A small question about Delta-V

[Martian Music]

Hey,

My question is basically about the Delta-V map. According to the map, we need 3400 dV (or m/s) to get a low 80km orbit around Kerbin. But a simple small rocket (like the one in the "Go For Orbit" tutorial) only requires 2000 dV to get a LKO.

So I concluded that dV is not a standard value and it depends on the weight of your craft, and the amount of thrust it has, right? If I am correct, does anyone know how to do the maths for that? (yes I'd love to use papers and calculate it!) If not, just how do you know your craft can get in LKO? Do you look at your dV in general or do you also consider other factors and decide based on them?

Thanks!

 

Edited April 17, 2017 by Martian Music

[nascarlaser1]

the calculation for delta-v (and other fun facts) can be found here on the wiki .

http://wiki.kerbalspaceprogram.com/wiki/Cheat_sheet#Basic_calculation

[Numerlor]

How do you know about 2000dV? If it is from launch KER info it is displaying dV in your current attitude which is lower than vacuum one

[StahnAileron]

1 hour ago, Martian Music said:

So I concluded that dV is not a standard value and it depends on the weight of your craft, and the amount of thrust it has, right? If I am correct, does anyone know how to do the maths for that?

The equations and calculations needed to derive dV already take that into account. They've been simplified to you just needing a vessel's dry and wet masses along with the ISP of the engine.

It's not quite standard in relation to atmospheric pressure as ISP varies with that.

The dV vales are average/typical number and are not absolute. The values for Kerbin (and other atmospheric planets) can vary greatly depending on drag, TWR, and ascent profile.

2,000dV is about what you'd need if Kerbin did NOT have an atmosphere. (The extra 1400 would be accounted for from piggybacking off Kerbin's rotation and drag/gravity losses.) So I kinda question your claim there. (LKO orbital speeds is around is around 2300m/s.)

Could you tell us how you came up with that number/conclusion? As @Numerlor notes, KER's dV readouts differ depending on circumstance (and are barely useable with unorthodox designs.)

[GoSlash27]

StahnAileron,

 DV values for atmospheric bodies vary greatly depending on launch profile, drag, t/w, engine atmospheric efficiency, and the assumptions upon which the calculation is based. 3,400 m/sec is a good ballpark figure, but only an estimate.

For vacuum values, the math is greatly simplified and the DV values are absolute minimums; your expenditures will be somewhat higher than what appears on the map.

For the math, start here:

http://www.braeunig.us/space/orbmech.htm

Best,
-Slashy

 

[Red Iron Crown]

Delta V is mass- and thrust- independent, to a large degree that is the point of using it as a measure. 

[Spricigo]

 

4 hours ago, Martian Music said:

... a simple small rocket (like the one in the "Go For Orbit" tutorial) only requires 2000 dV to get a LKO.

lets ignore the atmosphere  and suppose that we can apply ridiculous amount of deltaV instantly

Orbiting Kerbin 'at ground level' would requires a velocity ~2400m/s , lets consider starting at ~200m/s (Kerbin's rotation) , then we need 150m/s for a hoffman transfer (again with super-dupper-hiper-mega-quantum-relativistic engines and no atmosphere).

By my bad maths its 2350m/s even before accounting for the losses caused by gravity, drag and piloting.

 

https://instacalc.com/42836        https://en.wikipedia.org/wiki/Vis-viva_equation     http://wiki.kerbalspaceprogram.com/wiki/Kerbin

[Harry Rhodan]

12 minutes ago, Spricigo said:

a hoffman transfer

Hohmann.

[GoSlash27]

Kerbin's rotation is 175 m/sec. Subtract that from the 2,426 m/sec required to orbit at sea level leaves a DV of 2,251 m/sec.

The Hohmann transfer to 70 km is 66 m/sec for the kick burn and 64 m/sec to circ. Adding these to the sea level DV comes out to 2,381 m/sec.

Your math isn't bad, @Spricigo.

Best,
-Slashy

 

[Spricigo]

Just now, Harry Rhodan said:

Hohmann.

Its not my fault so much of the important rocket science was done by Germans and Russians.  Thanks for the correction.

[Martian Music]

Hey all, thanks for the responses :]

Yes I used KER and the tutorial spacecraft showed a 2200 m/s so I questioned myself. Yeah it does display dV in current altitude but doesn't it also display the dV in vacuum? I would've saw the 3000+ m/s. Dunno I'll check again.

Thanks for the math links, reading them atm. So let me make sure I understood this correctly, dV already takes mass and ISP into account, right? It doesn't take the TWR into account in the equation though..

But I think TWR affects dV? I mean why is it more efficient when we use a good TWR value?

[Spricigo]

If that was in flight the readout was porbably the remaining deltaV, also its possible that was in atmospheric mode (a estimative of the effective deltaV taking in account drag/gravity loss). Also, as @StahnAileron pointed, KER sometimes gives inaccurate number depending on circumstances.

[StahnAileron]

14 minutes ago, Martian Music said:

Hey all, thanks for the responses :]

Yes I used KER and the tutorial spacecraft showed a 2200 m/s so I questioned myself. Yeah it does display dV in current altitude but doesn't it also display the dV in vacuum? I would've saw the 3000+ m/s. Dunno I'll check again.

Thanks for the math links, reading them atm. So let me make sure I understood this correctly, dV already takes mass and ISP into account, right? It doesn't take the TWR into account in the equation though..

But I think TWR affects dV? I mean why is it more efficient when we use a good TWR value?

The rocket equation takes into account dry and wet mass, so it already accounts for varying TWR by extension (because it takes into account the varying mass between wet and dry mass). TWR as a statistic isn't that important; it's only useful for knowing how QUICKLY you can apply dV. In this case, the TIME varies for a given amount of dV dependent on current vessel mass. e.g. Applying 100dV when you're full takes longer than the same 100dV when you're near empty. (It's most pronounced when you have a high fuel fraction.)

Also, two near-identical engines/vessels with the only difference being thrust levels will have the same usable total dV; one will just be able to use it faster (high thrust) than the other (low thrust). (Oberth Effects can factor into this in actual practice though.)

KER has 2 readings. You have to click on the "Atmosphere" button to switch modes. (Which only appears in the Detailed mode, IIRC. I use Detailed all the time versus Simple.)

Edited April 17, 2017 by StahnAileron

[GoSlash27]

6 minutes ago, Martian Music said:

So let me make sure I understood this correctly, dV already takes mass and ISP into account, right?

Martian Music.

 Aye.

7 minutes ago, Martian Music said:

It doesn't take the TWR into account in the equation though..

But I think TWR affects dV? I mean why is it more efficient when we use a good TWR value?

The equation assumes infinite t/w ratio. Anything less than that will incur a penalty in the form of gravity losses.

 DV can be confusing because it refers to 2 different things; The change in velocity required to accomplish a job, or the change in velocity your rocket is capable of generating. When we look at the map, we're only talking about the first definition. The fact that your rocket doesn't actually generate infinite thrust isn't the rocket equation's fault. You will get to orbit with a lower total DV (talkin' about the second definition now) if you have infinite thrust, zero drag, attain orbital velocity at sea level, and pilot it absolutely perfectly. Anything other than that carries a penalty in the form of gravity and drag losses, which the rocket equation does not account for.

 I must caution you to be careful about how you define "efficiency". Getting the job done with less DV doesn't necessarily translate to a lighter, cheaper, or more fuel efficient design.

Best,
-Slashy

[Martian Music]

1 hour ago, StahnAileron said:

The rocket equation takes into account dry and wet mass, so it already accounts for varying TWR by extension (because it takes into account the varying mass between wet and dry mass). TWR as a statistic isn't that important; it's only useful for knowing how QUICKLY you can apply dV. In this case, the TIME varies for a given amount of dV dependent on current vessel mass. e.g. Applying 100dV when you're full takes longer than the same 100dV when you're near empty. (It's most pronounced when you have a high fuel fraction.)

Also, two near-identical engines/vessels with the only difference being thrust levels will have the same usable total dV; one will just be able to use it faster (high thrust) than the other (low thrust). (Oberth Effects can factor into this in actual practice though.)

KER has 2 readings. You have to click on the "Atmosphere" button to switch modes. (Which only appears in the Detailed mode, IIRC. I use Detailed all the time versus Simple.)

I actually never knew that about KER xD thanks!

So why do I hear people saying that using too high thrust is a "waste"? I'm sure I heard this numerous times in videos. Do they just say that about using a high thrust engine? Oh god I will slap myself so hard if that's the case and I understood that as something complicated xD

1 hour ago, GoSlash27 said:

The equation assumes infinite t/w ratio. Anything less than that will incur a penalty in the form of gravity losses.

 DV can be confusing because it refers to 2 different things; The change in velocity required to accomplish a job, or the change in velocity your rocket is capable of generating. When we look at the map, we're only talking about the first definition. The fact that your rocket doesn't actually generate infinite thrust isn't the rocket equation's fault. You will get to orbit with a lower total DV (talkin' about the second definition now) if you have infinite thrust, zero drag, attain orbital velocity at sea level, and pilot it absolutely perfectly. Anything other than that carries a penalty in the form of gravity and drag losses, which the rocket equation does not account for.

 I must caution you to be careful about how you define "efficiency". Getting the job done with less DV doesn't necessarily translate to a lighter, cheaper, or more fuel efficient design.

Thank you for the dV info, it was confusing me indeed. And yes less dV doesn't mean better. Thanks!

[StahnAileron]

49 minutes ago, Martian Music said:

So why do I hear people saying that using too high thrust is a "waste"? I'm sure I heard this numerous times in videos. Do they just say that about using a high thrust engine? Oh god I will slap myself so hard if that's the case and I understood that as something complicated xD

That's in relation to atmospheric drag. Too high a TWR means you accelerate very quickly and can possibly suffer excess losses from drag. Going too fast too quickly means the air can't get out the way fast enough for efficient launch. It's a concern in the lower (thicker) atmosphere.

It's compounded with trying to avoid gravity losses (i.e. fighting gravity unnecessarily.) Higher TWR means you can pour on speed quickly, meaning the ideal profile is flatter. (i.e. You want to point closer to the horizon earlier/more aggressively.) This is detrimental in atmosphere due to air density and the drag it creates: you spend more time in the atmosphere (especially the thick lower layer). High TWR is fine outside an atmosphere and to some extent preferred. (For launches from the surface of an airless world or large dV burns; extreme precision burns favor lower TWRs.)

TWR concerns matter mainly during launches from Kerbin, Eve, and Laythe (all have atmospheres) and either how patient you are about maneuver burns and/or how big a window you have to complete one (extreme case: a burn that lasts longer than the current orbital period.) Airless-world launches just need a (relative) TWR greater than 1. (Higher is better as you don't spend as much time fighting gravity.)

However, excess TWR can be worked around if you're willing to get really hands on during a launch by controlling the throttle the whole way. Most players seem to just go full throttle all the way. For RSS/RO players, yeah, that's all they can really work with, but stock KSP lets you unrealistically throttle your engine back all the way back to zero. I build and fly "High TWR" rockets to get to speed quickly then throttle back as needed. I learned this from using and observing the mod GravityTurn. KER's TWR readout is just a max-case number. You technically have a TWR of zero up to that max number if you care enough to actually control the throttle. (It's not just for planes!)

Edited April 18, 2017 by StahnAileron

[GoSlash27]

24 minutes ago, Martian Music said:

So why do I hear people saying that using too high thrust is a "waste"? I'm sure I heard this numerous times in videos. Do they just say that about using a high thrust engine? Oh god I will slap myself so hard if that's the case and I understood that as something complicated xD

Martian Music,

 Yeah, they're mainly talking about the engine itself. Thrust comes at a price. In pure vacuum, it's additional engine mass, lower exhaust velocity, and/ or a higher price tag. In the atmosphere, you add drag, heating, aerodynamic instability, and more complicated assembly,

 We try to strike a happy balance that minimizes cost, mass, and complexity while maintaining acceptable t/w for safety, player sanity, and minimal cosine losses.

Generally speaking, my rockets look like this:

Booster (LFO): 1,800 m/sec (1/2 ATM), 1.4:1 initial sea level t/w at liftoff

Booster (SRB): 1,800 m/sec (1/2 ATM), 1.2:1 initial sea level t/w at liftoff

Transstage: 1,700 m/sec (vac), 0.7:1 t/w (vacuum) at ignition.

Ejection/circ: 0.5:1 t/w (vac) with respect to local body

Lander: 1.5:1 t/w (vac) with respect to local body

Launcher: 1.5:1 t/w (vac) with respect to local body.

 

HTHs,
-Slashy

[FullMetalMachinist]

43 minutes ago, Martian Music said:

So why do I hear people saying that using too high thrust is a "waste"?

@StahnAileron gave a great explanation for why that's the case during launch. There is a different reason to shy away from too-high TWR when you're in orbit. 

Basically it means that you have more engine mass than you need, which causes your dry weight to be higher than it needs, which causes you to have less dV available for going places. 

For (a very rough, off the cuff) example, imagine a ship is in orbit and it has 2000 m/s dV and a TWR of 4. That means that the engine it has is way overpowered for orbital maneuvering. If you took that same ship and replaced the engine, and that was the only thing you changed, you could possibly see the available dV go up to 2500 m/s or more. Yes, this lowers the TRW which means that any burns you have to make take a little longer. But IMO that's a small price to pay for 500 m/s of free dV. A TWR of 0.5 is usually fine once you're in orbit. 

[Spricigo]

46 minutes ago, Martian Music said:

So why do I hear people saying that using too high thrust is a "waste"?

There is a few reasons,  among it:

> More powerful engines are also heavier,  so your dry mass is higher, so you need more propellant to get the same deltaV 

>A high thrust engine can change the velocity quickly but also run out of fuel quickly. How high is your thrust have the same importance as how quickly you use the fuel for your deltaV budget. 

>Too high thrust may cause drag and control issues resulting in less theorycal deltaV turning into effective deltaV. 

[StahnAileron]

1 minute ago, FullMetalMachinist said:

@StahnAileron gave a great explanation for why that's the case during launch. There is a different reason to shy away from too-high TWR when you're in orbit. 

Basically it means that you have more engine mass than you need, which causes your dry weight to be higher than it needs, which causes you to have less dV available for going places. 

I considered mentioning the whole engine mass thing, but I think my post was long enough  I left it out in the end so he could focus on understanding one aspect at a time.

[Martian Music]

Thanks you so much, everyone! I believe I fully understood it now. Yes I knew a high TWR in atmosphere is really bad, but I couldn't explain what it does in vacuum.

Thanks again :] btw how do I mark the question as answered?

[Spricigo]

@StahnAileron interesting your reference to Gravity Turn as 'learning tool'. Myself,  I design my rockets to launch then in "fire and foeget"  mode and use GT as benchmark for evaluation of the effectiveness. 

[StahnAileron]

4 hours ago, Spricigo said:

@StahnAileron interesting your reference to Gravity Turn as 'learning tool'. Myself,  I design my rockets to launch then in "fire and foeget"  mode and use GT as benchmark for evaluation of the effectiveness. 

My rocket are high TWR with SRB sometimes. GT has issues with the non-throttling SRB at high TWRs during initial launch. Also, this is like the only way I can play and not get completely bored of launching rockets. Otherwise, I wish KSP had a "Space Tycoon"/"Space Program Manager" mode where launches can abstracted if I wanted using a launch history of a particular rocket design. (Be great if you could use GT's database to just "cheat" the rocket to orbit with the approximate correct amount of dV left and the necessary stages discarded.) That and KSP letting you do maneuver nodes without needing to be the active vessel. (i.e. "Background" burns while on rails.)

5 hours ago, Martian Music said:

Thanks again :] btw how do I mark the question as answered?

Look at each answer on the left. There should be a button to mark a post as the answer. Pick the post you think is the best answer and click on it. (And I find it funny you can LIKE posts for questions in this forum software but it won't add to the VOTE count for that post.  Like and Vote are two different things here )

[Numerlor]

5 hours ago, Martian Music said:

How do I mark the question as answered?

There is grey check above all answers just click it to green it out

[Red Iron Crown]

15 hours ago, Martian Music said:

Buy I think TWR affects dV? I mean why is it more efficient when we use a good TWR value?

TWR doesn't affect how much dV a craft has, but in some circumstances it can permit more efficient use of it. Launching is an example of this, higher TWRs reduce gravity losses and thus the total dV expenditure to reach orbit. 

Edit: Didn't realize there was another page where this was covered more thoroughly. 

Edited April 18, 2017 by Red Iron Crown