• 0
MunsterIII

How much Delta-V required to escape the atmosphere?

Question

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"

Share this post


Link to post
Share on other sites

Recommended Posts

  • 0

I remember this video about testing different ascend trajectories, maybe it's of interest here

 

Share this post


Link to post
Share on other sites
  • 0
27 minutes ago, OhioBob said:

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.

Mine only exceeds 1 minute at about 50 km altitude.

28 minutes ago, OhioBob said:

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.

I go with 1.7 myself and dial the engines down in the VAB to be no more than that at 100% throttle. Sometimes it's less, when I'm launching a heavier payload that doesn't merit using a bigger launcher.

29 minutes ago, OhioBob said:

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.

I start gravity turning at 100 m/s vertical speed and target a parking orbit just above 120 km (for time warp convenience). I'm completely incapable of launching on the same trajectory twice but I usually hit the target apoapse altitude while prograde is between 10-20 degrees above the horizon and either cut the engines then circularize at apoapse, or I pitch down and start burning somewhere halfway between prograde and radial-in to try and maintain apoapse while circularizing if I'm still far away.

Share this post


Link to post
Share on other sites
  • 0
16 minutes ago, Fraktal said:

Mine only exceeds 1 minute at about 50 km altitude.

To be honest, I don't remember at what altitude it starts to exceed 1 minute.  I've been playing scaled-up solar systems (when I do play, which isn't that much anymore); haven't played stock size in quite some time.  I seem to recall that at 50+ km I'm still trying to keep it at <60 s.  I usually throttle back at that point and slowly push the apoapsis up to my target altitude.  I'm thinking it's more like 60-65 km before the time to apoapsis starts to grow, but I could be misremembering.  Of course if the rocket is lofted into a steep arc with a large circularization burn, you're going to be cutting off the engine sooner and at a lower altitude.  The fact I'm still burning at ~65 km is what happens when you minimize the circularization burn.

It's not like it's a huge difference.  I just know from past experience that, for a given rocket, my ascent is more efficient if my circularization burn is 100 m/s versus 500 or 1000 m/s.

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)

Especially with early career mode, it's very tempting to design tight tolerances with your rockets. This generally results in kerbals stranded in space and needing rescue. Sometimes several times.

I would design a first mun mission like so:

LKO: 4k dV. Yes, this is too much, at least on paper. If you have the dV, you will find uses for it, and if it turns out this was not too much, then you were prepared. A science mun trip tends to be draggy and hard to control, meaning you'll need extra dV for drag and corrections.

Lunar transfer: 900 dV.

Lunar circularization: 300 dV.

Lunar descent: 900 dV. Yes, this is too much, on paper, but landings are hard.

Lunar ascent: 900 dV.

Lunar return: 900 dV.

Kerbin descent: 300 dV.

This is intentionally over-engineered.

If you're using a solid rocket first stage, I'd actually suggest a different mission profile first, if you haven't already.

Launch two unmanned science satellites with a science jr or two and 2000 or so dV apiece. Send one to low munar orbit, and one to either minmal orbit or solar orbit (or both!). Return them to LKO and send an astronaut up to retrieve the science.

This will give you tons of exploratory cash, tons of science, and get you in a good position to make a really over-engineered manned lunar lander. Then you could just swap one of the satellites for a transfer stage and the other for a lander and you should be set.

Edited by dire

Share this post


Link to post
Share on other sites
  • 0
1 hour ago, dire said:

Especially with early career mode, it's very tempting to design tight tolerances with your rockets. This generally results in kerbals stranded in space and needing rescue. Sometimes several times.

I would design a first mun mission like so:

LKO: 4k dV. Yes, this is too much, at least on paper. If you have the dV, you will find uses for it, and if it turns out this was not too much, then you were prepared. A science mun trip tends to be draggy and hard to control, meaning you'll need extra dV for drag and corrections.

Yes, my experience is that early career rockets typically require more delta-v to attain orbit than later rockets.  1.25-meter parts produce greater drag losses than the 2.5+ meter parts.*  Also early rockets are often launched before fairings are available.  While conventional wisdom says 3400 m/s is required for orbit, I usually figure at least 3600 m/s for 1.25m rockets, and even more if I have a draggy payload and no fairing.  4000 m/s may be overkill, but it's a wise precaution for somebody planning their first Mun mission.

* It's not that 1.25-m rockets produce more drag, but because of their small size, they have less mass per cross-sectional area (i.e. a lower ballistic coefficient) than larger rockets.  This means they have a large ratio of drag force to mass, so they are slowed down more by the drag they do produce in comparison to their larger cousins.

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)
On 8/19/2019 at 11:48 PM, Harry Rhodan said:

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.

 

I was planning on using the second stage to turn 90 degrees east at apoapsis and just burn straight until my periapsis was above 70 km.  Based on all these comments, I guess I am going to change my design to do a more gradual turn.

2 hours ago, dire said:

Launch two unmanned science satellites with a science jr or two and 2000 or so dV apiece. Send one to low munar orbit, and one to either minmal orbit or solar orbit (or both!). Return them to LKO and send an astronaut up to retrieve the science.

I completed a couple "test while landed" missions, and a mission popped up to "plant a flag on the Mun" for about 123k funds.  I was considering using drones, but I can't pass up that easy money if I am going to the Mun anyway.

Edited by MunsterIII

Share this post


Link to post
Share on other sites
  • 0
44 minutes ago, MunsterIII said:

I was planning on using the second stage to turn 90 degrees east at apoapsis and just burn straight until my periapsis was above 70 km.  Based on all these comments, I guess I am going to change my design to do a more gradual turn.

I completed a couple "test while landed" missions, and a mission popped up to "plant a flag on the Mun" for about 123k funds.  I was considering using drones, but I can't pass up that easy money if I am going to the Mun anyway.

Generally, the closer your nose is to prograde, the less dV you lose from steering losses. These steering losses apply whether you are in an atmosphere or not (although losses are higher in atmo due to drag) because the hypotenuse of a triangle covers a shorter distance than the legs.

I'm not saying "Don't plant a flag on the Mun," I'm just saying "Get the easy money in low munar orbit first, then get the easy money on the munar surface." If you have to send three rescue missions because you brought 6400 dV instead of 7400 dV, at a cost of 300k, your 123k funds of easy money are going to look like a huge loss.

Share this post


Link to post
Share on other sites
  • 0
3 hours ago, MunsterIII said:

I was planning on using the second stage to turn 90 degrees east at apoapsis and just burn straight until my periapsis was above 70 km.  Based on all these comments, I guess I am going to change my design to do a more gradual turn.

Good! A good cheap reliable lifter will have good atmosphere engines (with an atmo isp that's close to their vacuum isp) on the bottom stage with a TWR of around 1.3-1.6 and a total dV of somewhere around 2000 vacuum (even though you're in atmosphere, use the vacuum numbers, it's how we all do it and it lets us all talk about the same thing. And the numbers aren't that different if you're using the right engines). Then for your second stage have good vacuum engines with 1500+ m/s of dV and a middlingly good TWR. Doesn't need to be 1.0 even but that's a good number to start with. Feel free to use solid boosters to help that first stage, but count their dV added to the total dV of the first stage, don't consider them the first stage themselves. And of course drop them as soon as they run out of fuel.

If you do your gravity turn correctly, you'll still be in the atmosphere when you drop that first stage but you'll be high enough that vacuum engines will work at almost maximum efficiency (anything over 10km is basically space) and you'll be burning almost sideways, pushing your Ap ahead of you and slightly raising it until you get it to 80-100km. Then cut the engines, coast, and give it the last 50-500m/s you need to actually reach orbit.

Expect yourself to take multiple tries to get this right, and then multiple months of flying as you tweak your gravity turn with experience. It's not something you get right the first time. You're not ticking off an achievement from a list, you're learning a skill that you'll use on almost every single mission from here on out.

Share this post


Link to post
Share on other sites
  • 0
Posted (edited)
10 hours ago, 5thHorseman said:

and a total dV of somewhere around 2000 vacuum

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

Edit: My bad! @5thHorseman was only talking about the 1st stage, not the whole way to orbit.

Edited by AHHans

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.