(potentially) stupid question about aerodynamics and atmospheric heating of Launch Vehicle

[Orc]

Hi all

 

I've been playing KSP for a long time but still consider myself a noob. I am using Version 1.4.1.

 

I have a tendency to built rockets that produce a lot of ''aerodynamic FX'' in the 0 to 20km altitude range. Often (but not always) I end up generating heating on the leading edges. Occasionally I manage to get a Mk1 Capsule to explode before 12 km. I've even managed to get the Mk 1 to explode at 35 km.

 

Obviously the rocket is going too fast. Thrust to Weight Ratio obviously is too high, right? 

 

When I was a complete noob I read somewhere on these forums that exceeding 300 m/s before 10,000 m meant you were wasting too much energy pushing atmosphere out of the way and should throttle back. Is this correct? Another source said that pushing your way through the atmosphere ASAP was the best strategy because aerodynamics forces represent less of a problem than gravity, which is the real energy thief. 

 

Who do I believe / follow? 

 

On a related matter, I use SRBs a lot in the early game, and when a rocket with SRBs is going too fast I often revert to the VAB and use the ''thrust limiters'' on the SRBs (anything from 55% to 80%). This slows the stack down a bit, forcing the SRB to burn for longer, but am I actually getting the optimal benefit by doing this? Dragging the SRBs higher means that their dead weight stays with the stack for longer. 

 

Any help, advise, correct or clarification is welcome.

 

Thank you.

 

Kind regards

Orc

[Gargamel]

40 minutes ago, Orc said:

Obviously the rocket is going too fast. Thrust to Weight Ratio obviously is too high, right? 

Yup, slow down.  Most likely your TWR is too high, but you also have a throttle for slowing down.   I like to keep my acceleration under 20 m/ss, but that's hard to do in stock.  But the G meter on the Navball will tell you your acceleration, just keep throttling down to keep the G meter under 2gs of force, and you shouldn't encounter explody burny ships.  

 

42 minutes ago, Orc said:

When I was a complete noob I read somewhere on these forums that exceeding 300 m/s before 10,000 m meant you were wasting too much energy pushing atmosphere out of the way and should throttle back

That number may have been accurate for an older version with a different atmospheric makeup, but the logic is still sound.  I remember seeing that number back in version .18 when I started, so I don't know how accurate it is anymore. 

42 minutes ago, Orc said:

Another source said that pushing your way through the atmosphere ASAP was the best strategy because aerodynamics forces represent less of a problem than gravity, which is the real energy thief. 

While they're not wrong, completely, you are most likely doing a gravity turn at launch.  So you are spending a lot more time in atmo than if you were launching straight up and taking a hard left at apogee.    You need to balance drag and gravity losses on your ascent. 

 

44 minutes ago, Orc said:

On a related matter, I use SRBs a lot in the early game, and when a rocket with SRBs is going too fast I often revert to the VAB and use the ''thrust limiters'' on the SRBs (anything from 55% to 80%). This slows the stack down a bit, forcing the SRB to burn for longer, but am I actually getting the optimal benefit by doing this? Dragging the SRBs higher means that their dead weight stays with the stack for longer. 

That's what I do.  I limit their thrust so I get an overall TWR of 1.7 ish at launch, and keep my throttle back as I mentioned above to stay under 2g's.    But as the SRB's burn fuel, their mass is decreasing, increasing their TWR.  So while they are burning, they are becoming the complete opposite of dead weight.  But when they're done burning, ditch them boosters, as they are now useless. 

 

Other's will come along with more detailed and much better and accurate replies, but this is how I fly, and it works for me. 

[bewing]

2 hours ago, Orc said:

I have a tendency to built rockets that produce a lot of ''aerodynamic FX'' in the 0 to 20km altitude range. Often (but not always) I end up generating heating on the leading edges.

This is perfectly normal, and not a problem at all.

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Occasionally I manage to get a Mk1 Cockpit to explode before 12 km. I've even managed to get the Mk 1 to explode at 35 km.

Obviously the rocket is going too fast. Thrust to Weight Ratio obviously is too high, right? 

Exploding before 12km usually means you are going faster than 1300m/s and that's too fast, yes.

Exploding at 35km usually means that you are staying at too low an altitude for too long -- things tend to get too hot if you are going faster than 1700m/s for a long time below 35km.

Quote

When I was a complete noob I read somewhere on these forums that exceeding 300 m/s before 10,000 m meant you were wasting too much energy pushing atmosphere out of the way and should throttle back. Is this correct?

That's a bit slow. I'd say 800m/s, but it depends on your aerodynamics. If your ship is super-draggy, then you want to be going slower through the thick stuff. Also, the low atmosphere is where you want to be doing your gravity turn. If you are going super fast in the low atmosphere, your ship won't turn. Super slow and it will turn too much.

Quote

Another source said that pushing your way through the atmosphere ASAP was the best strategy because aerodynamics forces represent less of a problem than gravity, which is the real energy thief. 

Who do I believe / follow? 

Pushing your way through without exploding.  And this also assumes that your ship is aerodynamically good, and that you are doing a nice gravity turn.

Quote

On a related matter, I use SRBs a lot in the early game, and when a rocket with SRBs is going too fast I often revert to the VAB and use the ''thrust limiters'' on the SRBs (anything from 55% to 80%). This slows the stack down a bit, forcing the SRB to burn for longer, but am I actually getting the optimal benefit by doing this? Dragging the SRBs higher means that their dead weight stays with the stack for longer. 

No. If you do a test you will see that throttling back the SRBs always results in poorer performance. You will be going slower, you will use more fuel, and you will be at a lower altitude when the SRBs burn out.

 

Edited May 4, 2018 by bewing

[Lego_Prodigy]

Use fairings for more heat resistant nose cones, and try not to go so fast mate

 

[blakemw]

4 hours ago, Orc said:

 

 When I was a complete noob I read somewhere on these forums that exceeding 300 m/s before 10,000 m meant you were wasting too much energy pushing atmosphere out of the way and should throttle back. Is this correct? Another source said that pushing your way through the atmosphere ASAP was the best strategy because aerodynamics forces represent less of a problem than gravity, which is the real energy thief. 

 

With the modern aerodynamic model throttling is never efficient (except to reduce overheating) unless your rocket is super absurdly draggy which would be a bad design.

With one exception that has nothing to do with aerodynamics: If you have a "falcon heavy" style rocket or just generally a core with side boosters design, it actually can be beneficial to throttle back just the core if the boosters are providing plenty of thrust (i.e. a TWR at least of 2.5). The reason this can be beneficial is that it saves fuel in the core to burn *after* the side boosters have been decoupled, that fuel then goes only to accelerating the mass of the core and not also the mass of the side boosters. The fuel will also be burned at a higher altitude at better ISP, which is relevant to some engines, but you'd do it even if launching from a vacuum world.

This kind of core throttling is and has been done in real life and the reason is as much getting more deltaV as reducing pressure on the rocket, i.e. from the wiki for Delta Heavy rocket:

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At lift off, all three cores operate at full thrust, and 44 seconds later the center core throttles down to 55% to conserve fuel until booster separation. The boosters burn out at 242 seconds after launch and are separated as the core booster throttles back up to full thrust.

1

1

 

Edited May 4, 2018 by blakemw

[CrashyMcCrashFace]

I kind of just asked this same question but worded different, Throttling engines that are too big. Here's my experiment from last night which turned out the opposite results as I was expecting. Controlling speed by limiting Q (pressure) vs unlimited. The exact same ship and MechJeb flying to keep it consistent. It turned out unlimited used slightly less fuel.

[Snark]

1 hour ago, blakemw said:

With the modern aerodynamic model throttling is never efficient (except to reduce overheating) unless your rocket is super absurdly draggy which would be a bad design.

"Never" is a pretty strong word.  "Usually not" would be a better term.  There are a few scenarios where this can happen, of which you've called out one of them.

Some of the possibilities, and how to deal with them:

• The rocket is really draggy, and doesn't need to be (i.e. flawed design):  This can be solved by redesigning the rocket to be more aerodynamic, thus eliminating the need to throttle.
• The rocket is really draggy, unavoidably so.  This can happen once in a while if there's an especially awkward payload that there's no good way to make aerodynamic.  In cases like this, a slower ascent to reduce drag can help.  Depending on the rocket design, it may make sense to throttle for a bit... though if possible, I'd prefer to solve this just by putting smaller engines on.
• The rocket is really overpowered (e.g. TWR significantly higher than 2).  This can be solved by putting smaller engines on.  There's generally never a reason to launch with a TWR much higher than 2.

6 hours ago, Orc said:

When I was a complete noob I read somewhere on these forums that exceeding 300 m/s before 10,000 m meant you were wasting too much energy pushing atmosphere out of the way and should throttle back. Is this correct?

No.  Bear in mind that before KSP 1.0 came out in April 2015, the game's aerodynamic model was completely different, so any advice you read from before April 2015 is going to be egregiously wrong.

As long as a rocket is reasonably aerodynamic and doesn't have a TWR higher than 2, it's generally always better to stay at full throttle.  Gravity losses (which get worse when you're going slow) outweigh aerodynamic losses (which get worse when you're going fast) nearly all the time.

I routinely launch my rockets at TWR 2, and I hit a 45 degree angle and around 400 m/s when I'm still only 6 or 7 km, and they work great.

Also, bear in mind that the square-cube law works in large rockets' favor, where drag is concerned.  The bigger the rocket, the less you have to worry about drag.

But in general?  As long as you have a reasonably streamlined rocket that's not absurdly overpowered (i.e. don't go over TWR 2), and you're following a reasonably efficient ascent path, then there's no reason to throttle.  Put the pedal to the metal.

6 hours ago, Orc said:

Another source said that pushing your way through the atmosphere ASAP was the best strategy because aerodynamics forces represent less of a problem than gravity, which is the real energy thief.

For the very large majority of cases, this is the correct answer.  Just make sure that you're reasonably streamlined and that your TWR doesn't go over 2.

6 hours ago, Orc said:

On a related matter, I use SRBs a lot in the early game, and when a rocket with SRBs is going too fast I often revert to the VAB and use the ''thrust limiters'' on the SRBs (anything from 55% to 80%). This slows the stack down a bit, forcing the SRB to burn for longer, but am I actually getting the optimal benefit by doing this? Dragging the SRBs higher means that their dead weight stays with the stack for longer.

In general, with SRBs you'll get more benefit from higher TWR (due to reducing gravity losses)... up to a point.  That point is about TWR 2.  Beyond that, you run the risk of aero losses exceeding gravity losses.  So, if you've strapped on your SRBs and it's pushing you significantly past 2, then you may want to consider dialing back their thrust a bit.

One thing that can easily happen with SRBs is that you get a brutal TWR spike near the end, which happens for two reasons.  First, their Isp reacts pretty strongly to atmospheric pressure, meaning that they gain quite a lot of thrust as they climb out of the denser part of the atmosphere.  Second, they're heavy and low Isp, meaning that they drop a lot of mass as they burn, so that the craft gets a lot lighter and the acceleration goes through the roof.

Depending on ship design, that huge TWR spike at the end may or may not be an issue for you.  One way that can help smooth out the ride a bit is to use a technique I refer to as "poor man's asparagus".  Let's say that I've got a ship I'm launching, and in order to hit my target TWR I decide to strap on eight radial Thumpers.  But putting them at 100% thrust would push me over my desired TWR.  Let's say I do the math and figure out that if I set them all at 90%, I'll get my desired launchpad TWR.  Well, I could do that... but instead, I do this:  I don't put on one group of 8 SRBs, set to 90%.  Instead, I put two groups of 4 SRBs each.  One of these groups is set to 100%, and the other is set to 80%.  All of them activate together on the pad.  The 100-percenters burn out first, and I jettison them.  Then the 80-percenters continue burning for a while longer.

This smooths the ride out somewhat, and also helps to add dV by ditching the dead weight of SRBs sooner.

 

[Zeiss Ikon]

5 hours ago, Snark said:

Depending on ship design, that huge TWR spike at the end may or may not be an issue for you.  One way that can help smooth out the ride a bit is to use a technique I refer to as "poor man's asparagus".  Let's say that I've got a ship I'm launching, and in order to hit my target TWR I decide to strap on eight radial Thumpers.  But putting them at 100% thrust would push me over my desired TWR.  Let's say I do the math and figure out that if I set them all at 90%, I'll get my desired launchpad TWR.  Well, I could do that... but instead, I do this:  I don't put on one group of 8 SRBs, set to 90%.  Instead, I put two groups of 4 SRBs each.  One of these groups is set to 100%, and the other is set to 80%.  All of them activate together on the pad.  The 100-percenters burn out first, and I jettison them.  Then the 80-percenters continue burning for a while longer.

This smooths the ride out somewhat, and also helps to add dV by ditching the dead weight of SRBs sooner.

I've done this and gotten very good results.  Instead of two groups, I used three (six boosters total), set to 60%, 80%, and 100%, for one of the earliest Mun launchers in my current career (didn't have fuel ducts yet, and hadn't yet understood the utility of decoupler crossfeed for asparagus staging)

[Geonovast]

20 minutes ago, Zeiss Ikon said:

(didn't have fuel ducts yet, and hadn't yet understood the utility of decoupler crossfeed for asparagus staging)

If I'm not mistaken, you can't use crossfeed until you unlock the node with the ducts anyway.