Help me escape the atmosphere

[zyco187]

 

I am having trouble reaching the mun bc i cannot find a rocket configuration that will escape the atmosphere at 4600 delta v. i am currently using this map. and by my current calculations it takes 4600 delta v to get into orbit around kerbin. adn then another 2600 to get to the moon.

 I am short by about 1k i believe and im having to burn too much fuel getting into 80k orbit. Take a look at my current rocket config and provide any useful feedback.

Edited October 1, 2018 by Deddly
Thread of the month, congrats :)

[HebaruSan]

Hi, and welcome to the forum! I see two problems with that rocket.

1. The only control it has is the command pod's reaction wheels. You'll usually need control surfaces (the fins you've got don't move), engine gimbal, or RCS thrusters to control your rocket well enough to perform a decent gravity turn (unless you get really lucky with the initial launch angle).
2. The atmospheric TWR of the second stage is less than 1 (click ATMOSPHERIC in your KER window to see this), which means it would fail to take off on its own. Since you're starting out with those side boosters, they'll give you some initial velocity, but once you drop them and turn on the central engine, gravity will be more powerful than the engine, so you'll slow back down and start falling despite having some thrust. That second stage needs more thrust or less mass.

2 hours ago, zyco187 said:

by my current calculations it takes 4600 delta v to get into orbit around kerbin.

It's currently about 3200-3400 m/s for a rocket with sufficient thrust.

Quote

adn then another 2600 to get to the moon.

The diagram says 860 to get an intercept, 310 to capture, 580 to land = 1750 total.

Of course it's hard to land totally efficiently so you can add an extra 100 or 200 of padding for that. Then 580 to get back to orbit, 310 to return to Kerbin = 2640-2840 for a round trip.

Edited September 19, 2018 by HebaruSan

[OhioBob]

2 hours ago, zyco187 said:

I am having trouble reaching the mun bc i cannot find a rocket configuration that will escape the atmosphere at 4600 delta v. i am currently using this map. and by my current calculations it takes 4600 delta v to get into orbit around kerbin. adn then another 2600 to get to the moon.

I don't know where you're getting those numbers.  Based on the looks of your design, I assume your mission is to orbit Mun and return.  That should take,

Launch to Kerbin orbit:  3400 m/s
Transfer to Mun:  860 m/s
Mun orbit insertion:  310 m/s
Return to Kerbin:  310 m/s

TOTAL:  4880 m/s (plus any safety margin you'd like to add)

Your rocket has 6674 m/s, which is over designed.  You don't need a rocket anywhere near that size for a Mun orbit mission.  I'd start by removing most of the fuel in the upper stage.  You probably don't need more than one FL-T400 tank, which would give your upper stage a delta-v of about 2000 m/s.  And the reduced weight up top will increase the delta-v and thrust-to-weight ratio of the first stage and SRBs.  It wouldn't surprise me also if you could reduce the number of strap-on SRBs from 3 to 2.
 

Edited September 19, 2018 by OhioBob

[Scarecrow]

I built a duplicate rocket to yours as best I could from the image you posted.  I couldn't quite make out what scientific instruments you have attached to the capsule, but performance figures are very similar.

As built, I found that there was insufficient control to make an adequate gravity turn, which meant that all of the initial burn was spent going almost straight up.  After the solid boosters were jettisoned, I subsequently struggled to build up sufficient horizontal velocity to achieve orbit.

I made 2 changes, and was able to do a Mun flyby with fuel to spare.  The first change was to swap the fins you have for AV-R8 winglets, which are steerable.  A gravity turn will have you heading at 45 degrees from vertical by the time you reach 10,000m, give or take.  The winglets will allow you to achieve that.

The second change I made was to move one of the FL-T400 tanks from the upper stage to the lifting stage.  Whilst this reduced the overall dV of the vehicle slightly, it has 2 benefits.  It allows the engine on your lifting stage to burn for longer.  I was able to achieve an 80km orbit with this stage with a small amount of fuel remaining.  The second benefit is that it improves the TWR of your upper stage, giving shorter burn times.

The vehicle, as amended, was perfectly capable of doing a Mun fly by.  I did a free return trajectory, making a correction burn on the way back to drop my Pe in to the atmosphere of Kerbin.  The upper stage still has way more fuel than is actually required.  In fact, just to prove that it is actually carrying too much, in a second flight I removed the FL-T200 tank from the upper stage.  This improved the TWR of all stages of the vehicle, and actually allowed me to achieve an 80km orbit with 600 m/s dV left in the lifting stage, which is almost enough to reach the Mun.

Adding to what @OhioBob said, it is possible to achieve your Mun flyby with even less.  To show that lighter and smaller can often be better, I further modified your craft - removing one of the FL-T400 tanks from the lower stage and one of the SRBs.  To maintain symmetry, I increased the number of fins from 3 to 4.  In this configuration the vehicle is capable of reaching an 80km orbit with just the smallest amount of fuel left, and the upper stage still has over 2000 m/s dV, which is more than enough to get to the Mun and back.

Edited September 19, 2018 by Scarecrow

[OhioBob]

8 hours ago, Scarecrow said:

Adding to what @OhioBob said, it is possible to achieve your Mun flyby with even less.  To show that lighter and smaller can often be better, I further modified your craft - removing one of the FL-T400 tanks from the lower stage and one of the SRBs.  To maintain symmetry, I increased the number of fins from 3 to 4.  In this configuration the vehicle is capable of reaching an 80km orbit with just the smallest amount of fuel left, and the upper stage still has over 2000 m/s dV, which is more than enough to get to the Mun and back.

That sounds about right to me.  My first reaction when looking at the design was to take away fuel tanks from both the upper and lower stages and remove one SRB.  I'm glad you took the extra step to actually test it.

[Scarecrow]

For the OP, if my explanation of what I eventually did to your craft is a bit confusing, this is what the final version looks like.

Edited September 19, 2018 by Scarecrow

[Draalo]

That design looks better but i recomend some tweaks:

Ignite the swivel together with the boosters. Switch KER to atmospheric mode, rightclick the booster and adjust the thrust till you have a TWR of 1.5 (NOT! the thrust of the swivel)

(If you used the reliant exchange it with a swiwel)

The swivel has gimbal which means it can steer. Note also that the thrust of all solid fuel booster can only be altered in the VAB/SPH

 

Remove the surface scanner, its useless in space  (It also provides no sciencepoints - it shows the local ore density when landed)

You should add some solarpaneels or add at least a bunch of Batteries, otherwise the ship will be uncontrolable when all EC has been used - resulting in a failed mission.

Edited September 20, 2018 by Draalo

[swjr-swis]

1 hour ago, Draalo said:

Remove the surface scanner, its useless in space  (It also provides no sciencepoints - it shows the local ore density when landed)

1 hour ago, Draalo said:

You should add ... at least a bunch of Batteries

I get a feeling you are confusing the batteries (which are there, several of them) for a surface scanner (which I can't see, not even after extreme zooming).

 

To @zyco187

Assuming the scanner is there: keep the surface scanner! It's not as useless in space as some people might think. If you rightclick it and pin its PAW window, it will show you 'live' what biome you are flying over, which in turn allows much more precise timing to capture extra science from orbit. Even in a perfect equatorial Mun orbit, you will pass over a good number of different biomes, so the benefit in science points will easily be worth taking it with you.

I'm more puzzled at why you decided to surface-attach so many fragile and draggy things on the outside of the pod, when you have a service bay right there to protect and shield them. All that stuff should be in the bay - that's what it is for.

Personally I'd stuff it all in the service bay, move the bay up above the heatshield, and land both pod and bay on the return. Even if you decide to save only the pod, condemning all the stuff in the bay to a fiery death, you can still move all data into the pod before separation, and the pod's internal EC and antenna are quite enough to work through reentry until recovery. So there's no need to suffer all that extra drag or risk losing them to heating through the initial ascent. Move it all into the bay.

[Draalo]

KER shows on the bottom left EC = 50 - thats the EC from the MK1-pod

KER also shows inflight the names of the biome

 

(But beside that, the surfacescanner weights only 5kg and produces only a tiny amount of drag - doesnt matter if its there or not)

2 hours ago, swjr-swis said:

I'm more puzzled at why you decided to surface-attach so many fragile and draggy things on the outside of the pod, when you have a service bay right there to protect and shield them. All that stuff should be in the bay - that's what it is for.

agreed

[Scarecrow]

I just put two instruments on the capsule to emulate the extra payload on the OP's craft, as I couldn't quite make out exactly what he had used.  I agree that if you are going to light the main engine at the same time as the boosters then you will need to reduce the thrust of the boosters as the boosters alone have plenty of thrust for the initial part of the launch, and lighting all three engines at full thrust together would be overkill.

[Xd the great]

I am definitely the laziest person in here, so i say 8 solid rocket boosters. If insufficient, do 16. Should be more tyan enough to launch you to a high orbit.

[swjr-swis]

On 9/20/2018 at 8:25 AM, Draalo said:

KER shows on the bottom left EC = 50 - thats the EC from the MK1-pod

KER also shows inflight the names of the biome

I'm not sure we're looking at the same screenshot. In the screenshot linked in the OP, the KER readout at the bottom says EC = 350, which indicates three Z-100 batteries have been added, probably in the same 3-way symmetry used in the rest of the craft. The one on the hatch may get in the way of EVA, although that may not be part of the mission plan.

I hadn't considered KER also showing the biome - I don't use it. The only stock ways to keep track of the biome you're flying in/over are KerbNet (he has no probe core for that), rapid-fire running/cancelling of one of the science instruments (gets tedious very fast), or the surface scanner PAW (add the part, pin the PAW, done). It's a pretty handy functionality of that part that many people seem to be unaware of.

In any case, it looks like @zyco187 is playing career, from the looks of it still stuck in tier 4 tech or lower, so the surface scanner (T7) is likely not available yet and so irrelevant in this discussion.

 

Taking another look at the OP and the screenshot, I don't see any mention of wanting to land on the Mun (other than the oddly overcalculated dV numbers), and the craft is called 'munar flyby'. If a flyby is really all you wish to achieve, this craft can go a LOT smaller still - you can stop counting at just the intercept, with a tiny amount extra to correct your Pe when on your way back to Kerbin.

That last stage needs only a single FL-T200 tank to be able to get a Mun intercept from LKO and get back to Kerbin, at a total of around 3.1 t. Which means the lifter stage can be a lot smaller too:

Spoiler

This 3.1t upper stage can circularize itself, transfer to a Mun flyby and return safely to Kerbin, with one FL-T200 tank. There's enough fuel margin to add some more parts to the service bay, which I can't see from the OP screenshot.

The lifter stage can be kept very simple, and doesn't even need fancy schmancy gimbal, fins, or SRBs. It takes advantage of the higher TWR offered by the Reliant, and a 5 degree tilt east to 'semi-automate' a very predictable gravity turn requiring minimal manual control.

See the full album on imgur for additional commentary about craft design and flight profile.

[Draalo]

7 hours ago, swjr-swis said:

I'm not sure we're looking at the same screenshot.

Oh, my first post was related to the design from Scarecrow, not that one from OP.

 

zyco187 newer answered or had additional questions - so this thread may be just chat between the Elders

 

Offtopic: if you dont use any mods similar to KER consider using it. I really like the display of AP and time-to-AP in the mainscreen during ascent. I hate switching to map and back while controlling the craft.

 

These displays are configurable, add all info you like (or remove that ones you dont want to see)

Usefull are:

- dV staged

- TWR

- inclination relative to target

you may even find time to suicide burn, or orbital period usefull in some cases.

[Xd the great]

Or, launch straight up, then burn sideways. Build a bigger rocket.

Use more liquid fueled gimbal engines. Use less solid rocket boosters.

[swjr-swis]

4 hours ago, Draalo said:

if you dont use any mods similar to KER consider using it.

Oh I've modded KSP before, including KER and MJ; had games with close to 200 mods, customized KER displays etc. I just stopped at one point and haven't yet felt the need to do it again.

 

4 hours ago, Xd the great said:

Or, launch straight up, then burn sideways.

Stop already. This stopped being a 'smart' ascent profile years ago. It only compounds the problems and confuses new players.

[Xd the great]

Just now, swjr-swis said:

.Stop already. This stopped being a 'smart' ascent profile years ago. It only compounds the problems and confuses new players.

He has problem of getting into orbit, not building a rocket. The launch profile is incredibly inefficient, but it is too simple.

[steve_v]

4 minutes ago, Xd the great said:

The launch profile is incredibly inefficient

Then why bother suggesting it? Unless you have zero control authority in atmosphere, it's just a stupid thing to do.
If you must use an all-SRB first stage, steerable fins are cheap and unlocked early in the tech tree.

[swjr-swis]

1 hour ago, Xd the great said:

The launch profile is incredibly inefficient, but it is too simple.

Really? Simpler than giving the rocket a small nudge (or a tilt) at the start and letting the balance of thrust, gravity and aerodynamic forces do the steering for you?

As opposed to going straight up and then having to fight to switch to horizontal, as literally every one of those forces is at that point working against the direction you now want to turn to?

[steve_v]

9 hours ago, swjr-swis said:

As opposed to going straight up and then having to fight to switch to horizontal

A properly designed lifter likely won't have the TWR in the final stage to circularise in time anyway. which leads to:

10 hours ago, Xd the great said:

He has problem of getting into orbit, not building a rocket.

A bad launch profile will probably cause design problems.

 

Edited September 23, 2018 by Vanamonde

[Vanamonde]

We can disagree with each other without attacking one another. 

Some comments have been removed from this thread. 

[Deddly]

Congrats on Thread of the Month