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jnbspace

Maximum launch velocity?

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In the real world there is a maximum launch velocity for a spacecraft before stress / strain / heating from air etc would destroy a craft. Looking at the tutorials in KSP Wernher Von Kerman advises throttling down the craft on take off presumably to avoid those problems.

However in KSP I have almost never found any of my spacecraft show problems even if I leave the throttle on maximum. Generally by the time I am hitting 1200 ms-1 which is the point at which I would start to see problems I am high enough that I don't see problems. Is there a practical reason (in KSP not the real world) to not just put the throttle to 100% and go for it?

 

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The tutorials have been around, unchanged, for quite a long time. In the olden days when the atmosphere was thicker, it was a waste of fuel to blast along at full throttle.

That no longer really applies.

If you try really really hard, you may be able to get a rocket to blow up on ascent from heating. But to accomplish that you'd need a very non-optimized, inefficient rocket. You can still build airplanes that will overheat and explode during normal flight at full throttle with the two most advanced engines.

If you turn off your autostruts, then you will also see more stress/strain effects from aero forces on ascent. It's certainly possible to build a deliberately weak airframe, and watch it crumple from aero forces.

 

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41 minutes ago, Daniel Prates said:

I may be wrong here but IIRC the Saturn V would launch way below maximum thrust, and then slowly increase thrust till it reached maximum power only when it was quite high already - the reason being just that: if it were going full blast and started turning, the rocket would simply breakup.

The Saturn V used F-1 engines which have no throttle. It would launch under full thrust (it needed it to get off the pad) and fly at maximum power until fairly close to burn out, at which point the centre engine was switched off to limit g-forces.

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18 minutes ago, jnbspace said:

Is there a practical reason (in KSP not the real world) to not just put the throttle to 100% and go for it?

Not for most rockets, no, there is no practical reason not to start at full throttle.  The reason for that has more to do with the physics of the planet than the capabilities of the rocket.  Kerbin has an atmosphere approximately the same height as Earth's (to the extent such a description makes sense; Earth's atmosphere doesn't just stop) and equal gravity, but is ten percent the size.  This means that, due to curvature alone, you escape the atmosphere and orbit much more quickly than you would in real life.  It was made this way for gameplay reasons; real rocket launches can take twenty minutes or more to make orbit, but KSP rockets can do the same in five minutes.

Because you spend comparatively little time in the atmosphere, you don't spend enough time for heating to be a major concern--assuming a more-or-less 'standard' gravity turn profile.  Heating is a concern if you're using a spaceplane, where you need air for the jets, but even then, you tend to go fast enough to cause catastrophic heating only when you're high enough that there isn't much air to heat catastrophically.

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2 hours ago, jnbspace said:

Is there a practical reason (in KSP not the real world) to not just put the throttle to 100% and go for it?

I think it's not only the speed/heat limits, its also important to consider the maneuverability of your rocket. When you launch you are flying in a straight line, piercing it like a pencil against an apple, but as soon as you start your gravity turn, your rocket's side will be put under an atmospheric pressure vector too. If a rocket is too fast, it may breakup altogether. So the ability to maneuver in the atmosphere could be considered another reason to avoid maximum speeds.

I may be wrong here but IIRC the Saturn V would launch way below maximum thrust, and then slowly increase thrust till it reached maximum power only when it was quite high already - the reason being just that: if it were going full blast and started turning, the rocket would simply breakup.

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  • Early space planes using mk1 cockpit tend to overheat to quickly if using a to shallow ascent profile during the rocket phase. But you should try to pilot and stay below that so you would only accidentally do so. Some noobs might but I was never bad at this playing flight sims so I can't judge.
  • Using LY-01 and LY-05 landing gear on early rocket planes or even SSTO's will heat up on most ascent paths using juno/wheesley and reliant/swivel engine combinations.
  • Accidentally have large gimbal on a engine that is directed at a fuel tank, especially when it's already hot when going fast in the atmosphere.
  • Using early rockets without fairings and a high velocity shallow ascent profile can heat up most stayputnik's and therefore destroy the vessel. On low fund/science reward career plays you often want to make efficient low tech rockets so fast high velocity rockets can be such a mission.
  • Using early rockets there are no struts, earodynamic drag from control surface deflection and engine gimbal using multiple fuel tank segments will then be able to tumble the rocket at high speed causing RUD due to the loose joints, but that only happens on such rockets.

 

2 hours ago, jnbspace said:

*snip*

However in KSP I have almost never found any of my spacecraft show problems even if I leave the throttle on maximum. Generally by the time I am hitting 1200 ms-1 which is the point at which I would start to see problems I am high enough that I don't see problems. Is there a practical reason (in KSP not the real world) to not just put the throttle to 100% and go for it?

1200m/s and problems? Do you mean heating effect? What do you mean?

As for the last question you asked. The best efficiency during a launch comes from something called a gravity turn. That is a curve the rocket follows from the moment it would want to start pitching which is usually at 100+ m/s after lift off. When past that point you want the rocket to follow prograde until reaching the desired apoapsis with the engines burning continuously. The idea being that you'd need next to no control correction and that gravity itself rotates the rocket as it keeps holding prograde. That involves you learn how your rocket works optimally and that you understand where and how much to pitch or use Mechjeb or GravityTurn mods which are available.

However, as you burn your fuel on the way up there TWR (thrust) increases as you burn fuel, so in order to follow the ascent curve during a gravity turn to orbit you want to lower your throttle at some point. A gravity turn is the best method of a efficient launch. The more aerodynamic your rocket the shallower this curve can become and thus the less Delta-V (fuel) used to get to orbit.

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For well-designed rockets using suitable parts, no, there really isn't any reason not to floor it.

For poorly-designed rockets -- such as ones you will likely have to deal with early in career mode, with just a few parts available to put them together -- yes it is possible to go too fast. Specifically: if you have a draggy payload in the nose, aero forces will tend to want to flip your rocket upside down, in which case you will no longer be headed towards space. This can be avoided by climbing at a fairly slow rate -- say, 300 m/s, tops -- until you're out of the lower atmosphere, and only going full throttle when the air is thin enough that it doesn't want to backflip anymore. 

It must be emphasised that this does indicate a design problem, and if you know what you're doing you shouldn't encounter it except in the early game. 

(BTW I believe the Saturn V rocket engines weren't throttlable at all, nor restartable after being shut down and were certainly firing at full power on lift-off. I think they shut down the first stage centre engine towards the end of the first stage burn however, to avoid going too fast.)

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I use the full throttle during all of my launches, the only problem I have, fuel, you run out of it faster at higher throttle levels. I try to get an orbit but still run out of fuel because I trade fuel for speed and altitude. The only reason I do use full throttle is my load, normally a lab+ all of the batteries and communotrons and solar panels. I also don’t really have enough brains to make a highly complex lifter, I use a two to three stage lifter using Rhinos’ and MainSail with two or four boosters but still need more as I don’t have the brains to rendezvous multiple payloads and dock them. Besides, it’s more efficient and effective to launch one smaller rocket with everything on it.

Edited by Mikenike

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10 minutes ago, Mikenike said:

I use the full throttle during all of my launches, the only problem I have, fuel, you run out of it faster at higher throttle levels. I try to get an orbit but still run out of fuel because I trade fuel for speed and altitude.

Full throttle should give the most efficient ascent, as you get out of the atmosphere faster so lose less energy due to atmospheric losses. A correct gravity turn makes a bigger difference than throttle though.

Edited by Flibble

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22 minutes ago, Flibble said:

The Saturn V used F-1 engines which have no throttle. It would launch under full thrust (it needed it to get off the pad) and fly at maximum power until fairly close to burn out, at which point the centre engine was switched off to limit g-forces.

I thought my memory was shady about this. I am pretty sure about the story, it could be another rocket though!

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Just now, Daniel Prates said:

I thought my memory was shady about this. I am pretty sure about the story, it could be another rocket though!

Might be the shuttle? They would launch that at full thrust and then almost immediately throttle it back until past max Q.

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No you guys got it correct about the shuttle.

Spoiler

Wahts this do?

 

 

Edited by Mikenike

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1 hour ago, Aeroboi said:
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1200m/s and problems? Do you mean heating effect? What do you mean?

 

Exactly that at above about 1200 ms-1 the craft is prone to overheating. But for most craft that I launch by the time I hit that velocity I am so high that it will still not overheat.

32 minutes ago, Flibble said:

Might be the shuttle? They would launch that at full thrust and then almost immediately throttle it back until past max Q.

The shuttle would throttle back as it passed the sound barrier. That has more to do with shock waves across the craft than atmospheric heating which is only really an issue on descent and not ascent.

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36 minutes ago, Flibble said:

Might be the shuttle? They would launch that at full thrust and then almost immediately throttle it back until past max Q.

Not immediately.  MaxQ was at about 60-70 seconds, they throttled down just before that

 

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44 minutes ago, Flibble said:

Might be the shuttle? They would launch that at full thrust and then almost immediately throttle it back until past max Q.

Oh yeah! That's it. 

 

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On 5/8/2019 at 6:06 PM, linuxgurugamer said:

Not immediately.  MaxQ was at about 60-70 seconds, they throttled down just before that

 

They usually throttled back around 30-35 seconds. Shortly after the roll was complete. They throttled back in a three step fashion to about 65% and throttled back up around 70 seconds.

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I find that the optimal TWR on the launchpad is around 1.5 - 1.8. Lower than that and you are wasting more fuel due to gravity losses. Higher than that and you begin to have stability problems unless you throttle back in the lower atmosphere. A TWR of 2.5+ runs the risk of your rocket melting before it gets to space.

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1 minute ago, sturmhauke said:

I find that the optimal TWR on the launchpad is around 1.5 - 1.8. Lower than that and you are wasting more fuel due to gravity losses. Higher than that and you begin to have stability problems unless you throttle back in the lower atmosphere. A TWR of 2.5+ runs the risk of your rocket melting before it gets to space.

I see a lot of people launching at full throttle and maintaining it all the way to orbit. Without a Q indicator in stock its not easy to judge when it would be good to throttle back and reduce drag losses and heat. That becomes much more important on Eve. 

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1 minute ago, Foxster said:

I see a lot of people launching at full throttle and maintaining it all the way to orbit. Without a Q indicator in stock its not easy to judge when it would be good to throttle back and reduce drag losses and heat. That becomes much more important on Eve. 

I use KER which has a temperature readout. But even if you don't have that, you can eyeball it with the plasma shock visual effect and the overheat bars on individual parts.

This is a supersonic plane, not a spaceplane or a rocket or anything. But I was keeping a close eye on its speed to keep it just barely below critical temperature.

lKwsai6.png

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In real life rockets may throttle down during ascent for two reasons. One is aerodynamic forces. These reach a peak, "Max Q", due to the tradeoff between increasing speed and decreasing air pressure with height, and throttling back reduces Max Q. The other reason to throttle down is to reduce peak acceleration when a stage is nearly empty, so that it doesn't crush the payload or astronauts.

But Kerbal rockets and spacecraft are built strong and heavy. They can take abuses that would utterly destroy any real rocket, like pulling a 360 flip during the ascent, or using the engine bell as landing gear. Kerbals are pretty tough too, and in any case if the rocket has a probe core then Kerbal blackouts mean little. So in Kerbal, we're usually free to go full throttle all the way.

The main case we have to throttle back in Kerbal is because of aerodynamic instability, if we're launching something ridiculous. In real life that doesn't really happen - payloads are required to fit in standard fairings or otherwise be aerodynamic.

The small size of Kerbin doesn't come into it much. I've spent a lot of time playing RSS with unaltered stock parts and most of my launches don't require throttling back. Even with launchers that hit 7-8 g at MECO, I might throttle back just to better control my trajectory but I don't need to.

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9 hours ago, cantab said:

The small size of Kerbin doesn't come into it much. I've spent a lot of time playing RSS with unaltered stock parts and most of my launches don't require throttling back. Even with launchers that hit 7-8 g at MECO, I might throttle back just to better control my trajectory but I don't need to.

Indeed, I have successfully flown (unmanned) rockets which pulled well over 10g in RSS/RO. I'm not sure if that would be realistic in real life, but the maximum reentry deceleration of any real spacecraft is 34g so maybe it's not that crazy.

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On 5/8/2019 at 4:24 PM, Daniel Prates said:

IIRC the Saturn V would launch way below maximum thrust, and then slowly increase thrust till it reached maximum power only when it was quite high already

[nope]

On 5/8/2019 at 5:28 PM, Daniel Prates said:

I am pretty sure about the story, it could be another rocket though!

Not the way you describe. Some more recent LVs throttle down some time after liftoff, but at launch time you typically want all the thrust you can get.

On 5/8/2019 at 1:46 PM, jnbspace said:

Is there a practical reason (in KSP not the real world) to not just put the throttle to 100% and go for it? 

The key phrase is "terminal velocity" -- going faster than TV on your way up is a waste of fuel. Ideally, you want to accelerate to terminal velocity instantly (requiring very high thrust at launch) and then just stay at TV (requiring less thrust).

Under the old aerodynamic model (when the tutorial was written), terminal velocity was an everyday concern for any rocket. These days, it would need to be very un-aerodynamic to begin with. If you ever want to move a large space station to orbit in a single launch, terminal velocity may still be a practical concern.

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1 hour ago, Laie said:

Not the way you describe. Some more recent LVs throttle down some time after liftoff, but at launch time you typically want all the thrust you can get.

Right. It would seem the space shuttle was an exception, bur for very particular reasons, as discussed above.

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On 5/30/2019 at 12:23 AM, cantab said:

 I've spent a lot of time playing RSS with unaltered stock parts

How so? I play in x10 resize and use SMURFF

I still struggle to build an Apollo 11 analog, atleast in the 2.5m techtree era.

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