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MunsterIII

How much Delta-V required to escape the atmosphere?

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Posted (edited)

I am trying to design the a rocket that has the exact right amount of Delta-V required to get to the Mun and back.  The first stage should be capable of getting the rocket clear of the atmosphere.  I have looked everywhere for how much Delta-V the first stage needs and have found nothing.  How much Delta-V should I be aiming for for my first stage to be able to get above 70 km?

Edited by MunsterIII
Misspelled "first"

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I don't have an exact value for you - it will vary depending on your gravity turn, etc - but probably around 2800-3000 should get you above 70km.  I'm sure someone will come along with a more accurate number

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Posted (edited)

Just getting above 70 km isn't sufficient information - how fast do you want to be going when you reach 70km?

The rule of thumb is that you need 3400 m/s to get into low Kerbin orbit (80 km). A typical launch might use 2400 m/s to get an Ap of 80km, then a 1000 m/s circularization burn.

There are an infinite number of variations possible.

 

 

 

Edited by FloppyRocket

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Posted (edited)

You'll need about 3400 dV for a LKO, ending with a speed of about 2300 m/s 

For my first stage, I usually go with about 1200-1500 dV, which brings my rocket to about 10-15 km altitude and a speed of about 500-700 m/s

You might want to check out this dV map:

 

Edited by VoidSquid

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1 hour ago, Fraktal said:

Um... all of my designs do 800-900 m/s circularization burns. If I don't, I spend too much fuel while in the atmosphere.

That's very different from my experience.  How much is needed for the circularization burn depends very much on the design of the rocket.  But regardless of the design, I find that for any particular rocket, the smaller I can make the circularization burn, the less delta-v I use overall.

I follow a standard set of guidelines for all my launch vehicles, therefore I get consistent performance and behavior out of all of them.  When I fly them manually, I can often get the circularization down to 100 m/s or so.  When I use an autopilot mod, like Gravity Turn, I can easily get it below 50 m/s.  For some rockets, however, we may not be able to achieve numbers that low.  I've had designs in the past where maybe a few hundred m/s was the best I could do.

In pretty much all cases, the lower the circularization delta-v, the less fuel is used.  If the circularization burn is large, then the rocket is being lofted into too steep an arc.  The rocket is accelerating too much vertically and not enough horizontally.  One of the most important things to keep an eye on is "time to apoapsis".  I try to keep the time to apoapsis to not more than about 60 seconds.  If the time to apoapsis starts to run away, pitch down, or even throttle back if necessary.  Just keep the apoapsis slightly out in front of the rocket and slowly push it higher and higher.  As you near the end of the burn, the time to apoapsis will start to rapidly increase, but that's OK at that point.  Just don't let it get out of control during the mid part of the ascent.

I find that high TWR rockets are much more difficult to control.  The time to apoapsis often gets away from me and it becomes difficult to rein it in.  I like a launch TWR of about 1.3-1.5.  Of course that's just my personal preference, doesn't mean it's right.

Of course there is also a limit to how flat you can make the trajectory.  If too flat, drag will be too great and you may not get to orbit at all.  I think there's a sweet spot where the trajectory is just flat enough to minimize gravity loses, and just steep enough to keep drag losses from getting out of control.

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17 minutes ago, AHHans said:

Considering that LKO orbital velocity is close to 2300 m/s this sounds a bit low. Did you mean 3000m/s?

Nope. Your first stage should get you up into the higher atmosphere and moving maybe 1000m/s. Then you used those massive inefficient but powerful engines and should toss them. Your 2nd stage should get you into orbit.

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Ideally my apoapsis would be just above 70 km, since the first stage is just to get the rocket into space.  I guess that means my speed would be about 0.

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1880 m/s is plenty to get an Ap over 70km, if you launch straight up and your TWR is near or above 2. For launching straight up, you want a high TWR.

Also if you launch straight up, then you know you will come back down very near KSC for a good high recovery percentage. It's silly to launch downrange if all you want is to break out of the atmosphere. It takes more dV to go downrange too.

 

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10 hours ago, MunsterIII said:

I am trying to design the a rocket that has the exact right amount of Delta-V required to get to the Mun and back.

Remember that in general, the rule here is that the amount of delta-V that works is the 'right' amount.

10 hours ago, MunsterIII said:

The first stage should be capable of getting the rocket clear of the atmosphere.  I have looked everywhere for how much Delta-V the first stage needs and have found nothing.

That's most likely because people don't generally single-stage a Mun rocket to orbit.  They usually reach orbit in two, or possibly three, stages.  Thus, aside from the usual advice that the initial stage should have between approximately 1.2 and 2.0 thrust-to-weight and each stage should have approximately two minutes' burn time, there really isn't much.

10 hours ago, MunsterIII said:

How much Delta-V should I be aiming for for my first stage to be able to get above 70 km?

That being said, if you want to optimise your rocket to the fullest extent possible and reach the absolute minimum required delta-V, then I think it can be done with 2,800 or 2,900 m/s.  However, I believe that the rockets that accomplished this had almost no payload; that's not an option for a Mun rocket.  The general advice of 3,400 m/s is about the best that you'll get.

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12 hours ago, VoidSquid said:

You'll need about 3400 dV for a LKO, ending with a speed of about 2300 m/s 

For my first stage, I usually go with about 1200-1500 dV, which brings my rocket to about 10-15 km altitude and a speed of about 500-700 m/s

You might want to check out this dV map:

 

My second stage is to get me from space into orbit.  3400 dV would be useful for getting into orbit, but right now I am just focused on how much the first stage needs.

3 hours ago, Zhetaan said:

Remember that in general, the rule here is that the amount of delta-V that works is the 'right' amount.

That's most likely because people don't generally single-stage a Mun rocket to orbit.  They usually reach orbit in two, or possibly three, stages.  Thus, aside from the usual advice that the initial stage should have between approximately 1.2 and 2.0 thrust-to-weight and each stage should have approximately two minutes' burn time, there really isn't much.

That being said, if you want to optimise your rocket to the fullest extent possible and reach the absolute minimum required delta-V, then I think it can be done with 2,800 or 2,900 m/s.  However, I believe that the rockets that accomplished this had almost no payload; that's not an option for a Mun rocket.  The general advice of 3,400 m/s is about the best that you'll get.

I am trying to minimize the size of the rocket, so the smallest amount of delta-v that still works would be what I am trying to find.  My Mun rocket is actually 4 stages: get into space, orbit and set up an encounter, get into Mun orbit and descend, and land/return.  I have already found good answers to how much dV the last three need, and this question is looking for how much dV the first stage needs.

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14 hours ago, MunsterIII said:

I am trying to design the a rocket that has the exact right amount of Delta-V required to get to the Mun and back.  The first stage should be capable of getting the rocket clear of the atmosphere.  I have looked everywhere for how much Delta-V the first stage needs and have found nothing.  How much Delta-V should I be aiming for for my first stage to be able to get above 70 km?

You should design the rocket as whole as required to complete the mission objectives.  The first stage will end up with whatever it needs.  Don't worry about trying to make it equal some specific number.

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Just now, OhioBob said:

You should design the rocket as whole as required to complete the mission objectives.  The first stage will end up with whatever it needs.  Don't worry about trying to make it equal some specific number.

I don't want to go overkill on the first stage, primarily because I am early on in career mode and don't have lots of cash lying around, and secondly because I am using SRBs to get to space and I don't want to over/undershoot.  I want to know exactly how much I need.

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12 hours ago, FloppyRocket said:

A typical launch might use 2400 m/s to get an Ap of 80km, then a 1000 m/s circularization burn.

That's not really a very efficient ascent.  If your circularization burn requires 1000 m/s, then you're lofting the rocket into too steep an arc.  You should try to flatten out the trajectory.  I try to limit my circularization burn to about 100 m/s.

3 minutes ago, MunsterIII said:

I don't want to go overkill on the first stage, primarily because I am early on in career mode and don't have lots of cash lying around, and secondly because I am using SRBs to get to space and I don't want to over/undershoot.  I want to know exactly how much I need.

Exactly how much you need is how much you need for the entire mission less how much the upper stages can deliver.

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Just thinking to myself...

Getting into a stable LKO requires a minimum dV (=energy). Getting for that LKO to Mun is another given amount of dV (=energy).

Now, is going directly from the Launchpad to Mun requiring significantly less energy than of going to LKO and, from there, to Mun?

Thinking about Oberth and that...

 

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30 minutes ago, OhioBob said:

That's not really a very efficient ascent.  If your circularization burn requires 1000 m/s, then you're lofting the rocket into too steep an arc.  You should try to flatten out the trajectory.  I try to limit my circularization burn to about 100 m/s.

It does depend on the rocket;  if there is a lot of draggy bits, then you do want to higher first.  But just as an example, I've been launching my shuttles (for the Shuttle Challenge) and have been doing a final circularization burn of about 25-50 m/sec

1 minute ago, VoidSquid said:

Just thinking to myself...

Getting into a stable LKO requires a minimum dV (=energy). Getting for that LKO to Mun is another given amount of dV (=energy).

Now, is going directly from the Launchpad to Mun requiring significantly less energy than of going to LKO and, from there, to Mun?

Thinking about Oberth and that...

Significantly, probably not.  Measurable, probably.  But if you go for a vertical launch, you aren't going to get the advantage of the rotational velocity of Kerbin at launch

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6 minutes ago, linuxgurugamer said:

if you go for a vertical launch, you aren't going to get the advantage of the rotational velocity of Kerbin at launch

Just what I'm thinking here, yes. LKO, then to Mun. Numerous advantages, imo.

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Posted (edited)
6 hours ago, OhioBob said:

That's not really a very efficient ascent.  If your circularization burn requires 1000 m/s, then you're lofting the rocket into too steep an arc.  You should try to flatten out the trajectory.  I try to limit my circularization burn to about 100 m/s.

I never claimed it was efficient ("typical" was the word I used), and I also said said that there are infinite possibilities.

I wasn't the one asking the question on this thread, just trying to be helpful.

Needless badgering is one of the reasons that people avoid posting on the forums.

 

Edited by FloppyRocket

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On 8/19/2019 at 6:07 AM, MunsterIII said:

Ideally my apoapsis would be just above 70 km, since the first stage is just to get the rocket into space.  I guess that means my speed would be about 0.

14 hours ago, MunsterIII said:

My second stage is to get me from space into orbit.

14 hours ago, MunsterIII said:

I am using SRBs to get to space and I don't want to over/undershoot.  I want to know exactly how much I need.

Are you doing a gravity turn at all? Because all you said sounds like you're trying to just launch straight up with your first stage.

 

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14 hours ago, FloppyRocket said:

Needless badgering is one of the reasons that people avoid posting on the forums.

What you call badgering I call trying to be helpful.  Pointing out a simple helpful tip on how to measure the efficiency of one's ascent trajectory (minimizing circularization delta-v) is not "needless".

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23 hours ago, linuxgurugamer said:

Significantly, probably not.  Measurable, probably.  But if you go for a vertical launch, you aren't going to get the advantage of the rotational velocity of Kerbin at launch

More or less an academic question, I know. But what weights more then, which one is more dV efficient? Oberth plus rotational velocity (ofc, both are related)? Or direct vertical ascend, avoiding some air friction, shorter path?

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On 8/18/2019 at 7:09 PM, MunsterIII said:

I am trying to design the a rocket that has the exact right amount of Delta-V required to get to the Mun and back.  The first stage should be capable of getting the rocket clear of the atmosphere.  I have looked everywhere for how much Delta-V the first stage needs and have found nothing.  How much Delta-V should I be aiming for for my first stage to be able to get above 70 km?

I usually design shorter missions based on staging events -- asking myself when I want the stages to happen, and then design each stage's delta-V around that. Generally for a Mun landing and return, I use 2 main staging events, plus the launch stage. After launch, the first staging event is when I switch to a vacuum engine. This lets the first stage fall back into Kerbin. The second event is when I switch to the lander's engine while landing at the Mun. This lets the second stage crash into the Mun. Then that final stage finishes landing on the Mun and returns.

Then, I design the rocket backwards from the final stage. I'll construct the lander keeping in mind that I'll need at least 1100 m/s (110 minimum for the final part of the landing + 580 Mun orbit + 310 return to Kerbin). If I have more, then that's fine too, because I can stage at any point during the landing burn and still let my previous stage crash into the Mun.

I then design my second stage. I know I'll need around 1800 m/s (100 minimum Kerbin circularization + 810 Munar Injection + 310 Mun circularization + 580 Mun de-orbit and land - Lander Stage Excess). If my lander stage has more than 1100 m/s, then I know that I can have a bit less in my second stage, since I can use the lander stage to perform a bigger part of the Munar landing. I also know that if my second stage has more delta-V, that is fine too -- because I can then make my first stage smaller and complete more of the Kerbin circularization using the second stage.

Finally I'll design my first stage (though it's not always just one stage). I know I'll need around 3400 m/s minus excess from the second stage. In my usual designs, my second stage can provide around 1000 m/s of the Kerbin orbit, so my first stage needs around 2400 m/s, which can be easily done with a single liquid engine or a liquid engine + side SRB boosters providing an initial extra kick.

In practice, I then ensure that each stage has around 100-200 m/s extra delta-V, just so I can deal with poor piloting or unexpected things.

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On 8/19/2019 at 10:05 AM, VoidSquid said:

Just thinking to myself...

Getting into a stable LKO requires a minimum dV (=energy). Getting for that LKO to Mun is another given amount of dV (=energy).

Now, is going directly from the Launchpad to Mun requiring significantly less energy than of going to LKO and, from there, to Mun?

Thinking about Oberth and that...

 

Yes, this has been discussed and analyzed quite extensively on this forum. It's generally considered a poor idea because burning straight up causes you to suffer more from gravity losses, which will more than offset any savings you might get. If you try to minimize gravity losses by using a rocket with very high TWR, then the increase in aero losses and the fact that you're carrying way more heavy engines means that you still lose out compared to just launching normally, especially in career mode.

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12 hours ago, OhioBob said:

What you call badgering I call trying to be helpful.  Pointing out a simple helpful tip on how to measure the efficiency of one's ascent trajectory (minimizing circularization delta-v) is not "needless".

I wasn't asking for help.

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Posted (edited)
2 hours ago, FloppyRocket said:

I wasn't asking for help.

Although I quoted you, I wasn't responding to you specifically.  I was addressing the forum at large, and anyone who would benefit from what I said.  You're not the only one reading.

 

Edited by OhioBob

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On 8/19/2019 at 6:34 PM, OhioBob said:

That's not really a very efficient ascent.  If your circularization burn requires 1000 m/s, then you're lofting the rocket into too steep an arc.  You should try to flatten out the trajectory.  I try to limit my circularization burn to about 100 m/s.

Um... all of my designs do 800-900 m/s circularization burns. If I don't, I spend too much fuel while in the atmosphere.

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