Air Brakes on Rockets

[chrisb2e9]

6 hours ago, SessoSaidSo said:

I have [Removed by the Moderation Team]

 

Good day. Sir.

Haha, but you still had the time to try and get in the last word. Might as well have said "I'm taking my bat and my ball and I'm going home"

[StrandedonEarth]

I read somewhere on these forums that the lift/drag from the fairing is applied way out in front of the rocket, which caused me no end of grief and required some hugeS fins to keep it straight. I heard there's a fix for that but I haven't downloaded that yet. Airbrakes sound like a great way to keep the flamey side down, I'll have to try that if and when I play again before 1.1 drops.

[SessoSaidSo]

1 hour ago, chrisb2e9 said:

Haha, but you still had the time to try and get in the last word. Might as well have said "I'm taking my bat and my ball and I'm going home"

He tried. It deserves a response. C+ for effort.

[Sirad]

4 hours ago, chrisb2e9 said:

Haha, but you still had the time to try and get in the last word. Might as well have said "I'm taking my bat and my ball and I'm going home"

You know where this is running. He is right. just admit that he is right (even if he is wrong in all ways) Average low Level Logic dont stick to those.

[sal_vager]

Some posts were edited under rule 2.3B.

When responding to members you disagree with it is advised that you avoid making remarks that would lead to argument.

[Tig]

Lot of anger in this thread...

If I can help a bit, I think one of the major reasons we see smaller (or no) fins on real world rockets is because they have significantly higher gimbal than stock KSP.

The shuttle's main engines had 10.5 degrees of gimbal range.  I mean some engines in stock KSP have like 8 degrees, but its only the little ones.  Primary launch engines, i think the largest gimbal is on the Rhino at 4 degrees.

Even the massive F1 engines on the Saturn V had about 6 degrees of range (i think the center one was fixed and it was only the outer 4 that had that range, but don't quote me on that).

I couldn't find for certain if the modified rs-25s from the shuttle that they plan to use on the SLS will have the same range as they did on the shuttle, but i think its safe to assume so, or at least that they'll have substantial gimbal range.  Its just generally more efficient than the drag created by fins, or any other aerodynamic system. 

In any case, real world gimbal creates a substantial amount of force that doesn't exist in stock KSP.

@OP if you wanted something more realistic i think the earlier advice about procedural tanks is a good idea + modding the engines to something similar to real world engines would let you go finless without airbrakes.  I mean if the goal is realism then real space agencies found ways to reduce/remove fins to reduce drag, obviously airbrakes would add a bit of drag

That said, it would make the game a bit easier... not sure if you really want that or not.

 

PS.  I love fins.  Reminds me of building rockets as a kid.

Edited February 19, 2016 by Tig
rewording for clarity

[Xyrus]

IMHO, if you need to add airbrakes to your rocket for launch stability then you likely need to re-examine your rocket design. The only times I've had rockets flip out on me is when I've done something stupid (incorrect COM, uneven drag, too much thrust, turning just a bit too much, etc.).

That being said, it's a sand box game. Your free to do whatever you want. You can build a lunar mission using nothing but boosters. You can use a tower of components for re-entry instead of a heat shield. It doesn't really matter so long as it works for you and you think it's fun.

[Geschosskopf]

On 2/16/2016 at 11:00 AM, ment18 said:

2.  The rockets have RCS thrusters and use them during ascent.  I don't use them in KSP and assume others don't either.

Hmm.  I make simple lifters, just a core LFO sustainer stack that takes the payload from wherever the SRBs die to finishing circularization.  As payloads (and their transfer stages) increase in mass, I just make bigger lifters but on the same plan (using stock 3.75m and then even Space-Y 5m parts as necessary.  Anyway, at the top of the ascent, the whole long thing of sustainer + transfer stage + payload has to orient to circularize, and this needs RCS.  For which I now use 4x Vernors on the bottom of the sustainer stage.  They work like champs, so much that I usually turn them on immediately on liftoff for attitude control and stabilization during the whole gravity turn without noticing any major change in the sustainer's dV.  They're WAY better than using mono RCS for the same job, which I used to do until Vernors appeared.

[The Optimist]

On 2/16/2016 at 10:31 PM, SessoSaidSo said:

So either way I'm breaking rules. The Soyuz also uses grid fins, your point? They are for atmospheric control, they are not true lift generating apparatus. 

I'm talking about requiring large lift producing fins for regular stability. I didn't start this post to be told I'm building wrong, I'll go to the steam forums for that. I asked a fun questions. Go be a jerk somewhere else.

Nope.

Edited February 20, 2016 by The Optimist

[Mad Rocket Scientist]

On 2/16/2016 at 7:23 PM, SessoSaidSo said:

I don't like fins on my rockets. When was the last time you saw a fin on a launch vehicle? [...]

This is an interesting question.  Most completely new designs use the much larger gimbal ranges that IRL rocket engines have, plus the fact that KSP aero calculations aren't quite realistic.  Some rockets also use RCS during launch, especially for roll control.  However, the soyuz rocket does use small fins:

Because the only gimbal authority comes from the verniers. 

Nonetheless, whatever building method works best for you is right one. 

[Beowolf]

Yes, I've done it, though not initially for stability during ascent. I built a family of vertical launch/recovery SSTOs that used airbrakes (modded to have a higher maxtemp) for trajectory control during reentry.

It was just serendipity that I later noticed the airbrakes gave me unparalleled steering authority during launch, and could put absurdly wide, even unbalanced, payloads into orbit easily.

Despite what the other players here have claimed based on their prodigious common-sense, when I actually measured airbrakes vs fins* there was no difference in efficiency. That makes sense when you watch an airbrake-stabilized launch. With a normal, balanced payload, the airbrakes rarely move with anything more than the tiniest twitches. A proper gravity turn is, after all, aerodynamically just going straight.

-----

*5 trials with each, 20t payload, MJ ascent with 40% curve, 1.4 TWR at launch. Conclusions based on remaining dV upon reaching 100km circular orbit. Results were scattered randomly within a 2% band. First stage was a single huge Procedural Parts tank.

edit: Crap, these tests might have been done before 1.0.5. I didn't include the KSP version in my spreadsheet.

Edited February 20, 2016 by Beowolf
Well, crap.

[Tig]

On 2/20/2016 at 3:38 AM, Beowolf said:

Despite what the other players here have claimed based on their prodigious common-sense, when I actually measured airbrakes vs fins* there was no difference in efficiency. That makes sense when you watch an airbrake-stabilized launch. With a normal, balanced payload, the airbrakes rarely move with anything more than the tiniest twitches. A proper gravity turn is, after all, aerodynamically just going straight.

That's an interesting point, Beo.  I hadn't thought of that, the airbrakes although producing more drag when deployed, would only deploy at the moments they are needed rather than the constant but less drag provided by fins which would provide drag even when not required... interesting.

My point was more directed at your last sentence, modern rockets are obviously much more thoroughly tested than our kerbal creations, and for safety that the lack of fins/brakes on most modern rockets is due to larger gimbals on the engines than we see in stock KSP.  As others have pointed out, the fins on the Saturn V were probably unnecessary as well, but were there for safety margins.

 

I don't know about the rest of you, but I have been known on occasion¹ to place my kerbal's on vehicles with, shall we say, "sub-standard" safety protocols?

¹Occasion shall be defined in this context as most of the time.

[godefroi]

On 2/20/2016 at 1:38 AM, Beowolf said:

Despite what the other players here have claimed based on their prodigious common-sense, when I actually measured airbrakes vs fins* there was no difference in efficiency. That makes sense when you watch an airbrake-stabilized launch. With a normal, balanced payload, the airbrakes rarely move with anything more than the tiniest twitches. A proper gravity turn is, after all, aerodynamically just going straight.

So, for a rocket which requires no aerodynamic stabilization, airbrakes work as well as fins?

[Beowolf]

15 hours ago, godefroi said:

So, for a rocket which requires no aerodynamic stabilization, airbrakes work as well as fins?

Haha, well phrased, sir!

[KerBlammo]

Is this the right room for an argument?

[Vanamonde]

2 hours ago, KerBlammo said:

Is this the right room for an argument?

No. 

[godefroi]

So I did SCIENCE. I used MechJeb (limit AoA to 5 degrees, no other options enabled), and tested the difference between fins and brakes for a very aerodynamically-unstable rocket. I call it, the PANCAKE ON A STICK. It features crazy TWR (starts at 1.85, ends >8) and bad aerodynamic design.

Here are the results:

• No stabilization (apart from the Mainsail's gimbal):
  • failed to make orbit
  • failed to make orbit
• Fins (4x AV-R8):
  • made orbit with 957 m/s dV left
  • made orbit with 936 m/s dV left
• Airbrakes (4, on bottom):
  • failed to make orbit
  • failed to make orbit
• Airbrakes (4, on top):
  • failed to make orbit
  • failed to make orbit
• Airbrakes (top+bottom):
  • failed to make orbit
  • failed to make orbit
• Airbrakes (many airbrakes all over the rocket in various configurations):
  • failed to make orbit
• Airbrakes + lots of reaction wheels:
  • made orbit with 884 m/s dV left

 

Edited February 23, 2016 by godefroi

[Sirad]

5 hours ago, godefroi said:

• Airbrakes (many airbrakes all over the rocket in various configurations):
  • failed to make orbit

 

Inside my Head there appeared an Image of a artichoke shaped rocket. I dont know why

[Nukeknockout]

I honestly like Airbrakes. Even if they explode on re-entry, as long as they slow the craft down in that situation and help keep it firmly pointed retrograde as long as possible they did their job.

I also like them as control surfaces for large planes and I absolutely do use them for extra control authority for certain atmospheric designs.

A concept I've been playing with is mounting wings, fins or airbrakes around a circular part to force it to spin on launch, hopefully generating torque without needing electric charge, or force it to spin during re-entry to maximize drag (I did not say it was a good concept).

Edited February 23, 2016 by Nukeknockout

[Archgeek]

Another reason real-life rockets often don't need fins too much is that they possess reasonable gimbal response.  They move according to the constraints of physical motors, often from 5-20deg/s.  Most of our launch engines only have 2-4 degrees of gimbal, save the vector with 10.5. in both directions, so the engines can move 4-8 degrees or 21 degrees, which excepting the vector, shouldn't be a problem...but they can twitch across their whole range between physics deltas.  With a delta of .04s, that's an absurd 100-200 deg/s or 525deg/s for the vector.

Leaping from extremes like that all the time for the tiny corrections SAS is trying to make makes for a shaky ascent that loses stability much more easily than it would in real life.  Install Claw's stock bug fix Plus, fire up ModuleGimbalPlus, set gimbal rate to around 10deg/s, and you'll be surprised at what doesn't need fins anymore.

Edited February 23, 2016 by Archgeek
Replaced a missing 's'.

[Friend Bear]

36 minutes ago, Archgeek said:

Another reason real-life rockets often don't need fins too much is that they posses reasonable gimbal response.  They move according to the constraints of physical motors, often from 5-20deg/s.  Most of our launch engines only have 2-4 degrees of gimbal, save the vector with 10.5. in both directions, so the engines can move 4-8 degrees or 21 degrees, which excepting the vector, shouldn't be a problem...but they can twitch across their whole range between physics deltas.  With a delta of .04s, that's an absurd 100-200 deg/s or 525deg/s for the vector.

 

In addition, real life engines have HUGELY more pounds of thrust than in KSP on the highly responsive, gimballed engines. It is unfortunate, but it is what it is. Also, keep in mind we have huge cross-winds in real life, unlike kerbal. Just throwing that out there.

 

You could consider using SAS, as stated. But it only aides a good ascent and doesn't do the work for you.

Also, the problem you are most likely facing is turning outside your prograde too aggressively when ascending and simultaneously attempting to start your horizontal movement. While turning, even if immediately after launch, keep your forward movement within your compass's recorded prograde. Almost guaranteed you won't topple unless you are cranking it at a high turn speed. While sometimes you need to hold the directional key down to keep it where you want, don't do aggressive turning where you can avoid it.

 

Then, you do not need fins.

 

P.S. - Remove all salt from your comments before replying to this post. Thank you.

Edited February 23, 2016 by Friend Bear