# Max G of STS, CLV, HLV ?

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##### Guest
Anyone have a rough estimate of the relative Max G loads, vibration levels, etc. between the current STS, the CLV, and the HLV?

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#### propforce

##### Guest
3 G on the STS. I think the CLV should be the same as the STS (human rating), while the HLV approaches 4 Gs (cargo only) <div class="Discussion_UserSignature"> </div>

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#### barrykirk

##### Guest
<font color="yellow">HLV will be about 4 to 5 G's</font><br /><br />What will be the G's at liftoff?<br /><br />Is this thing going to take a major hit from Max Q?

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#### propforce

##### Guest
typical liftoff G's range is 1.2 ~ 1.4<br /><br />Max Q for each vehicle is at different number, but it simply means it was at the maximum dynamic pressure (Q) for a given trajectory. <div class="Discussion_UserSignature"> </div>

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#### barrykirk

##### Guest
Yes, I understand that liftoff G's don't directly relate to max G's.<br /><br />It depends a lot on what type of rockets, solid/liquids etc. and a lot of other factors.

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#### propforce

##### Guest
So you also realize Max Q varies.... even for rockets ? <div class="Discussion_UserSignature"> </div>

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#### tap_sa

##### Guest
And Max Q alone doesn't tell the whole story, right? Because HLV doesn't have sidemounted orbiters it's frontal area is probably smaller. Therefore max Q can be bigger but drag loss the same or even less.

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#### barrykirk

##### Guest
Correct me if I'm wrong, but I was under the impression that Max Q was a term that referred to the point of Maximum Aerodynamic Drag?<br /><br />HLV would have a smaller frontal area than a shuttle, because the shuttle has the SRB, the external tank, and the orbiter.<br /><br />The HLV put the payload inline with the tank so they share the same frontal area thus reducing the drag.

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#### henryhallam

##### Guest
Q stands for dynamic pressure and is defined as (1/2) * (air density) * (velocity^2)<br /><br />For a given vehicle, drag is proportional to Q.<br /><br />Two differently-shaped vehicles flying the same trajectory will experience the same Q at a given time after liftoff, but they will have different amounts of drag depending on their surface area and drag coefficients.

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#### tap_sa

##### Guest
Max Q means maximum dynamic pressure and is calculated ( rho * v<sup>2</sup> ) / 2 (rho = air density). Because it's pressure the usual unit in US is psi or psf (pounds per square foot). To get the actual drag force you multiple max Q with rocket's frontal area A and drag coefficient Cd. Maximum drag does happen during max Q.

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#### barrykirk

##### Guest
Thank you for clearing me up on that misunderstanding.<br /><br />I can now post with more accuracy.<br /><br />It makes sense that the HLV would have a lower frontal area than the shuttle.

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#### henryhallam

##### Guest
Question: do the definitions Q= ( rho * v^2 ) / 2, and the drag equation F=QACd hold at supersonic velocities?

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#### tap_sa

##### Guest
Not 100% sure, haven't studied the subject enough yet, but I'd lean towards answering yes. Those who know for sure please confirm or refute. AFAIK equations stay the same, Cd varies with speed. This was discussed while ago. Interestingly the Cd 'bump' is not at transsonic area but around mach two.

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#### propforce

##### Guest
Now that we went from G to Q <img src="/images/icons/smile.gif" /><br /><br />Here's another way to look at Q = 1/2 * p * M^2<br /><br />where p is the air pressure at a given altitude<br />M is the Mach number <br /><br />You can derive from the previous definition of Q to this by letting the Rho = P/zRT for air; where z = compressibility factor.<br /><br />What this tells you is that you can lower the value of your Max Q by NOT accelerating your vehicle too fast so that you can get to an altitude where air is less dense (lower pressure) befor going supersonic. You do this by reducing your inititial lift-off Gs. By doing so, each launch vehicle has it's own value of "max Q".<br /><br />So that's the scoop on G - Q <img src="/images/icons/laugh.gif" /> <div class="Discussion_UserSignature"> </div>

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#### propforce

##### Guest
The Shuttle Cd bump is around Mach 1.5 <div class="Discussion_UserSignature"> </div>

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