Advice on ascend profile for my ship

[MaxKot]

I built a delivery vehicle for my scanner probe. Stage 2 is somewhat under-powered and my first launch had some problems with the craft reacting to much to pitch controls so to make it gain some some speed I was turning towards horizon very early on. I didn't expected it to make it to the orbit and I was very surprised to see my apoapsis reaching 90 km. Then I managed to circularize the orbit with as few ass 110 delta-V and I had about 70 delta-V for de-orbiting and landing after separating the probe.

The delivery vehicle craft had the battery packs and solar panels placed on the outside and they burned on re-entry.  Other than that the delivery vehicle landed nicely on the KSC runway.

My delight was ruined when I switched to the probe to see that I completely forgot to place some solar panels on the probe. I loaded an earlier save, added the solar panels on the probe and now I can't make it to the orbit again.

You can see the updated ship here. The ship was built in KSP 1.11.2 with  MechJeb 2 2.12.0.0, SCANsat v20.4 and Kerbal Engineer Redux 1.8.3. The changes after reaching the orbit on the first attempt are the following:

1. I moved two OX-STAT panels and two Z-100 battery packs to the inside of the cargo bay.

2. I added six OX-4L 1x6 solar panels on the probe.

3. I removed monopropellant from the cockpit.

4. I removed all items from the pilots inventory (EVA jetpack and EVA experiments kit).

I only had one crew member, a pilot, on both the successful attempt and the later attempts.

Could someone please explain what ascend profile should I use for this craft? The added weight from the solar panels is small and somewhat compensated by removing the monopropellant and inventory items so I think the reason I can't even reach a circular orbit now is the ascend profile.

[Streetwind]

Welcome to the forums!

Unfortunately, you're very unlikely to get the help you seek in this way. At first glance, it may seem like providing the craft file is a good idea, because then people can see exactly what you see. But in practice, you are asking that people stop whatever they're doing, create a file and copy it around, and launch their KSP client just to see what you are talking about. This may well take 5-10 minutes, depending on how fast the person and the person's computer are.

And then you've provided a modded craft file. That means that in order to help you, people now need to clone their KSP installation to a new folder, browse a mod repository, download and install those mods, then create and integrate your craft file, and launch the client. Even the most helpful-minded person is not going to do that.

Instead, try providing a screenshot or two. Host them on imgur.com or a similar site, and link to the gallery (don't embed, that tends to break). You'd be surprised how much a veteran player can tell at a glance when presented with the image of a rocket.

For example, a single glance would be enough to determine whether or not you have room to tape two basic SRBs to the sides of your launch stage. If the mass of some solar panels is enough to make the difference between getting to orbit or falling short, then you need only a small amount of extra dV to compensate. And that, in turn, means you can most likely eschew all complicated analysis, and just fall back on the most famous of all rules of the thumb in KSP: moar boosters. It may be a band-aid fix, but if you just want to get to orbit...

If you're interested in improving your rocket construction and flying skills in general, let me link you to a few walls of text I've written in the past:

On common best practices for building rockets

On learning to consistently launch well

[AHHans]

First a very warm welcome to the Forums, @MaxKot

3 hours ago, Streetwind said:

At first glance, it may seem like providing the craft file is a good idea, because then people can see exactly what you see. But in practice, you are asking that people stop whatever they're doing, create a file and copy it around, and launch their KSP client just to see what you are talking about.

Well, you might be surprised by what the people here are willing to do.

3 hours ago, Streetwind said:

And then you've provided a modded craft file.

That was my problem. When I have the time I'm perfectly willing to download a craft file and have a look at it. (And improving it... ) But I'm not installing any mods to do that.

But @Streetwind is right, that more often than not the problem can already be diagnosed from images alone. In your case I would say that in addition to meaningful images of the craft itself an image with the staging setup that shows the dV, TWR and Isp of the first few stages is useful. (I.e. don't remove the GUI from the images. And don't crop images when asking for help!)

[Rhomphaia]

Not sure if the craft provided is the one you are talking about since it is a probe controlled craft with no cockpit...

Assuming it is...  I am not exactly an expert on spaceplane ascents, so while I could not get this into orbit I am sure someone else could, but  the problem to me is not so much the ascent profile, but too many panthers. The rockets seem anemic because they are trying to move too much dead weight after the panthers give up.

By switching the bi-coupler to a short mk2- to size one adapter and ditching one jet it went form a spaceplane that could almost make orbit to one that had just under 200m/s left before decoupling the probe.  this had changed the balance though and the wings would need re-worked to make it controllable on the way back down.

[MaxKot]

Thanks for looking into my question!

2 hours ago, Rhomphaia said:

Not sure if the craft provided is the one you are talking about since it is a probe controlled craft with no cockpit...

Sorry, I copied an earlier version from SPH instead of the latest one from VAB. So silly of me Here is the craft I was talking about.

Here are the screenshots of the craft. I guess strapping a booster under the rocket engine should do the job and that's what I would do if the first attempt failed. But since the first launch showed it can get to the orbit, getting the craft as is to the orbit has become a challenge for me to beat

[FleshJeb]

@MaxKot I would reduce the thrust of the SRBs until KER gives you a starting TWR of around 1.7 or lower. Try 75%. This will give your second stage a higher TWR at SRB burn-out. While this will actually slightly decrease the effective dV, It will allow you to fly a more conventional profile, which may be a lot more efficient, and almost certainly easier to control.

[herbal space program]

That thing looks like it has plenty of dV to make orbit,  but those giant SRB's don't have any gimbals so they will be really tough to steer. You also have lots of draggy parts in front of your CoM, which will make your rocket inherently flippy under high aerodynamic pressure. So in addition to lowering the TWR at takeoff like @AHHans said, you need to add MOAR TAIL FINS to the bottoms of your SRB's, like at least 3 of those per booster instead of just one. You could also make it more steerable by mounting the SRBs on radial decouplers  to the side of your fuel tanks rather than underneath them, although this will increase drag a bit. As to your ascent profile,  it looks like you're using an aero mod, but in regular KSP a good rule of thumb is to start your gravity turn at around 150 m/s, be boosting at about 45% by the time you hit 10km, and more or less horizontally by the time you reach 25-30km.  The closer you can stay to prograde during all this the better, especially since you have wings. You should also cut thrust right as your AP reaches >70km, and boost prograde in small increments to keep it there as you climb out of the atmo. In my most efficient ascents, I only have to add  a few tens of meters of dV at AP to circularize.

[Streetwind]

2 hours ago, MaxKot said:

Thanks for looking into my question!

Sorry, I copied an earlier version from SPH instead of the latest one from VAB. So silly of me Here is the craft I was talking about.

Here are the screenshots of the craft. I guess strapping a booster under the rocket engine should do the job and that's what I would do if the first attempt failed. But since the first launch showed it can get to the orbit, getting the craft as is to the orbit has become a challenge for me to beat

Hmmm. 3453 m/s total - that should make it to orbit. My rockets generally tend to make it in 3300-3400 m/s. Of course, you're carrying a winged orbiter, so you're going to have more drag than a typical pencil-shaped rocket. But drag is only a very minor factor in dV cost to orbit, in the end.

The key to spending as little dV to orbit as possible is, counterintuitively enough, ignoring aerodynamic drag entirely and being super aggressive with your TWR and your ascent profile. So I would actually go against other people's suggestion here. Stick with your high TWR, and pitch over right the moment you leave the pad.

A good rule of the thumb, for sane TWR values, is to aim to be at 45° pitch angle by the time you cross the 10km mark. But in this case, I want you to try to be at 45° pitch by 5 km. And just be full throttle all the time until you get an apoapsis outside the atmosphere. At that point, just coast, and maintain your apoapsis at your desired altitude by gently nudging it upwards every so often after drag has pulled it down. Finally, circularize at AP.

If 5km ends up being so flat that you can't get your apoapsis outside the atmosphere at all, try 6km - meaning, don't sweat the exact number, just be as aggressive as your TWR possibly lets you be. Your first stage unfortunately burns out very early, so you'll have to experiment with what your second stage can manage.

I hope you put all your exterior greebling on the inside, because you're going to be on fire nearly all the way to space

 

EDIT: actually, since you have lift, a very flat, aggressive trajectory might suit this vehicle even more than a normal rocket. never tried anything like that myself. But is certainly shouldn't hurt you.

Edited June 4, 2021 by Streetwind

[herbal space program]

2 hours ago, Streetwind said:

EDIT: actually, since you have lift, a very flat, aggressive trajectory might suit this vehicle even more than a normal rocket. never tried anything like that myself. But is certainly shouldn't hurt you.

Although what you say is true from a technical standpoint, that philosophy has its limits, as in if you get going too much sideways and not enough upwards at 10km you will actually take a significant hit from the drag, not to mention that you will probably explode!  Having said that, 45 degrees at 5km and maybe 25 at 10km is probably OK from a not-exploding standpoint, but I wouldn't push it much beyond that.  Even though drag per se is not such an issue above 10-15km, excessive heating definitely is. Also, I think OP would have to do a very significant redesign of that vehicle before it ever could do such a thing without flipping. It's pretty clear the CoL is in front of the CoM as currently designed, and that is just not going to work in that kind of scenario.

[swjr-swis]

A few small tweaks to your craft, and an ascent profile much like @Streetwind suggests, can get you back to a 80km orbit with a good bit of margin to play with.

I did have to strip the non-stock parts to be able to load your craft and test/tweak it. But I think you'll find there's enough margin to re-add them and still get to orbit (and back) easily.

Spoiler

List of tweaks done are in the craft description text.

Ascent is almost hands-free... just remember to switch SAS to prograde when you pass 30 m/s.

And stage to decouple SRBs when empty.

Cut throttle when you reach your Ap.

Should be able to make it up there with 250-300 m/s left. Re-adding the ScanSat/KER parts will eat some of that, but there's enough to make it comfortably.

Full album of test flight: https://imgur.com/a/gpmYdYv

Tweaked craft file: https://www.dropbox.com/s/qo793yjv7bvf5oj/_Scanner 02 minmus.craft?dl=0

 

[Streetwind]

8 hours ago, herbal space program said:

Also, I think OP would have to do a very significant redesign of that vehicle before it ever could do such a thing without flipping. It's pretty clear the CoL is in front of the CoM as currently designed, and that is just not going to work in that kind of scenario.

So long as it flies as a rocket (as in, thrust dominates all other forces), a vessel shouldn't care much about where its center of lift is. It's the center of aerodynamic pressure that must be behind the center of mass for the vessel to be stable. CoP and CoL are not the same thing, and the little blue ball in the editor only ever shows CoL.  You can pretty much ignore it 100% of the time in the VAB, unless you're specifically designing something that is meant to do aerodynamic flying at some point. In which case you only pay attention to it for that specific stage.

As there's no way to see CoP in the editor, you have to eyeball the stability of your rocket. OP's rocket did not strike me as overly unstable from the image, and since both he and others have successfully flown it into orbit, that first impression appears to hold out.

[herbal space program]

9 hours ago, Streetwind said:

As there's no way to see CoP in the editor, you have to eyeball the stability of your rocket. OP's rocket did not strike me as overly unstable from the image, and since both he and others have successfully flown it into orbit, that first impression appears to hold out.

CoL, CoP, whatever. It was quite obvious from looking at OP's craft that with those big SRB's positioned where they are, that the center of aerodynamic forces was both forward  of the center of mass and far forward of the center of thrust, at least at liftoff. Maybe not so much after the SRB's are empty. At any rate, the profile that AP posted was only a tad more aggressive than the standard one, and OP did also put significantly more control surface all the way at the bottom than there was before, considerably mitigating the instability as compared to the original vessel, so I think there is room for us both to be right! 

[MaxKot]

I made it! Thanks everyone for the help, especially @Streetwind! Aggressive turn to 45° pitch by 5 km did the job. I'm so used to the 45°-by-10 km trajectory for rockets that I wouldn't figure it out on my own.