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Andrew Ridgely

Ways to add more Delta V?

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Hey everyone, I have a quick question regarding Delta V.  A lot of times my rockets wont have enough to make a full burn, and I'll have to use the next stage to complete it, which puts my whole mission out of whack.  Any tips for getting more Delta V?  Thanks! 

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DV= 9.81*Isp*ln(Mw/Md).

Ways to increase your DV basically boil down to a) increasing the efficiency (Isp) of your engine, reducing the mass of your rocket that is not fuel, and staging.

Reducing the mass that is not fuel can be further sub-divided into reducing payload and eliminating unnecessary mass in your lifter. Staging is basically chucking engines and empty tanks once they are no longer needed, thus reducing the mass of your vehicle part of the way through the flight.

I generally stage my lifters mid-way through the flight in order to take advantage of the optimized nature of the parts. The first 1,800 m/sec DV is essentially atmospheric flight. Engines that excel in this regime have relatively high atmospheric Isp and low vacuum Isp. They are light, powerful engines. This regime also requires aerodynamic streamlining and control surfaces. This stage has the bulk of the fuel tanks.

Beyond the halfway point, you get into the opposite. Efficient, weak engines, small fuel supply, no aerodynamics necessary. It makes sense to stage at this point because there's so much dead weight to shed and the optimal engines are so different.

So here's what I do...

Instead of trying to "add DV" to an existing design, I design each stage to have the DV and t/w it needs from the outset and *then* build it.

If you have MechJeb, this is straightforward. Minimum 0.7g t/w and 1,700 m/sec DV in vacuum for the upper stage and 1.4g t/w (1 atm) and 1,800 m/sec (1/2 atm) for the lower stage. If you don't have a mod to calculate this for you, you will have to calculate it yourself.

Best,
-Slashy

 

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6 hours ago, Andrew Ridgely said:

Hey everyone, I have a quick question regarding Delta V.  A lot of times my rockets wont have enough to make a full burn, and I'll have to use the next stage to complete it, which puts my whole mission out of whack.  Any tips for getting more Delta V?  Thanks! 

I once wrote a post about mass fractions that I keep linking to because it keeps being relevant :) When it comes to dV, the mass fraction is the number one thing to know about. If you understand this concept, you will have no problem getting exactly as much dV as you like.

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One thing often overlooked - good initial TWR. You can afford much lower TWR later during the ascent, but you should really spring off the launchpad.

Oberth Effect is your friend, and it can be your friend from moment one. TWR of 4 for the first 20 seconds is NOT too much.

Even launching huge stuff, with the whole tree unlocked, I still use Hammers, the early, simple SRBs. I attach whole clusters of them to the outside of the bottom-most tanks. In KER they read as extra 120m/s of delta-V. But they result in good 400m/s more on the orbit - simply, as you are moving upwards faster, you reach the altitude where you begin your gravity turn sooner, and you reach the orbit in a shorter time. With low TWR, on the other hand, you spend ages climbing arduously up, fighting against the gravity and wasting tons upon tons of fuel making very little progress towards the orbital speed.

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What engines and fuel tanks are you using? How big of a payload are you carrying?

For transfer stages, I reccomend using engines with high vacuum ISP. The LV-909 Terrier is a good engine for starters when doing transfers. The RE-L10 Poodle is another good transfer engine. If you're sending small probes, the 48-7S Spark and the LV-1 Ant are good for small probe/small vehicle transfer stages. I highly suggest using the LV-N Nerv engine, as it's very good for transfer stages. Keep in mind that it only consumes Liquid Fuel though.

 

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There are four basic techniques to increase the delta-V of a spacecraft:

  1. Use more efficient engines (ones with higher Isp).
  2. Increase fuel mass by adding more tanks, though this has diminishing returns.
  3. Reduce dry mass (anything that is not fuel, reducing upper stage mass is most important).
  4. Use staging (related to 3, get rid of empty tanks or other unneeded parts during flight to reduce dry mass).

 

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One other thing to keep in mind is that you don't need to wait to stage in between burns or activities.  It makes burn time a little harder to predict, but it's often necessary or preferable to get efficient stages.

For example, you could plan for your boosters and core stage to get you to 50km altitude.  Then a vacuum stage (say a poodle) takes over, burns to orbit, burns to the mun and then starts a lunar injection burn.  Then a smaller vacuum stage (say a terrier) finishes the burn and lands on the mun.  (Those are just made up points; where you'll break depends based on payload size, thrust needs, etc.)

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Everyone else has done an excellent job answering the question you asked, but that likely doesn't solve the problem you are trying to solve. We need a better question! And that question is, given that you think you have enough dV, and then you wind up short, why are you winding up short?

The most likely answer is that your initial stage is taking more dV to get you out of the atmosphere than you expect it to because you are getting too many losses due to either drag or gravity (probably gravity losses, but if you are flying a monster with a bunch of radial parts it might be drag also).

This prompts us to chase another question. Why are you getting too many gravity losses? Probably your gravity turn is not as efficient as you think it ought to be. So! My question for you is how are you turning your spaceship from vertical to horizontal?

It used to be that the approved method was to hit 10km and then topple over to 45 degrees. And if you use this method you will see the extra dV losses due to gravity that I expect you to have seen!

The new, hip way to do it is to nudge the rocket over a little bit as soon as your control surfaces are responsive. You should still hit 10km at 45 degrees. But you'll have built up quite a bit of horizontal momentum along the way.

Let me know if that helps.

Edited by dire

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12 hours ago, GoSlash27 said:

DV= 9.81*Isp*ln(Mw/Md).

It would make sense to record ISP in relation to Kerbin surface 'g' (9.81 m/s/s). In earlier KSP versions though, engine ISP ratings were given in relation to a 'conversion factor' of 9.82 N/kg. Do you know if the constant was changed?

Edited by Rodhern
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8 minutes ago, Rodhern said:

It would make sense to record ISP in relation to Kerbin surface 'g' (9.81 m/s/s). In earlier KSP versions engine ISP ratings where given in relation to a 'conversion factor' of 9.82 N/kg. Do you know if the constant was changed?

Yes, the conversion factow was changed from 9.82 to 9.81 some versions ago.

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1 minute ago, mhoram said:

Yes, the conversion factor was changed from 9.82 to 9.81 some versions ago.

Thanks.

Btw. How do you guys notice these changes? Do you carefully comb through the change logs every time?

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Btw. How do you guys notice these changes? Do you carefully comb through the change logs every time?

The original old number I got from http://wiki.kerbalspaceprogram.com/wiki/Specific_impulse#Conversion_factor
There is also a method described how the number was determined experimentally.

The change to 9.81 was mentioned in a thread about physics here on the forum.

Edited by mhoram

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Note that when they changed it, they didn't just take 0.01 off of the number. They actually input the RL standard gravity of 9.80665 m/s². Most people round that to 9.81, but hey, if you ever want to be really, really precise about your dV budget... :wink:

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1 hour ago, Streetwind said:

They actually input the RL standard gravity of 9.80665 m/s².

Ahh! Thanks. I was going to ask about the exact value. I was getting annoyed that the indicated fuel flow didn't match up. (by using 9.80665 it does match up).

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1 hour ago, Sharpy said:

Oberth Effect is your friend, and it can be your friend from moment one. TWR of 4 for the first 20 seconds is NOT too much.

Is this really Oberth in play?  Seems like avoiding gravity drag is the bigger issue - Kerbin will steal almost 10 m/s every single second you're going vertical, so you want to clear the heavy atmo and get horizontal ASAP.  But yeah, you want all the TWR you can reasonably get until your speed is high enough that thermal or control/stability issues come into play.  

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2 minutes ago, Aegolius13 said:

But yeah, you want all the TWR you can reasonably get until your speed is high enough that thermal or control/stability issues come into play.

That is true if you are optimizing for minimum dV to orbit. If optimizing for payload fraction or cost per unit of payload to orbit then lower initial TWRs work better.

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28 minutes ago, Red Iron Crown said:

That is true if you are optimizing for minimum dV to orbit. If optimizing for payload fraction or cost per unit of payload to orbit then lower initial TWRs work better.

Except when higher dV means bigger tanks - and more engines to lift these tanks.

Short-burn SRBs are CHEAP. A Hammer costs 400 funds. An initial set of two clusters of five hammers hanged on two small hardpoints is 4120. That's pocket fluff. Compare that with cost of extending the stack so that a Mammoth runs 30 seconds longer.

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1 minute ago, Sharpy said:

Except when higher dV means bigger tanks - and more engines to lift these tanks.

Short-burn SRBs are CHEAP. A Hammer costs 400 funds. An initial set of two clusters of five hammers hanged on two small hardpoints is 4120. That's pocket fluff. Compare that with cost of extending the stack so that a Mammoth runs 30 seconds longer.

Worth your time to have a look at rockets tightly optimized for cost to orbit, like you can find here. Spoiler: High TWRs do not help reduce cost, none of the top entries have >2 off the pad.

 

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45 minutes ago, Red Iron Crown said:

Worth your time to have a look at rockets tightly optimized for cost to orbit, like you can find here. Spoiler: High TWRs do not help reduce cost, none of the top entries have >2 off the pad.

 

But does any of them have <1.4?

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3 hours ago, Red Iron Crown said:

That is true if you are optimizing for minimum dV to orbit. If optimizing for payload fraction or cost per unit of payload to orbit then lower initial TWRs work better.

Yeah, by "reasonably," I meant more along the lines of "keep your LF engines at full throttle, use a few SRBs and don't set your SRB thrust limit too low."  

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3 hours ago, Sharpy said:

But does any of them have <1.4?

IIRC, my "cheep36" lifter used 1.2 t/w off the pad. Having an all- solid first stage means there's no throttling through that portion of the flight. Even with tapering the thrust, the T/W gets a little hot which makes it difficult to follow a proper gravity turn.

The larger stages used parallel boosters, allowing for a little more aggressive T/W off the pad in the region of 1.6.

RIC is correct about cost and t/w. While high t/w allows for lower DV to orbit, this comes with a price of "moar boosters" and more aero/ reaction wheels required to fly an efficient course. The additional mass of high t/w designs also brings about a curiously paradoxical situation where you wind up expending more fuel, even though the DV expenditure is less.
 When chasing efficiency, using "DV expenditure" as a guide can be highly misleading.

Best,
-Slashy

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2 hours ago, GoSlash27 said:

 When chasing efficiency, using "DV expenditure" as a guide can be highly misleading.

Best,
-Slashy

Well... TIL.

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