# How great is the stress on a rocket at max-q [closed]

How much more stress does a rocket experience during max-q compared to other parts of the flight (such as when it is at rest on the pad)?

Use data for any 'cylindrical' rocket if a specific example is needed. By cylindrical I mean to exclude the Space Shuttle, Space Ship One and other exotic designs.

Note that I am not interested in the value of q. Only the stress on the rocket.

• I believe this is very much an "it depends on the rocket" answer. Could you specify a particular rocket family or model you're interested? Jul 6 '17 at 19:49
• Its the Max man, like max-q. Jul 6 '17 at 19:59
• @sarah Bailey SaturnV seems like a good option. But I don't want to require that. Jul 6 '17 at 20:02
• q-alpha is many times a much better measure.
– Erik
Oct 18 '17 at 20:49
• I'm voting to close this as unclear what you're asking. The stress in a rocket varies from point to point in its structure, and as @SarahBailey says, from rocket to rocket. Oct 18 '17 at 23:26

According to this answer Max q for the saturn V is 34kPa. Note that it is very easy to calculate q, as it depends only on velocity and atmospheric density. The aerodynamic stress depends on the shape of the rocket so is more difficult to calculate.

We can get a ball-park figure for the force as follows:

Force = pressure x area

= 34kPa x (10m)^2 * PI / 4

= 2670kN

= 267000 kgf approx or 587000lbf approx.

This is about a tenth of the launch weight of the saturn V.

However it is likely a significant overestimate as the rocket does not stop the air that hits it completely, but rather pushes it out of the way. In order to get the actual drag force acting on the rocket, you have to multiply by the coefficient of drag which is a function of the shape of the vehicle, and is usually somewhat less than 1 (though it can be greater than 1.)

Hope that helps.

• Stress is not measured in kgf, but in units of force/unit area. This does not answer the question. If you divided your answer by the area, you would just have q, and the OP says that is not what they want. en.wikipedia.org/wiki/Stress_(mechanics) Oct 18 '17 at 23:23
• @OrganicMarble I find your downvote disappointing though I appreciate the explanation. I am aware of the strict mechanical definition of stress but this would vary over the entire rocket hence giving a single value without context would be meaningless. Should I give that component of longitudinal stress in the sidewall of the rocket due to aerodynamic forces, or a profile of the bending stress in the conical section? I think OP is referring to stress in a more general sense, and I have given a comparison between the aerodynamic force and the launch weight. Oct 18 '17 at 23:46
• If its 101.325kPa at sea level, that makes 34kPa sound far too light. This calculator confirms the calculation though - kylesconverter.com/pressure/…. What am I missing? Jan 20 '20 at 2:12
• @MikeS at sea level the Saturn V was hardly moving, so the aerodynamic pressure was essentially 0 (note that the 101.325kPa of static pressure acting uniformly on the rocket is not included.) If you check the link in my answer you will see max q was at around 85 seconds into the flight and at about mach 1.8. The rocket would have been pretty high up by then. Jan 20 '20 at 2:21

Here's a graphic that shows dynamic pressure vs. mission time for the Shuttle:

(apologies for the awful units, this was the first graph I found)

• The OP says "Note that I am not interested in the value of q." Otherwise I would have answered it long ago. I don't really know what the OP is looking for. Units for stress are same as pressure, of course. Oct 18 '17 at 23:16
• The max q here is 33kPa, surprisingly similar to my answer for such a different vehicle. Also the vehicle's cross sectional area is not that different so the pressure x area product will be similar. The coefficient of drag and hence the overall stress will probably be quite different though. Oct 18 '17 at 23:25
• @LevelRiverSt where did you get your value for the frontal area of the Shuttle stack? Oct 18 '17 at 23:30
• @OrganicMarble external tank diameter is 8.4m as compared with 10m for the Saturn V. Add a bit more on for the orbiter and its close to the cross section of the Saturn V. en.wikipedia.org/wiki/Space_Shuttle_external_tank Oct 18 '17 at 23:34
• I tried to answer "How much more stress does a rocket experience during max-q compared to other parts of the flight", and a graph is the simplest way to give an indication of how dynamic pressure (one of the components of total stress, the other big one is acceleration) varies. That question is also impossible to answer without giving a value for q. Oct 19 '17 at 6:51