Space Shuttle Gravity Turn

[Kerbin Dallas Multipass]

I tried to measure the angle in photoshop and it's about 20-25 degrees.

Let's guess the altitude.. is that 700m (2000ft)?

Did the shuttle always turn so early?

Why did they turn so early? Isn't it counterproductive to fly through the atmosphere diagonally?

This mission put the Hubble telescope to space. Source: http://en.wikipedia.org/wiki/File:STS-31_Hubble_launch_roll_and_pitch.jpg

[CalculusWarrior]

When playing with FAR installed, you need to begin the gravity turn as soon as your rocket has lifted off. KSP's 'go straight up then flip 45 degrees' method not only doesn't work efficiently, but it will cause your ship to flip out. I assume that the space shuttle operated under similar constraints.

[Kerbin Dallas Multipass]

@CalculusWarrior Didn't know the shuttle was flown under ferram aerospace conditions. So sad they used deadly reentry, too! SCNR XD

[Themohawkninja]

The increased stabilization (as the first guy said) probably more than makes up for having to go through the atmosphere for a longer period of time.

That's just a guess though.

[Skyler4856]

@CalculusWarrior Didn't know the shuttle was flown under ferram aerospace conditions. So sad they used deadly reentry, too! SCNR XD

That's not funny...

[Kerbin Dallas Multipass]

That's not funny...

Real spaceflight is not a game. Explaining why things are done the way they are done by pointing at KSP mods IS funny,

This was not intended to be disrespectful towards real life engineers or real life victims of manned space flight.

[tavert]

Part of the reason for turning immediately is so if something bad does happen, you don't destroy your launchpad infrastructure with falling debris.

[AlamoVampire]

even with the early gravity turn, there have been some disasters where the fall out STILL can strike the pad and surrounding areas.

should be noted, NO life was lost, only a GPS satellite and some cars as burning fuel came down.

http://www.youtube.com/watch?v=z_aHEit-SqA

[Aghanim]

0:24 We had an anomaly... an anomaly of the Delta II rocket launch... we need to secure the area

[miracmert]

It's mainly because the atmosphere of the earth is way thinner than kerbins and the delta v needed to orbit is roughly 4 times more than kerbins. With so powerful boosters, you want to get horizontal speed ASAP so that's why the gravity turn is earlier. None of the reasons above are the main reasons. Hope this helps

[brooklyn666]

This is why KSP needs realisitic aerodymamics asap. What people think of a gravity turn in ksp is not actually a gravity turn, and the way people learn to do things during atmospheric flight in the game is usually the WORST way to do them in real life

[Kerbin Dallas Multipass]

I found this Saturn V launch profile simulation http://www.braeunig.us/apollo/SaturnV.pdf

For comparison:

At 700m altitude the pitch angle is 0°

Pitch reaches 20.38° at 5550m altitude

Pitch reaches 24.75° at 7185m altitude

I hope I read those numbers correctly. This is a completely different launch profile and much more how I would imagine it.

[tavert]

It's mainly because the atmosphere of the earth is way thinner than kerbins

A nitpick: the scale height of Earth's atmosphere is a bit higher than Kerbin's, and the sea level pressures are the same. Earth's atmosphere doesn't behave isothermally or have a hard cutoff at 1e-6 atm, but that doesn't make Kerbin's atmosphere thicker. It's the force of drag that is much higher in stock KSP than in reality, for aerodynamically shaped objects.

[miracmert]

A nitpick: the scale height of Earth's atmosphere is a bit higher than Kerbin's, and the sea level pressures are the same. Earth's atmosphere doesn't behave isothermally or have a hard cutoff at 1e-6 atm, but that doesn't make Kerbin's atmosphere thicker. It's the force of drag that is much higher in stock KSP than in reality, for aerodynamically shaped objects.

True, but Earth's atmosphere also doesn't have cut-ends like in KSP. IIRC the atmosphere drag is most effective in first 40kfeet i might be wrong though. Since earth has the same gravity with kerbin even though earth has way more surface area, high density atmosphere is more scattered at lower levels meaning closer to the surface. The reason Saturn V turned later than the shuttle is that shuttle is aerodynmacally more effective than Saturn v at gaining horizontal speed at lower altitudes. Of Saturn v tried to do the same maneuver, it would break apart because of the pressure.

(PS: I might sound a little douche with grammar mistakes and all I'm writing from mobile so please don't mind ^ ^)

[PakledHostage]

(PS: I might sound a little douche with grammar mistakes and all I'm writing from mobile so please don't mind ^ ^)

Oh don't worry about it... Your English is far better than my Turkish and we can tell that you don't know what you're talking about even with the grammar mistakes.

[miracmert]

A nitpick: the scale height of Earth's atmosphere is a bit higher than Kerbin's, and the sea level pressures are the same. Earth's atmosphere doesn't behave isothermally or have a hard cutoff at 1e-6 atm, but that doesn't make Kerbin's atmosphere thicker. It's the force of drag that is much higher in stock KSP than in reality, for aerodynamically shaped objects.

I'm sorry that I totally misread and misunderstood your post, mine was a pretty ignorant reply. Damn phones!

Oh don't worry about it... Your English is far better than my Turkish and we can tell that you don't know what you're talking about even with the grammar mistakes.

Darn it I couldn't hide my ignorance once again!

Whatever, I finally got home and now I have time to bs about science some more! Yay!

So the question is why Space Shuttle isn't doing gravity turns like we do in KSP. Well, what I'm trying to say is that in KSP the atmosphere is much more denser than it is in real life. Let's think about the formula we use for gases: P.V=n.R.T where P is the Pressure and V is the Volume, n is mol (m/M) and other part is needless for now. In order to provide us a realistic physics simulation, SQUAD kept 1atm pressure and ~10 g gravitational acceleration so we could feel like flying a rocket in real world. However, Kerbin is much more smaller than Earth in volume, therefore in surface area. In order to keep P=1, since V is similar to Earth's (actually smaller than Earth's atmosphere volume) we assume the RT part is a constant (even though T is not, we assume T is same in both situations) we need to bring n up. That means we need to put more mass to keep the pressure same.

So when we compare the radius of Kerbin and it's atmosphere, there's already a lot more room for atmosphere as ratio compared to Earth. Still, to keep the pressure close to 1 atm we put more mass in so we brought up the density.

In real life, dense part of the atmosphere is about 40k feet high(which is the trophosphere level) and then the atmosphere suddenly starts getting less dens. However, if you look at Kerbin, it's pretty dense until 30k meters. It's because you have to share the density to more volume in order to keep the pressure Earth-like. In KSP, you need to get higher first because you don't want to mess with high drag while trying to get horizontal speed because that would cost you fuel. IRL, you need to get horizontal speed as soon s possible because you need a lot of delta v. So the thing is: Go little bit straight up so you will have enough vertical force to keep you out of the troposphere quickly, and start turning to gain more and more horizontal velocity.

And about Saturn V again: Saturn V was not designed to have quick and easy manuevers like Shuttle was so if it were start turning too early, either it would lose control or the external material would brake apart due to high resistive force in the air. Instead, it just got high enough for itself to have an easier manuever, and then fire horizontal. Even at some point of the burning sequence, there is a part where one of the stages burn towards closer towards earth to gain horizontal velocity and drag apogee back to wanted altitude.

I know I suck at telling some stuff in English even though I studied one year in US, but I hope I could give you a little idea about why they do it

Edited January 21, 2014 by miracmert

[ferram4]

As a side note, since we're talking about aerodynamics in KSP vs. real life, the actual density, pressure and temperature parameters are close to correct for a real life atmosphere (though density and pressure following a linear relationship is wrong). The problem is that the drag area of parts in KSP is much, much higher than it should be; either the surface area is too high or the drag coefficient is too high. Whichever it is, it results in drag forces in KSP that are somewhere between 2-10 times as high as they should be in real life, at least for rockets and planes.

[Kerbin Dallas Multipass]

So the question is why Space Shuttle isn't doing gravity turns like we do in KSP.

My original question is not about KSP. But talking about KSP on a KSP forum is absolutely fine, too!

And about Saturn V again: Saturn V was not designed to have quick and easy manuevers like Shuttle was so if it were start turning too early, either it would lose control or the external material would brake apart due to high resistive force in the air. Instead, it just got high enough for itself to have an easier manuever, and then fire horizontal.

Wikipedia states the opposite

" The pitchover should also be carried out while the vertical velocity is small to avoid large aerodynamic loads on the vehicle during the maneuver."

Makes sense to me. Flipping a rocket sideways near mach 1 in dense atmosphere sounds like much more stress than doing it at racecar speed just above the launchpad.

They turned the Saturn V between mach .75 - mach 1 at the altitudes I quoted above, so I would assume the very early shuttle turn is to reduce forces.

Even at some point of the burning sequence, there is a part where one of the stages burn towards closer towards earth to gain horizontal velocity and drag apogee back to wanted altitude.

You mean accelerating towards a point below the horizon? Got a citation for that?

Edited January 21, 2014 by Kerbin Dallas Multipass

[Galacticruler]

Whichever it is, it results in drag forces in KSP that are somewhere between 2-10 times as high as they should be in real life, at least for rockets and planes.

wait, so if we were in the kerbalverse, on kerbin and tried to walk, by all means we would have 10x the force against us?(Or am I getting this wrong?)

[miracmert]

As a side note, since we're talking about aerodynamics in KSP vs. real life, the actual density, pressure and temperature parameters are close to correct for a real life atmosphere (though density and pressure following a linear relationship is wrong). The problem is that the drag area of parts in KSP is much, much higher than it should be; either the surface area is too high or the drag coefficient is too high. Whichever it is, it results in drag forces in KSP that are somewhere between 2-10 times as high as they should be in real life, at least for rockets and planes.

Well, you are ferram, so I trust your aerodynamic knowledge I can't understand though, if the pressure is staying almost same to Earth's at almost same altitudes, how come the density would be same? Volume is much smaller than Earth's atmosphere volume. d=m/V so I don't understand how d stays the same. I'm just asking by the way, not trying to be a smart-ass to you

My original question is not about KSP. But talking about KSP on a KSP forum is absolutely fine, too!

Wikipedia states the opposite

" The pitchover should also be carried out while the vertical velocity is small to avoid large aerodynamic loads on the vehicle during the maneuver."

Makes sense to me. Flipping a rocket sideways near mach 1 in dense atmosphere sounds like much more stress than doing it at racecar speed just above the launchpad.

They turned the Saturn V between mach .75 - mach 1 at the altitudes I quoted above, so I would assume the very early shuttle turn is to reduce forces.

Manuevering in high speeds get pretty difficult due to air resistance. So Saturn V was so heavy it could catch enough speed at 7km finally, and then start to turn. Makes sense

You mean accelerating towards a point below the horizon? Got a citation for that?

Yes, that was what I meant. I believe it was at 3rd stage, and it's been a while I read about it. I'm searching but I haven't found anything about where the spacecraft pointed towards while accelerating, so I couldn't cite it. Not cited information is not counted as information! So I hope I will be able to cite it soon.

Okay, I want to say something a little bit off topic. I'm just saying things from my little knowledge which I learned from basic physics classes previous semester of my freshman year at engineering. I might be wrong, and if I'm wrong, please correct my mistakes so I can learn. I would appreciate to be corrected rather than knowing the wrong info and maybe even giving wrong info to people.

There was a similar topic a year ago, and the answers were pretty much similar to what I said until now. Again, slap me with some science if I'm wrong at any calculation

Aaand, to end the post, I want to state my theory again: To keep pressure Earth-like, the density is higher in more areas of the Kerbin atmosphere but in Earth the dense part is pretty much located at the bottom.

And as ferram said drag is higher, I believe it's related to this formula which is again related to density of the atmosphere.

So that's why in KSP we try to skip the dense part first and then do the gravity turn to save fuel.

Edited January 21, 2014 by miracmert

[Seret]

Part of the reason for turning immediately is so if something bad does happen, you don't destroy your launchpad infrastructure with falling debris.

Tbh, I always tip over about 5° straight after launch in KSP for the same reason. Dropping boosters on the pad is bad manners.

[tavert]

I can't understand though, if the pressure is staying almost same to Earth's at almost same altitudes, how come the density would be same? Volume is much smaller than Earth's atmosphere volume. d=m/V so I don't understand how d stays the same.

Don't try to think of of the entire atmosphere as a control volume here, since the properties vary over altitude that isn't a very helpful way of looking at it.

The ideal gas law says P V = n R T as you said. Assuming the chemical composition of the atmosphere is more or less constant, n = m / M where m is mass, M is molar mass. Rearrange the idea gas law a little, you get P M / (R T) = m / V = rho. So density of an ideal gas is directly proportional to pressure, inversely proportional to (absolute) temperature. In KSP's atmosphere the temperature is completely independent of the aerodynamics, the pressure and density behave as if the atmosphere is at a constant temperature.

[softweir]

Well, you are ferram, so I trust your aerodynamic knowledge I can't understand though, if the pressure is staying almost same to Earth's at almost same altitudes, how come the density would be same? Volume is much smaller than Earth's atmosphere volume. d=m/V so I don't understand how d stays the same.

Because Squad don't put genuine gas formulae into KSP, they just shove some numbers in that make the game work the way they want. They don't simulate density, speed of sound, sonic/supersonic shockwaves or anything - they just have a graph that rather arbitrarily associates coefficient of drag with altitude, adjusted to reach zero at 69km altitude. Not to mention that KSP calculates drag for each and every part of a vehicle regardless of what parts are in front of them and ought to be "shielding" them from much of the drag.

[tavert]

they just have a graph that rather arbitrarily associates coefficient of drag with altitude, adjusted to reach zero at 69km altitude

Density, not coefficient of drag. Coefficient of drag in stock KSP is just a mass-weighted average of the drag coefficients of the parts contained in your craft. And density isn't totally arbitrary, it is a conventional barometric formula assuming constant temperature.

[rdfox]

OK, there's several reasons for starting the turn early. The first, and single most important reason, is range safety. NASA honestly isn't THAT worried about an exploding booster dropping debris back on the pad--after all, it could just as easily explode on the pad and completely wipe it out. However, the horizontal distance that debris will travel before hitting the ground depends on altitude, and if they launched straight up, it wouldn't take long for an exploding vehicle's debris to be able to travel far enough to land in Titusville or other open-to-the-public areas. Therefore, just after clearing the tower and completing the roll program, they initiate an early part of the pitch program to pitch over about five degrees and start the vehicle downrange. This actually costs them a little extra delta-V, but it's considered beneficial for range safety requirements.

Secondly, yes, the drag model in KSP is screwed up (and so is the *rate* of change in the atmosphere's thickness), resulting in the lower atmosphere being like soup compared to Earth. This means that you do want to start gaining downrange velocity as soon as possible (look, for example, at films of Polaris or Trident submarine-launched ballistic missiles being test-fired from submarines and starting pitchover almost immediately after ignition), though not as aggressively as the Shuttle did; that profile was almost entirely for range safety.

Third, as noted, an early start to the gravity turn is very important to avoid excessive aerodynamic loads on the vehicle. This is more important than you might think--most people think that the Challenger was torn apart by the force of the external tank exploding; it wasn't. When the ET ruptured and the LH2 and LOX burned, it generated very little force, at least compared to what the Orbiter's structure was built for. What caused the Challenger to break up were the extreme aerodynamic loadings caused by the Orbiter abruptly pitching towards the horizon due to the now off-axis thrust provided by the main engines (still burning on residual propellant in the lines). That accident happened at fairly high altitude and low velocity, yet the aerodynamic loads were still enough to break the vehicle apart. (The Saturn V started its turn later because a "single stick" configuration is more tolerant of aerodynamic loads than a winged vehicle--there's no big flat wings to build up massive loads and tear off.)

Side note: The Saturn V made the earliest pitch maneuver of any manned spacecraft ever. In the first few seconds after launch, the booster was within ten meters of the launch umbilical tower, and the inherent imprecision in the guidance system could have easily had it drift over into the tower in a vertical launch. Therefore, pretty much as soon as the engines were clear of the flame pit, the S-V was programmed to make a 2-3 degree pitchover *away* from the tower; it would then pitch back to vertical as soon as it had cleared the tower. This was internally known as the "lean," and was only there to decrease the odds of a catastrophic collision with the LUT, and is, to my knowledge, completely unique to the Saturn V (though I wouldn't be shocked if the Saturn I and IB also had a similar maneuver in their profile).