Delta V question

[Red Iron Crown]

5 minutes ago, wumpus said:

Note that the equation will naively insist on using low-Isp fuels first, without any checking on the efficiency changes due to the TWR changes.

I'm not sure what is meant by this, can you explain?

[OhioBob]

1 minute ago, Renegrade said:

My main points where A) it's possible and not too onerous to do it, and B) it's a good learning experience.

I totally agree with that.

Another good reason to learn how to do it by hand is because there are times when a mod simply can't provide the dV information needed.  This often happens when there is some unusual or complex sequencing involved.  For example, suppose we are going to recreate an Apollo-style moon mission in Real Solar System.  We know that the maneuvers the service propulsion system must perform require about 2000 m/s.  However, half of that is performed with a fully fuelled lunar module attached, and half is performed with no LM attached.  If we design the command/service module to give 2000 m/s without the mass of the LM, then we're under-designed.  If we design the CSM to give 2000 m/s with the mass of the LM, then we're over-designed.  The answer lies somewhere in the middle and the only way to figure it out accurately is to perform the calculations my hand.  A mod like KER just can't do it.

[wumpus]

You will always get more delta-v by burning low Isp fuels first.  The catch is, that when launching a rocket in a gravitational field you also have gravity losses.  So instead of getting all of your delta-v, you get a percentage of your delta-v equal to ((TWR-1)/TWR).  If this corrected delta-v is higher due to the increased TWR of your rocket (because you are burning both liquid and solid boosters), then you will get more delta-v out of your rocket.  Note that for higher TWRs this only works on airless worlds and that the "1" implies that TWR is localized (technically, the "thrust to weight ratio implies localization).  I'm guessing that there should be some simple relation between (TWR-1)/TWR and the ratio of the two ISPs (assuming you have boosters and liquid rockets) that tells you to throttle down the liquid rockets, but in my experience it merely give me an opportunity to botch the steering.

[regex]

17 minutes ago, OhioBob said:

Another good reason to learn how to do it by hand is because there are times when a mod simply can't provide the dV information needed.  This often happens when there is some unusual or complex sequencing involved.  For example, suppose we are going to recreate an Apollo-style moon mission in Real Solar System.  We know that the maneuvers the service propulsion system must perform require about 2000 m/s.  However, half of that is performed with a fully fuelled lunar module attached, and half is performed with no LM attached.  If we design the command/service module to give 2000 m/s without the mass of the LM, then we're under-designed.  If we design the CSM to give 2000 m/s with the mass of the LM, then we're over-designed.  The answer lies somewhere in the middle and the only way to figure it out accurately is to perform the calculations my hand.  A mod like KER just can't do it.

You can still use aids to help with that, though.  In your example, give the CM enough fuel to make it home without a payload then attach the fully-fueled LEM and add fuel for the maneuvers required with it.  Granted, you'll be changing the fuel tank size but that can be subsumed under "safety margin" (or using Procedural Tanks you can get quite exact).  I suppose you could consider that "by hand" calculations, but it's easier for me than dragging out the calculator.

Or use math, whatever's quicker for you.

Edited March 28, 2016 by regex

[Renegrade]

15 minutes ago, wumpus said:

Calculating the delta-v for simple craft is trivial.  Strap on some SRBs (while the main [liquid] rocket is also firing and you will either need to download KER or dig into the derivation to figure out where to plug in all the changes).  Note that the equation will naively insist on using low-Isp fuels first, without any checking on the efficiency changes due to the TWR changes.  This is pretty critical to understanding where delta-v fits in importance.  It is more or less everything between bodies (assuming you are willing to perform Mangalyaan maneuvers to get there) but you must balance TWR for take-off and (powered) landings.

Once you understand delta-v, get a load of this:

Pretty much a complete guide to the Kerbol system.  Just remember ULA's recent launch and have a good sized fuel margin.

Since the specific impulse / thrust relationships in KSP were fixed (in uh.... 0.90? 1.0? I forget when), it's a lot simpler now to calculate a hybrid rocket - fuel flow is now invariant with altitude for non-jets.  Just average out the thrust and ISP for as long as the SRBs are burning, and then calculate from there as a discrete staging event (er, well, I guess it IS a staging event).   Although with some designs I've seen and used, if the SRBs are a small enough percentage of the stage mass-wise, you can pretty much ignore 'em and be happy that you have an extra 50 m/s from 'nowhere'.

Um re: your latest post - I don't see that as craft delta-v changing due to gravity drag/losses.  I see that as the delta-v requirement changing instead.

Nice chart BTW, but if you're doing plane changes in LKO to go to Minmus, you're doing it wrong, heh.  It's better to do the plane change out beyond the Mun's orbit, or wait until KSC is at Minmus' AN or DN and then launch directly into Minmus' inclination (I prefer the former technique myself as I don't like waiting on AN/DNs, as that might result in an evening or before-sunrise launch, and/or might result in over-warping. KAC doesn't exactly have a "warp until KSC is at Minmus' DN" button AFAIK).

Here's a really hastily thrown together example of a post-Munar plane change (it's actually a terrible example, but it should illustrate the point I'm making well enough):

 

8 minutes ago, OhioBob said:

I totally agree with that.

Another good reason to learn how to do it by hand is because there are times when a mod simply can't provide the dV information needed.  This often happens when there is some unusual or complex sequencing involved.  For example, suppose we are going to recreate an Apollo-style moon mission in Real Solar System.  We know that the maneuvers the service propulsion system must perform require about 2000 m/s.  However, half of that is performed with a fully fuelled lunar module attached, and half is performed with no LM attached.  If we design the command/service module to give 2000 m/s without the mass of the LM, then we're under-designed.  If we design the CSM to give 2000 m/s with the mass of the LM, then we're over-designed.  The answer lies somewhere in the middle and the only way to figure it out accurately is to perform the calculations my hand.  A mod like KER just can't do it.

Ditto - totally agree here too   I've actually done some missions that are like that (early Mun tech BTSM missions pretty much require a semi-Apollo approach.  It's only a single stage lander, but you don't bring it back), and the utilities did indeed fall short there.

[OhioBob]

3 hours ago, regex said:

You can still use aids to help with that, though.  In your example, give the CM enough fuel to make it home without a payload then attach the fully-fueled LEM and add fuel for the maneuvers required with it.

That's a good idea.  I haven't tried that but it certainly ought to work, at least in the scenario described here.

Quote

Granted, you'll be changing the fuel tank size but that can be subsumed under "safety margin" (or using Procedural Tanks you can get quite exact).

We don't necessary have to add tankage, we can start out with an oversized tank and just increase or decrease the amount of fuel/oxidizer.  For step #1, we add a tank (best guess size) and use the sliders to decrease the propellant load until we have just enough Δv for the return trip.  In step #2, we add the LM* (or an equivalent mass) and use the sliders to increase the propellant load until we've added enough Δv for the maneuvers that will take place with the LM attached.  If we end up with a tank that is too small or way too big, we can either increase it or decrease it and repeat the steps.

* Note, that when the LM mass is added, the Δv indicator will go down, but that's OK.  We know from step #1 that, after jettisoning the LM, we'll have enough Δv for the trip home.  In step #2 we add to whatever value the indicator is showing the amount of Δv needed for the first part of the trip.  For example, say we need 950 m/s with the LM and 1050 m/s without the LM.  In step #1 we adjust the sliders until we have 1050 m/s.  Say that after adding the LM the indicator drops to 450 m/s.  We now adjust sliders until we have 450 + 950 = 1400 m/s, plus whatever margin we want to include.  If we now remove the LM, the Δv indicator might jump to something like 2700 m/s.  So the indicator is telling us either 1400 m/s or 2700 m/s, depending whether the LM is attached or not, but we know that we'll actually get the 2000 m/s we need when we fly the mission as we planned it.

 

Edited March 28, 2016 by OhioBob

[wumpus]

3 hours ago, Renegrade said:

Nice chart BTW, but if you're doing plane changes in LKO to go to Minmus, you're doing it wrong, heh. 

It is hardly my chart (this one came from KSP-wiki, with plenty of cites to reddit).  But once you understand delta-v (like the OP was learning), it is a great time to see it.  I have an older one printed out taped to the side of my monitor.

[Renegrade]

22 minutes ago, wumpus said:

It is hardly my chart (this one came from KSP-wiki, with plenty of cites to reddit).  But once you understand delta-v (like the OP was learning), it is a great time to see it.  I have an older one printed out taped to the side of my monitor.

I used to use such a thing (it was less accurate though, older, had some errors) - I actually run things through the vis-viva equation nowadays (actually a script that I scribbled up), or use the Transfer Window Planner for interplanetary (the mod version is much faster than the web version) with my script to estimate landing costs.    Such tools are definitely handy and I do recommend their usage.

[Lukaszenko]

On 3/28/2016 at 11:00 AM, Smorfty said:

Delta-v means velocity change. And the way you change your velocity is by applying acceleration so yeah, they're pretty much the same thing.

Acceleration denotes how fast velocity is changing. It doesn’t actually say how MUCH it changes, which is exactly what delta V specifies, while conversely not actually caring about how fast it does it. So no, they are not the same thing.

Equating delta V to acceleration would be akin to equating distance to velocity. Yeah you need one to do the other, but they are different concepts. You don’t have to drive 100 miles, for example, in order to drive 100 mph.