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20 minutes ago, sevenperforce said:

Controlling yaw with just two flaps in the coronal plane, actuating ventrally or dorsally, is impossible. You can control roll easily, though. Lift the left flap to decrease drag and roll left, and vice versa. Lift both to decrease aft drag and send the center of pressure forward of the center of mass, pitching up; extend both to increase aft drag and pull the center of pressure aft of the c enter of mass, pitching down.

The reason for four flaps is that you actually can control yaw aerodynamically. Order the forward flaps to roll left and the rear flaps to roll right, and you'll yaw left.

If you only have two flaps, you can't nudge yaw without a high AoA, but there's no barrier to using RCS for yaw with low AoA.

You could always use a split flap like the B2 apparently does. One side splits up and down and creates yaw in that direction like a speed brake. 
 

nevermind. I just remembered that the angle of attack is to great for this type of flap to really be effective. 

Edited by Ricktoberfest
Autocrrect.
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Just now, Wjolcz said:

I was about to say that yaw is kind of useless anyway but then realized that you can actually aim for the landing site using yaw. I guess it makes sense. Instead of carrying an extra pair of wings you go *poof* with RCS to yaw and that actually makes you a bit lighter during the landing.

Yep, changes to heading are 100% a matter of controlling yaw. A conventional plane uses roll and yaw in combination to change heading, but when you are using body lift, you use pitch to control the magnitude of the lift vector, roll to control sink rate and translation, and yaw to control heading.

With a conventional plane there is no aero resistance to roll. With a body lift regime, there is extreme aero resistance to roll but none to yaw, so RCS is fine. You just don't want to be in a situation where you are burning RCS continuously to oppose aero correction forces.

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1 hour ago, sevenperforce said:

With a conventional plane there is no aero resistance to roll.

I'm not sure exactly what you mean by this, but static roll stability is a thing. Wing dihedral, for instance, contributes a self-correcting roll stability.

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21 minutes ago, mikegarrison said:

I'm not sure exactly what you mean by this, but static roll stability is a thing. Wing dihedral, for instance, contributes a self-correcting roll stability.

Yes, you can design in static roll stability, but without this the aero corrective forces are not proportional to the angle of deflection from the passively stable orientation. 

1 hour ago, Ricktoberfest said:

You could always use a split flap like the B2 apparently does. One side splits up and down and creates yaw in that direction like a speed brake. 

It all comes down to angle of attack and the position of the lift vector. In a conventional aircraft, which flies close to prograde, the angle of attack is low and the lift vector is behind the center of mass, pointed dorsally. Roll is independent of yaw and you can roll without much if any rudder input. Once you are at a high angle of attack, like the Shuttle or a similar spaceplane, roll and yaw become coupled because your lift vector points both dorsally and forward. If you roll left, the lift vector tugs your nose to the left.

When you have a lifting-body spaceplane like the Shuttle Orbiter or the X-37, designed to enter within a very narrow aerodynamic regime and then glide to a landing, you can constrain the vehicle's pitch to the envelope where yaw and roll are coupled, and thus you need only the split body flaps to handle everything. If I remember correctly, the Orbiter's horizontal stabilizer and rudder/speedbrakes were completely out of the airstream during re-entry and only became useful during glide. This was the original planned design for the BFR as shown in the 2016 IAC.

However, this only works when you can constrain your lifting-body envelope to some very narrow parameters. If you intend to re-enter with varying cargo weights or on various worlds, you need much more flexible pitch control. On a vehicle that flies completely with body lift at an extremely high angle of attack, like a skydiving Starship, roll and yaw become uncoupled again and your roll no longer impacts your yaw at all. The good news is that at high enough angles of attack, you can rotate around the yaw axis with very little torque because there's no corrective aero force, just like you can rotate around the roll axis with low torque during level flight. 

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Actually, I think SS *could* control yaw with just the two flaps.  As it belly flops through the atmosphere, it could pull back one flap slightly. Let's pick the right-hand one.  This would cause the whole SS to roll to the right a bit before becoming stable again in the roll axis.  At that point, the (non-moving) forward flaps would be canted to the right, and would therefore try to push (yaw) the nose to the right.  At the rear, there would be no left/right force.  The result would be a yaw to the right.

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26 minutes ago, ThatGuyWithALongUsername said:

Am I the only one wondering why the livestream says its starting almost 2 hours before launch? :huh:

 

I'm sure it's just a small mistake somewhere but... eh, whatever

Different time zone?

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1 hour ago, tater said:

 

Am I the only one that is relieved that the modes of failure get more stupid by the times? Like is no design flaw, just someone who forgot "that switch" or something like that ( in the last years at least, not counting the dm-1 capsule explosion)

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27 minutes ago, Flavio hc16 said:

Am I the only one that is relieved that the modes of failure get more stupid by the times? Like is no design flaw, just someone who forgot "that switch" or something like that ( in the last years at least, not counting the dm-1 capsule explosion)

IKR?

 

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14 minutes ago, ExtremeSquared said:

Looks like they got the livestream start time wrong. Launch target is in an hour or so.

Launch time was originally supposed a half hour or so ago, had to push to 3:37 EDT

Edited by Kerballing (Got Dunked On)
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Launch time is 3:30 eastern, not 3:37.

7 minutes ago, ExtremeSquared said:

19:37 UTC

Not sure why EDT is used given it is unreliable to convert from.

Because us 'Muricans would then have to convert, and SpaceX is in 'Murica! ;)

 

 

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Just now, ExtremeSquared said:

I'm American and can't be bothered to learn what EDT means. UDT is just easier all around. Unless you're in one of those 30 minute offset timezones. Then you need a calculator regardless.

You still need to know what DST is, else UTC doesn't help, you would be off by an hour if you convert to ST vs DT (we should stop with the DST crap).

Also:

Music live.

 

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17 minutes ago, tater said:

You still need to know what DST is, else UTC doesn't help, you would be off by an hour if you convert to ST vs DT (we should stop with the DST crap).

That's the rub, I guess. I can't be bothered to keep track of this. I have definitely woken people up with 7am phone calls before because I knew they'd be up by 8am and they live in a place with spuriously changing timezones.

Edited by ExtremeSquared
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