV to achieve orbit and I was having a hard time putting together a decent ship under 30 parts with 3400. I finally tried to launch with the ship I built which was 3100 or so. I was easily able to achieve orbit with that vessel and had several hundred DV to spare. So am I reading that chart wrong? Or reading the DV on my ship wrong (I just look at the total DV under the staging on the HUD)? Or some other issue?
 

I am a total noob here, so just trying to figure out how to read these things. I imagine fully understanding planning and DV will be huge when it comes to going to the different celestial bodies here. 

Thanks,

 Rylant

[HebaruSan]

23 minutes ago, Rylant said:

Or reading the DV on my ship wrong (I just look at the total DV under the staging on the HUD)?

Are you aware that the delta V meter in the VAB has atmospheric and vacuum modes? The delta V maps generally use vacuum delta V, and it's likely you were looking at the atmospheric delta V, which can be significantly less for some engines. If you hover the delta V icon in the toolbar the options should be apparent.

[AlexinTokyo]

Welcome to the game and the forums, and congratulations on your first orbit

The Δv required to launch into Kerbin orbit is highly dependent upon both your craft and your launch flight profile.   As it says on the Δv map (I assume you mean this one), "More/less efficient ascents are likely, depending on ascent profile, TWR, and aerodynamic effects."
So if you have a small, aerodynamic rocket and fly a very efficient profile you can definitely get to orbit for less than 3,400 m/s.

That said, it's more likely that you were seeing the atmospheric Δv of your ship in the VAB, while the map uses vacuum Δv exclusively.
This difference, which can be quite large, is caused by the fact that rocket engines (especially those designed to work in a vacuum) suffer an efficiency penalty (represented by a lower specific impulse - I_sp and commensurately lower thrust) under atmospheric pressure.  So it's likely that you had significantly more vacuum Δv than the 3,100 m/s displayed.

I use KER (a mod) for my vehicle information readouts, so I'm not familiar with the stock game's Δv data in the VAB, but there should be an option to switch between vaccuum and atmospheric values.

Ninja Edit: What HebaruSan said

Edited April 13, 2021 by AlexinTokyo
Ninja - Then spelling!

[Guest]

Just to check when you say you’ve achieved orbit you do mean you’ve circularised as opposed to just left the atmosphere? 

[Wizard Kerbal]

On 4/13/2021 at 3:04 AM, FruitGoose said:

Just to check when you say you’ve achieved orbit you do mean you’ve circularised as opposed to just left the atmosphere? 

That reminds me my friend was trying to reach orbit.

So that white line is my orbit?

I’d think, wait. What are you looking at? Send me a picture.

He was going in a straight line to 500 km, looking at Mun.

Edited April 19, 2021 by Wizard Kerbal

[jimmymcgoochie]

3400m/s is an average value- you can do better if you fly a really efficient trajectory- and is based on vacuum delta-V; rocket engines have to fight against the atmosphere so their efficiency (and by extension their delta-V) is reduced, as well as their overall thrust. You can switch between surface and vacuum ISP by clicking the little delta-V icon in the bottom right of the screen when in the VAB (or SPH) editor; use surface level for your first stage so you can see the overall thrust to weight ratio (TWR) and make sure it’s above 1 or you’re going nowhere, and vacuum delta-V for everything else as you’ll probably be high enough that the atmosphere isn’t a problem efficiency-wise at that point.

Most booster engines e.g. the Mainsail are built for heavy lifting, with high thrust and a small difference in ISP in atmosphere or vacuum, but low overall ISP and high mass; in contrast a vacuum-optimised engine like the Poodle has low thrust and performs poorly in atmosphere, but has excellent ISP in space meaning you’ll get very misleading numbers for vacuum engines if you have the setting set to display surface level values.

[camacju]

The best way to measure ascent efficiency is in terms of mass fraction. For example, an ascent that reaches orbit with 86% of the original mass remaining (such as an efficient SSTO aircraft) is better than an ascent with 47% of the original mass remaining (an efficient single staged rocket).

This is because mass fraction directly computes the actual fuel expended by all engines instead of having to rely on vacuum and surface delta-v readings. This technique is even better when you have multiple types of engines, like a SSTO craft which uses Rapiers and nuclear engines, or a rocket which uses KS-25s and nuclear engines, so you can't use delta-v readings at all. So you'd have to actually fly the mission but you get better information that way.

You can see your craft's mass in the VAB, or in flight you can go to the map screen and go to the vessel info tab. However, I prefer to use Mechjeb for that because it's easier.

Edited April 20, 2021 by camacju