New player; How to get to orbit with the Dv suggested in the Dv Maps?

[katateochi]

This crazy thing is one of my early 1.0.5 career craft. It has a dv of 3500m/s | 3952m/s (atmo | vac) and is fairly low tech.  It can reach a 200km orbit with plenty of fuel left to de-orbit (or raise it's Ap to around 2Gm) 

It needs the first 5 tech tree nodes unlocked (ignore MJ in the above pic, just added it to measure the dV) and the first upgrade to the launch pad. VAB can still be entry level. pic of required tech nodes

It doesn't use any fins, but it does have two extra command pods which provide a bit more SAS which helps with the initial turn, once they're dropped it remains stable. It's still viable without the command pods, just has a much steeper ascent and requires a fair bit of counter steering to keep it from raising the Ap too high.
NOTE: lower stages separate by being overheated when igniting the stage above (so explosions are part of the process here).

At about 1.5km up start trying to turn (hold D down hard, it's sluggish at this point).  basically hold D constantly, except when staging just to be safe, until the command pods are dropped, then a lighter touch is needed.
First stage sep at ~2.7km (still fairly vertical, but pitched over a little)

By 5km you should be pitched to about 70 degrees, by 10km ~60 degrees (I'd ideally aim for 50 degrees, but this craft won't pitch over fast enough for that)
2nd stage (with the command pods) should sep around 15km (pitch is around 50 degs now)

Now it will start to be more responsive to its pitch control.  
At 20km pitch should be a bit under 40 degs, decreasing to 30 degs by 25km (next stage around this point).
At 30km be pitched over to 20 degs

At 40km pitched over to 10 degs (penultimate SRB stage around this point, which also tends to explode violently)

Now on the last SRB and the Ap should be around 60-65km.  Pitch over to 0, and as the Ap passes 70km start pitching down!! There is still booster to burn but you want to prevent the Ap from going too high while also getting as wide an ascent path as possible.  Pitch down to ~10-15 degrees (even steeper is ok if required, as is the case if you don't have the extra command pods to start with).
By the time the SRB burns out Ap should be around 90km and you should have a fairly wide ascent path.

You should be able to circularize, and then raise the Ap and Pe to 200km and still have about half the fuel left, way more than needed for deorbiting (is enough to raise the Ap to ~2Gm, but then you're stuck up there!).

 

This isn't my ideal ascent profile, it's a bit steeper at the start than I usually go for, but this craft is a bit sluggish.  My general guideline is; at 2km start G turn, by 5km be pitched to 70. At 10km pitch to 50.  Then I start focusing on my Ap rather than current alt.  When Ap is 20 pitch to 40, when Ap is 30 pitch to 30, when Ap is 40 pitch to 20, when Ap is 50-55 pitch to 10 and be pitched to 0 by the time Ap is 70km.  Basically decrease pitch by 10 degs for every 10km increase in Ap.  I launch pretty much all rockets on that profile, it's kinda a no-brainer and seems to work well for most things. 

Sorry for tons of small pics, Imgur was refusing to let me create an album, just click on em to see the details.

 

 

 

 

[MoridinUK]

Right, yes Jeb is pulling on the pitch to try to stop the whole thing leaning westward.  even when rotated it still leans westward flying straight up.

Making the grav turn at 10k is very very slow due to most pitch being used to even keep the craft straight.

Having said that I just got 80k ap with a -56k pe which is very close, and still running my own rocket.  I'm going to rebuild from scratch the one mentioned above and try with that...  

[MoridinUK]

35 minutes ago, WuphonsReach said:

I can get a reliable 3500 dV to 85km orbit out of MJ with the turn starting at 3500m, by around 30,000m it's at least 45-60 degrees and from 45km upwards you're pointed almost horizontally.  Starting TWR is usually around 1.3-1.6, second stage has a TWR of 1.0.

(MJ setting of 3.5km or 100 m/s to start the turn, final flight path angle of 1 degree and turn shape of 60% with a target altitude of 85km.  I've done enough by-hand launches, I just let MJ do it now.)
 

I still can't figure out how to get MJ to fly things for me, am I missing a mod? I have the Kerbal Engineer already?

[HvP]

Ok, so I just recreated your design in sandbox, MoridinUK, and it will make a stable orbit without modification.

The process was this:

Begin gravity turn as soon as you hit 30km/s by pulling hard over to the east. Your rocket is actually very aerodynamically stable at that point and thus resists turning over so you will want to keep pitching down - keep your navball centered on the lower edge of the yellow circle of your prograde marker for the entirety of your ascent.

When your SRB burns out at 6000m use the opportunity to pitch downward just a teeny-tiny bit below your prograde to drag your gravity turn a little bit more sideways before igniting that second stage. Keep it hard over following the lower edge of your prograde marker until the second stage burns out at about 30km.

This isn't a very efficient gravity turn though because a rocket with a little more control would ideally be well past the 45 degree mark by now. At this point your apoapsis should be in the mid 70km range. Jettison the second stage but don't ignite your final stage until you have exited the atmosphere and turned flat into a circularization burn which should put you into a circular orbit between 75 and 80km.

[MoridinUK]

HvP I have been trying to do this all evening! lol... I guess I'm just a bit naff at flying...

Did you leave the SRB at 100% thrust?  I think in my runs it gives up somewhere around 5k, not 6.  I'll give this a try soon though..

[HvP]

Or much more simply: Reducing the thrust on your SRB in the VAB down to 90% gives you much more control in the trans-sonic region and allows you to fine tune your gravity turn much better in the initial stages of the launch. In fact, you'll have to throttle back on you final stage to keep from exploding the goo canisters because you have so much velocity when staging your final engine.

I was able to make 80km by 90km orbit with 300m/s delta-v remaining on your design this way.

Edited December 9, 2015 by HvP

[HvP]

21 minutes ago, MoridinUK said:

HvP I have been trying to do this all evening! lol... I guess I'm just a bit naff at flying...

Did you leave the SRB at 100% thrust?  I think in my runs it gives up somewhere around 5k, not 6.  I'll give this a try soon though..

For that ascent profile the SRB was at 100% thrust, but you're fighting it the whole way which keeps your ascent pretty vertical.

Limiting the SRB throttle to 90% drastically altered the ascent but allowed me to turn towards 45 degrees much sooner. It also means that the second stage runs out much deeper in the atmosphere but at a higher velocity. I believe I ignited the final stage at 35km which as at 1100 m/s and exploded the goo due to overheating. I'm sure it's possible to moderate that last burn and ignite it a little later higher up when that speed is safer.

[katateochi]

12 hours ago, MoridinUK said:

I still can't figure out how to get MJ to fly things for me, am I missing a mod? I have the Kerbal Engineer already?

If you're in career mode then you gradually unlock the different MJ modules, you don't get the ascent autopilot module until later on.

You could take the rocket over to a sandbox save and try it out with full MJ there.  There's a deal of controversy around using MJ, but it's a good way to learn by observation (although I don't know how good the ascent autoP is since the atmo was changed).

Edited December 10, 2015 by katateochi

[GoSlash27]

http://wikisend.com/download/603504/orbit1.craft

A very simple early career example craft:

1) Launch vertically at full throttle with SAS engaged

2) Hold vertical until 90 m/sec velocity

3) Pitch East 10° so you're holding 90° pitch

4) Maintain 90° pitch until prograde marker matches 90° prograde as well

5) When prograde marker is at 90° pitch prograde, disable SAS and go get a snack.

6) At 30 km altitude, reengage SAS and maintain nose in the center of the prograde marker.

7) When apoapsis reaches 72 km, cut throttle.

8) when ship reaches apoapsis, throttle up.

9) when periapsis reaches 72 km, cut throttle. You are in orbit.

Best,

-Slashy

[Vanamonde]

How-to question moved to the how-to subforum. 

[MoridinUK]

HvP I'm in awe that you can get that thing into space!  I also never ran it with the srb on even 90%, I have the twr down to 1.5ish if I remember right! 

This does rather prove to me that it isn't my designs that are awful but my flying them!

 

MJ I don't think I'd use it much, but for things once they become routine, launches for supplies to space stations etc, anything big interesting or one off I'm gonna want to launch myself!  

[WuphonsReach]

2 hours ago, MoridinUK said:

MJ I don't think I'd use it much, but for things once they become routine, launches for supplies to space stations etc, anything big interesting or one off I'm gonna want to launch myself!  

That's the main reason I use MJ, even in career (where I'll give myself 500k funds and 5000 science because I've done the start-over way too much).  It relieves the tedium of getting yet another launch to orbit, or getting up to a certain Ap, or circularizing, or any other maneuver that I've done a few hundred times by this point.

[MoridinUK]

Right, I've got it.  I had replaced the lt-30 with an lt-45 so I'd have more attitude, this meant I ended up leaning way over and ending up to much in the think of it.  Also the extra punch out of the lt-30 seems to make a difference.

Starting the g turn at 30m/s with teh booster on 90% it gives out just before 5k, and I'm almost at 45 degrees by then.  so I hold it there for a while, 2nd stage burns out at 20km alt, apo of 37 ish.  I then do the 10 degrees per 10km until apo hit 77 then I wait till I leave orbit and try to circularise.  I have real trouble keeping the ap down, but got a 100km ap with a 80km pe, and 200 m/s in the tank!  I was happy with that.  Though this still isn't as efficent as HvP manages.  I also noticed that during the second stage, jeb was using almost full pitch to stop the craft tipping over eastwards too much, a real gravity turn I guess, should I have turned sas off and let it turn naturally or not?

My 2nd stage I'm buring at full throttle could I get more efficient if I backed off a bit? 

[Snark]

For SRBs:  Yes, you should definitely adjust the thrust, 100% is almost never the right amount.  However, the rule of thumb needs to be:  decide what TWR you want first, then adjust your SRBs to give you that much.

"What's the right TWR on the launchpad" is a whole topic in its own right, but the conventional wisdom is to aim for 1.3-1.5.  (I prefer 1.5 myself.)  You don't want it to be too low or you waste too much fuel on gravity losses.  You don't want it to be too high or you end up wasting fuel fighting drag while you're still low, and you're also wasting mass on having more engines than you need.

So let's say you've picked a target TWR you want to have on the launchpad, let's say 1.5 for the sake of argument.  How do you set your SRBs?

Well, first you need to know how much thrust you need.  Take your total ship mass in tons, multiply by 9.8 m/s2 (Kerbin gravity), then multiply by desired TWR (1.5, in this case).  This tells you how much total thrust you need, in kN.  Then add up all the thrust of your SRBs if they were 100%.  That tells you how much thrust you have available.  Divide the needed thrust by the available thrust, and this gives you the power setting for your SRBs.

For example, let's say your rocket has a total launchpad mass of 15 tons, and you're taking off with a single Thumper SRB.  The thrust you need is 15 tons, times 9.8m/s2, times TWR of 1.5, which gives you 220 kN needed thrust.  The Thumper gives 250 kN of thrust at sea level.  220 / 250 = 0.88.  So in this case you would set your SRB to 88% thrust to get the desired TWR.

If your calculated percentage is >100%, you don't have enough, add MOAR BOOSTERS.  If your calculated percentage is really low, like <60%, you may be over-supplied with SRBs and may want to consider reducing them.

A typical launch profile at TWR 1.5 looks something like this (at least, it works well for me):  Start the turn right away, as soon as I'm up to 20 m/s or so and barely off the launch pad.  That's a very gentle nudge eastwards.  Gradually nudge it a bit more and a bit more, the goal being to be at around 45 degrees off vertical by the time I'm at 10km, and going at least 300 m/s.  Then punch it, sticking prograde all the way.  Typically I'm at around 20-30 degrees above horizontal by the time I'm at 25 km.  Stop the burn when my Ap is at the desired height, then coast up to Ap and circularize.

[HvP]

Congratulations on taking your craft to orbit!

7 hours ago, MoridinUK said:

Right, I've got it.  I had replaced the lt-30 with an lt-45 so I'd have more attitude, this meant I ended up leaning way over and ending up to much in the think of it.  Also the extra punch out of the lt-30 seems to make a difference.

Starting the g turn at 30m/s with teh booster on 90% it gives out just before 5k, and I'm almost at 45 degrees by then.  so I hold it there for a while, 2nd stage burns out at 20km alt, apo of 37 ish.  I then do the 10 degrees per 10km until apo hit 77 then I wait till I leave orbit and try to circularise.  I have real trouble keeping the ap down, but got a 100km ap with a 80km pe, and 200 m/s in the tank!  I was happy with that.  Though this still isn't as efficent as HvP manages.  I also noticed that during the second stage, jeb was using almost full pitch to stop the craft tipping over eastwards too much, a real gravity turn I guess, should I have turned sas off and let it turn naturally or not?

My 2nd stage I'm buring at full throttle could I get more efficient if I backed off a bit? 

Just a few points to consider. If you are already at 45 degrees below 5k then you have turned over a little too quickly. Ideally, you should hit the 45 degree mark at about 10k, so that is probably where you are losing some efficiency. I'd guess that the extra weight of the LT-45 compared to the LT-30 may be the culprit here. You want to imagine your ascent as a smooth curve from the point you begin to turn towards a region where you are almost horizontal when you are leaving the atmosphere.

I'd say that you did really well considering that. I didn't test it with the swivel engine in the second stage, but you should have no problem if you wait maybe a little longer to bank over. Just a few seconds may be all it takes. Keep in mind that you will go faster the more horizontal you are at the same throttle level and risk your nose falling too flat if you throttle down at that point. Try to keep your nose up if you are overheating until you are past about 30km where you should have no problem throttling back if you find yourself pushing your apoapsis too high.

Every craft is a little different, and it's interesting how little changes early in the flight can dramatically affect the later part of the ascent. As always, I believe Scott Manley on YouTube can be of assistance here: First Orbit For Beginners

Edited December 10, 2015 by HvP

[Plusck]

I tend to dial down the thrust on SRBs quite often but I do wonder if it is really a smart move. So I'm starting to head the other way and consider that I want them to get my liquid fuel as high and as fast as possible before I ditch them.

I too tried the OP's first design. I tried it with a noob pilot too, just in case that made a difference.

10 tries later I eventually got a stable orbit - and down again with 2 goo cannisters on the roof. However, it was with the SRBs at 100% that it eventually worked. At 90% I fluffed the Ap and got it too high.

What worked was much as what HvP said. Only difference was I nudged east just a touch straight off the launchpad (i.e. got the chevron dot just off but touching the white dot), then let the prograde gently fall down to about 70° by the time the SRB gave out. I tried another few times and it seem you have to be in the 55° to 70° range at the end of the first stage, otherwise you won't make it. By the time the first Reliant is out of fuel it is massively overpowered, so I actually ended up coasting to 72km Ap from somewhere around the 42km mark after only three seconds of burn with the final stage.

After trying again with SRBs at 90%, it was harder at first but equally possible. It will fall towards the horizon much more, meaning the start of the turn has to be even gentler, but I actually managed to be even more efficient.

 

TLDR: it's ok, I just realised that Snark said essentially the same thing

Edited December 10, 2015 by Plusck
tldr

[Snark]

2 hours ago, Plusck said:

I tend to dial down the thrust on SRBs quite often but I do wonder if it is really a smart move. So I'm starting to head the other way and consider that I want them to get my liquid fuel as high and as fast as possible before I ditch them.

...

TLDR: it's ok, I just realised that Snark said essentially the same thing

heh.  Always fun to watch a bunch of folks violently agreeing with each other...

For a simple demonstration of the "too much thrust is bad" principle, try playing this little game:  Make a ship consisting of a Mk1 command pod, Mk16 parachute on top, Flea underneath.  The goal of the game is to reach as high an altitude as possible.  The only thing you can change is the power setting on the Flea.  At what power setting do you reach the maximum altitude?  (hint:  it's a whole lot less than 100%!) 

[WuphonsReach]

One of the reasons I run a 60% curve in MechJeb is due to overheating of the pointy part of the rocket in the 20-25km altitude range.  The default (and probably optimal curve) is around the 35% setting in MJ's ascent autopilot, but the nose will get really warm.

Once I hit 60km (going about 1500 m/s), I'm already deploying my Comm-16 or Comm-32 (due to needing communications via RemoteTech).  The fairing also gets ejected above 60km and since I tie the same button to my solar panels, those start to unfurl as well.

[Mister Dilsby]

Could the OP (original poster) maybe update the OP (original post) with a better title? "Newish player help" could mean anything. You actually are asking an interesting question, which others might be interested in seeing answered--a more descriptive title would help them find it

[Plusck]

2 hours ago, Snark said:

For a simple demonstration of the "too much thrust is bad" principle, try playing this little game:  Make a ship consisting of a Mk1 command pod, Mk16 parachute on top, Flea underneath.  The goal of the game is to reach as high an altitude as possible.  The only thing you can change is the power setting on the Flea.  At what power setting do you reach the maximum altitude?  (hint:  it's a whole lot less than 100%!) 

Absolutely, I've done the same tests myself.

However, the reason I started dialling back the SRBs so much was because I was going through a phase of putting seriously draggy and light payloads into orbit (typically hubs for my space stations). Even exceeding 140m/s under 12km or 180m/s under 18km would make them flip. So I got into a habit of taking it extra slow and extra straight up to about 14km or so.

At the same time my choices for gimballing engines were limited, so I absolutely needed to keep a significant level of thrust to keep control in the lower stages. That also meant balancing the SRBs to ensure they made a significant contribution while still leaving the main engines with enough thrust for control.

It was only later that I realised that I was really overdoing it and losing a lot of precious fuel. Now that all of the stock parts are available, I'm coming back round to square 1 - the SRBs are there to do their job quickly and then drop ASAP, leaving as much of the precious fuel as high as possible so that the more efficient engines can kick in usefully, while going faster (and therefore deriving more kinetic energy) and with a better Isp.

 

Edited December 10, 2015 by Plusck
typo

[MoridinUK]

10 hours ago, Snark said:

heh.  Always fun to watch a bunch of folks violently agreeing with each other...

For a simple demonstration of the "too much thrust is bad" principle, try playing this little game:  Make a ship consisting of a Mk1 command pod, Mk16 parachute on top, Flea underneath.  The goal of the game is to reach as high an altitude as possible.  The only thing you can change is the power setting on the Flea.  At what power setting do you reach the maximum altitude?  (hint:  it's a whole lot less than 100%!) 

I've been trying this flea experiement but with a payload to better simulate what we would actually want a booster to do. so mk1, parachute, a 200 tank and a lt-30 engine, decoupler and the flea.  

on 100% thrust it gets to 3500 before starting to come back to kerbin.
on a twr of 1.5 it only gets to 1700 before starting to come back to kerbin.  

I should do some more test, but I'm having problems with my simulation mod!

 

[Snark]

11 hours ago, MoridinUK said:

I've been trying this flea experiement but with a payload to better simulate what we would actually want a booster to do. so mk1, parachute, a 200 tank and a lt-30 engine, decoupler and the flea.  

on 100% thrust it gets to 3500 before starting to come back to kerbin.
on a twr of 1.5 it only gets to 1700 before starting to come back to kerbin.  

I should do some more test, but I'm having problems with my simulation mod!

 

Except that that test has issues, too.

The problem is that the Flea is such a pathetically useless SRB.  It's just too small.  It's like the Stayputnik:  better than nothing when you don't have any other alternative at all, but becomes completely useless as soon as you have a better option.  Which is soon.  The Hammer is a great workhorse, it's far more effective than the Flea, and becomes available very, very early in the tech tree.  Realistically, nobody would launch a Mk1 + 1t tank + Reliant with a Flea.  They'd use a Hammer.

The reason that you saw better performance with 100% thrust in that scenario is that the Flea runs out of fuel so quickly that it never gets even close to Mach 1.  At 100% thrust, it tops out at 240 m/s or so.

If air resistance weren't an issue, you always want to run all your engines at the maximum possible power.  When you take off from the Mun, for example, you blast off at full throttle, no pussyfooting around.  For that matter, if you launch a liquid-fueled rocket off the pad on Kerbin with no SRBs, you want to launch at max throttle, because you're doing nobody any good while you're putt-putting along under 200 m/s.  You want to get up to the 200s as fast as you possibly can, and then consider maybe throttling back for a little while as hit max Q before you throttle up again.

The only reason to ever run an engine at less than its max thrust is to avoid wasteful air resistance.  And if your SRB runs out of fuel long before you go very fast, you don't get any benefit from taking it easy-- instead, you pay the penalty of much higher gravity losses.

It's also worth noting that the test ship is a very streamlined one.  Not every rocket is going to be as streamlined as that, and draggier rockets have to care more about passing Mach 1 at low altitudes.

[MoridinUK]

Sigh, this was all a lot easier in 0.9!! lol... I'm not sure what you mean by max Q?  However, I did follow the rest, lol... I'll play with a hammer under it instead to see what's happening.  

You know what? It seems there is a very good reason they refer to rocket scientists as being somewhat clever huh?

[Snark]

10 minutes ago, MoridinUK said:

Sigh, this was all a lot easier in 0.9!! lol... I'm not sure what you mean by max Q?  However, I did follow the rest, lol... I'll play with a hammer under it instead to see what's happening.  

You know what? It seems there is a very good reason they refer to rocket scientists as being somewhat clever huh?

https://en.wikipedia.org/wiki/Max_Q

It's when the aero stress on a ship is at a maximum, therefore the point at which things are most likely to go kablooie, thus a point of lively interest from rocket engineers.

Rockets go faster and faster as they launch.  Initially aero forces are zero because the rocket's sitting still on the launch pad.  As it accelerates, the aero forces go up (because it's going faster).  On the other hand, the rocket is also climbing, which means the air is getting thinner, which tends to reduce the aero forces (and they reduce all the way down to zero, or very close to it, once the ship is in orbit).

So the profile of aero forces is to start at zero, go up to some maximum, then start to go down again and end up close to zero in orbit.

That maximum point (the point of "maximum dynamic pressure") is referred to as "max Q" for short.  Since it has the potential to rip the ship apart if the aero forces are too great, rockets may throttle down a bit to ease them through it (i.e. to experience a lower max Q) until the air gets thinner and they can really stretch their legs.

Max Q is less of a concern in KSP than in real life because KSP rockets generally don't need to worry about getting torn apart by aero forces, unless you're running a mod like FAR.  Yeah, things are harder post-1.0 than they were in 0.90... but still nowhere near as hard as real life.

[Plusck]

I just ran through the flea experiment to check.

Should have just trusted my intuition because the answer is... of course... 42.

 

This is with just a flea, a mk1 pod and nothing else. Valentina trusted me that she'd revert so she volunteered.

Of course the reason is you hit the sound barrier wall in any event, at low altitude. Put more thrust in and you lose, quicker, a battle you were never going to win anyway. Less thrust and you only just hit the barrier, and spend more time just countering that -9.81m/s per second thing sucking at your soles.

 

Final thing to add for efficiency: don't forget that you are moving fowards by chucking mass backwards. If the mass you are chucking backwards is hurtling back into the ground at high speed, that is energy you are putting into the ground for nothing. You always get more energy if your exhaust is doing nothing much. Therefore, the closer you are to sending exhaust directly behind your direction of travel, and the faster that travel is, the more energy you are sucking out of your exhaust and putting it into your ship.

Therefore you really do want to go as fast as possible and thrust in that direction. Starting off the launchpad slowly is blasting the earth with masses of energy for nothing. Driving into low atmo sound barrier is pushing pointlessly into highly resistant air. And burning in any direction other than purely prograde (if you want to go faster, retrograde otherwise) is giving your exhaust added energy for nothing. That last point can't be helped in a lot of cases, but shouldn't be forgotten either.

So you never start at anything less than 100% thrust, to get going as fast as possible ASAP. And if there is any time to save your fuel's potential energy, it is while you wait to get into thinner air where the sound barrier is much less of a big deal. Doing that at anything less than about 2/3 of the speed of sound is just giving more time to the 9.81m/s/s sucker under your feet to pull you down.

Edited December 11, 2015 by Plusck
plus point