• 0
Sign in to follow this  
amateur astronaut

What is total delta-v?

Question

In one of Matt Lowne's videos I heard that to go to eloo your rocket need to reach a total delta-v of 4000 m/s. But what this mean? Is this means that that's the max deceleration or acceleration what you'll need to make, or what? He also stated out that his fuel just ran out before reaching that speed. I wanna make some rockets and put some stations on distant planets and an ssto that can send crew on them and back to kerbin, and i was just curious what could this total delta-v mean, and how it could help me make it. 

Edited by amateur astronaut
I messed up

Share this post


Link to post
Share on other sites

7 answers to this question

Recommended Posts

  • 0

Delta-v is the amount by which you can change your velocity vector.  'Delta' (a triangle) is the symbol mathematicians use for 'change' and 'v' is just an abbreviation for velocity.  Velocity being a 'vector' just means it's speed-plus-direction; you can't be going "at 80mph" without going at 80mph in a particular direction.  All 'steering' in space is by accelerating in different directions, so changing the velocity.  It's all because in space there's nothing to slow you down unless you 'accelerate' the opposite way.

Standard example: Vehicle at rest.  Accelerate 100mps (meters per second is the usual KSP measure of speed, not mph) North.  Your speed is '100mps', your direction is North, your velocity-vector is 100mps North.  Now accelerate 150mps South; your velocity is 50mps South (150mps South minus the previous 100 North).  Accelerate 50mps West; v is now 70.71mps Southwest (the diagonal of 50 South and 50 West).  If you want to stop, you will need to make a 70.71mps delta-v burn facing NE.

dV (another way of abbreviating delta-v) is calculated using the Tsiolkovsky rocket equation (https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation) based on the efficiency of your engines and the amount of fuel available.  Because it's so important though people have already worked out the delta-v you need for most trips from Kerbin.  The community delta-v map ...gBoLsSt.png

... is pretty indispensable.  Enjoy :-)

Edited by Pecan

Share this post


Link to post
Share on other sites
  • 0
28 minutes ago, amateur astronaut said:

hmmm... interesting... can u see somehow how many dV your spacecraft can make?

There are several ways.  First, KSP from 1.6 on has a delta-V readout in the staging list.  Second, mods such as Mechjeb and Kerbal Engineer will give you a display.  Third, you can calculate it yourself.

Remember that high delta-V is not, of itself, a solution to the problem of reaching a destination in space.  To extend the car analogies, delta-V is somewhat equivalent to an automobile's range; the comparison isn't perfect, but it will do for now.  If I drive in circles or rev the engine in neutral, then I expend fuel without getting anywhere.  On the other hand, if I shut off the engine for long coasts downhill, then I can extend the range more than I'd be able to do otherwise.  With rockets, a similar effect works, as well:  you can burn at inopportune times or the wrong direction and never reach your intended destination.  On the other hand, you can choose your burns with enough care that you end up with fuel to spare.

Share this post


Link to post
Share on other sites
  • 0

This gameplay question has been moved to the Gameplay Questions subforum.

Share this post


Link to post
Share on other sites
  • 0
22 hours ago, amateur astronaut said:

hmmm... interesting... can u see somehow how many dV your spacecraft can make?

For a simple rocket - single stage, single engine - it's easy to calculate, as long as you have a spreadsheet programme or scientific calculator.  The tricky bit an ordinary calculator wouldn't be able to do is the logarithm that's needed.  All the figures you need are shown in the VAB on the parts list or as you assemble the complete rocket.

I'll use a straightforward 'starter' rocket as an example:
Mk16 Parachute
Mk1 Command Pod
TD-12 Decoupler
FL-T800 Fuel Tank
FL-T800 Fuel Tank (again)
4 x Basic Fin (around the base of the lower fuel tank)
LV-T45 "Swivel" Liquid Fuel Engine

a.  Tsiolkovsky's rocket equation is: dV = (Isp * g) * LN(Mf / Me) or, in English,
(dV =) the amount the rocket can accelerate is ...
(Isp * g) the engine specific impulse (shown on the parts list in the VAB) multiplied by the gravitational constant (9.81 mss)
(*) multiplied by
(LN) the natural logarithm of
(Mf / Me) the total mass of the rocket with all its fuel divided by the mass without the fuel

Since the engine Isp (efficiency) and fuel mass are the important parts here this comes down to - you can do more if you have a better engine or more fuel - who'd have thought!  Because of the logarithm function (LN) though you get diminishing returns so twice as much fuel won't give you twice as much dV, as fuel adds extra mass to be accelerated as well.

b.  So what you need to get from the VAB is; the engine Isp, the mass of the rocket full of fuel and the mass  of the rocket without any fuel.

Isp is shown in the VAB parts list 'ASL' (At Sea Level) and 'Vac.' (Vacuum, ie; in space) and they're usually different so the next question is which to use?  This is because of how rocket engine's efficiency is affected by the surrounding atmospheric pressure.  In practice we all tend to use the vacuum Isp for just about everything.  It'll give you a slightly-high result when you're launching from Kerbin but the atmosphere starts to thin out pretty quickly as you gain altitude during the launch.  Trying to get really accurate results for launch vehicles is much, much harder because that pressure is changing all the time plus exactly how you fly your launch trajectory affects it a lot (and is hard to do consistently by hand on a computer keyboard).

Mouse-over the swivel engine in the parts list and right-click for extra info.  "Engine Isp: 250 (ASL) - 320 (Vac.)"; we use 320

Mf, the mass of the rocket with all the fuel is easy - just mouse-over the 'Engineer's Report" at the bottom of the screen.  "Mass: 11.520t"

Me, mass without fuel, is a bit harder because you have to drain the tanks or use the delta-V calculator (in which case you might as well just use the results it gives you!).  Either way you should get the result 3.520t.

All that together gives us:

dV = (Isp * g) * LN(Mf / Me)
= (320 * 9.81) * LN(11.52 / 3.52)
= 3139.2 * LN(3.2727)
= 3139.2 * 1.1856
dV = 3721.9 m/s (vacuum)

(If, instead, you use the ASL Isp you will get only 2907.74 m/s dV)

c.  More complicated rockets

Extra stages just require you to calculate the dV for each stage separately.  Cross-feeding fuel between stages can make that difficult, otherwise it should be as straightforward as here.

Multiple engines are also easy, provided they have the same Isp.  If they don't then you have to work out the ratios of each engine type and get a weighted average of the Isps from that.  Hint: more trouble than it's worth unless you really love doing things by hand!

NOTE:  The command pod contains monopropellant but the rocket has no RCS thrusters that use it.  Wasted Mass!  Remove it and note the new dV ... 3746 m/s.  The reduced mass results in a better 'fuel ratio' (Mf / Me) which results in better performance.

Edited by Pecan

Share this post


Link to post
Share on other sites
  • 0
22 hours ago, Zhetaan said:

Remember that high delta-V is not, of itself, a solution to the problem of reaching a destination in space...

Point well made.

The perfect example is launching - if your engine isn't powerful enough to lift the vehicle in the first place you'll just burn all the fuel sitting on the pad and never go anywhere at all.

Share this post


Link to post
Share on other sites
  • 0
13 minutes ago, Pecan said:

The perfect example is launching - if your engine isn't powerful enough to lift the vehicle in the first place you'll just burn all the fuel sitting on the pad and never go anywhere at all.

Indeed; that was the image I wanted to conjure with my analogy about revving the engine in neutral.

Another similar situation is not staging the launch clamps, which gives me leave to ask whether you've left the parking brake on.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this