Structural analysis of a rocket ? 1

[Ionut Cosmin]

Hello everyone,

I am curious to know if anyone knows what are the steps performed in the structural analysis of a rocket ? What are the most important parts and what type of analysis are performed on them ?

If anyone has worked on building rockets for NASA or other space agency, maybe they can provide some information on what are the steps that are taken by the stress team in analyzing the rocket ?

Also it will be great if anyone could provide some good books, or reference papers, on this subject.

Thank you very much,
Ionut

 

[MikeHalloran]

AFAIK, because of the cost of lifting stuff with rockets, _everything_ is analyzed.

I'm not in the rocket business, but I interviewed for a job in a rocket factory a long time ago.
I found one thing particularly interesting.

The rocket had several internal tanks, for propellant, oxidizer, whatever.
The tanks were made of shiny metal that had been drawn and ironed, in a process very similar to that used for making beer cans, but on a much larger scale.
As a result of that process, the tank walls were very, very, thin.
In order to not wrinkle the walls, which could otherwise happen from accidental contact with dropped wrenches or even persons, the tanks were maintained under axial tension, or kept inflated with gas, throughout the manufacturing process. The axial tension was provided by a sort of cage around the tank, anchored to the tank ends, and jacked axially to maintain the walls in tension. Just that fixture alone had to cost a fortune, and surely someone analyzed the hell out of it, and also analyzed the tank to be sure there were a sufficient number of fasteners between the tank and the fixture, that the fixture didn't take up too much space or interfere with the work to be done.

They had fixtures all over the place, for all sorts of jobs.
So not only must every part of the rocket be analyzed, but also all sorts of fixtures that will never fly must be analyzed too, along with their interactions with the rocket in various states of assembly, and as it's completed.



Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

Mistakes are a problem with rockets -

http://www.pbs.org/wgbh/americanexperience/films/command-and-control/

Start small, like internal combustion engines.

 

[Sparweb]

Just things I've sponged up over the years:
Looking at the rocket as a whole, a dominant load on the entire structure is compression. The load starts as a static 1g weight supported by the structure at the base, then multiplies as it accelerates during launch. Aerodynamic drag works against the body moving through the air, then adds buffeting and sonic shock as the craft travels fast enough.
Thin sheets of metal do not behave nicely when they are compressed on their edges, making this particularly tricky. Various strategies are employed, such as long, thick reinforcing members running up the sides, or maintaining the pressure inside the integral fuel tanks to prevent the wrinkling/buckling. Example of the former, Titan (Gemini program) and the former, Redstone (Mercury program).

Looking at components in more detail, and the story changes depending on the component. Fuel tanks often carry cryogenic liquids, meaning they must maintain their shape through dramatic shifts in thermal expansion while holding the internal pressure. Engines, of course, experience tremendous heat, pressure, and vibration. Joints between stages must withstand all interface loads, yet separate reliably at the right moment.

There are also loads caused, ironically, by the very lightness of the rocket itself, or more accurately, by its flexibility. Made from such thin flexible materials, a rocket can experience many kinds of whole-body oscillations in lateral and axial directions. One of these is called "pogo" and just like the toy, the rocket's thrust makes it shorter, then it springs back longer, then back, and this oscillation doesn't damp down until the capsule on top it getting several extra g's of +/- acceleration. I read that the Titan II (Gemini program) had that problem.

There are also "pre-launch" loads, which occur before the rocket gets to the pad. If the machine is transported laying horizontally on a truck/barge/train/etc then you wouldn't want a component crushed by the weight.

As SpaceX is now proving, there will soon be "post-launch" loads to consider, as their rockets come down and land themselves. With the intent to re-use the rocket, this matters now.

STF

 

[tbuelna]

I spent a few years working in the mechanical systems engineering group at Rockwell/Boeing assigned to the Shuttle program. Every component and system was analyzed based on a set of design requirements issued by the program office, which in this case was NASA. There were numerous analyses and design reviews performed for stress, thermal, shock & vibration, fatigue & fracture, safety & reliability, corrosion control, etc. Most of the analysis and design work was validated by system level qualification testing.

Regarding your question, "What are the most important parts and what type of analysis are performed on them?", every component/system on the vehicle is subject to a Failure Mode, Effect, and Criticality Analysis (FMECA). This analysis assigns a criticality level to each component/system based on its fault tolerance capability. Something like a piece of structure that could experience a failure resulting in loss of life or loss of vehicle would be assigned the highest level of criticality. The components/systems assigned the highest level of criticality are placed on a Critical Items List (CIL). Anything listed in the CIL is given more rigorous analysis, manufacturing processing, quality controls & inspections, and maintenance procedures while in service.

Here are some NASA documents regarding Space Shuttle design requirements you can take a look at:

http://spacecraft.ssl.umd.edu/design_lib/Systems_Eng_Handbook.pdf

http://spacecraft.ssl.umd.edu/design_lib/STS21492.PL_users_guide.pdf

https://prod.nais.nasa.gov/eps/eps_data/161793-AMEND-002-002.pdf

Here is a document from the FAA covering safety-critical structural design requirements for launch vehicles:

https://www.faa.gov/about/office_or...ety_Critical_Structures_Guide_v2.3_112205.pdf

Hope that helps.

 

[Ionut Cosmin]

3DDave thanks for the suggestion, I will look into it.

SparWeb Great insight, thank you ! Yes, SpaceX is really changing the game, it would be nice to investigate those "post-launch" loads but I guess you have to work there to have access to it

tbuelna Wow, first of all, awesome job man ! I wish I could someday be part of some awesome project such as the Space Shuttle program. Second, thank you very much for the information regarding NASA requirements. I am working on some European projects now and we are mostly using the ECSS handbooks, but it is interesting to compare them with FAA requirements. Thank you again !

 

[rb1957]

from my experience the most important loading is vibration.

as noted above everything is analyzed to the nth degree, and everything is tested.

the flip side of that is that margins are low ('cause weight is so critical).

SpaceX reusable rocket "nonsense" ... shuttle showed reusing components is very expensive (and complicates design analysis significantly ... it's no enough to test one firing, now you need to test re-firing). I think what they are really doing is learning how to land a rocket (for when they go to Mars)

another day in paradise, or is paradise one day closer ?

 

[WKTaylor]

Ionut Cosmin ...

'A LITTLE OF THIS, A LITTLE OF THAT'...

1. First...

Tactical, short range, medium range, intercontinental or space-launch [unguided or guided] rocket or missile???

Solid propellant, liquid propellant, hybrid propellant???

2. Regardless... Might find some of the following useful...

ANALYSIS AND DESIGN OF MISSILE STRUCTURES - BRUHN, ORLANDO, MEYERS [1967]
Old but not necessarily outdated... Especially from the perspective of missile structures introduction.

US ARMY GUIDE TO AMATEUR ROCKETRY aeroconsystems.com/tips/a-guide-to-amateur-rocketry-1.pdf



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[tbuelna]

Great points rb1957.

"from my experience the most important loading is vibration." One project I worked on for the Shuttle was design of a heavy weight latching system for the large ISS payloads. Normally, to save cost the qualification test articles were refurbished after testing and used for flight, typically with no restrictions. However, the qualification level vibration test procedure for the heavy weight payload latch used up a significant amount of the latch frame's fatigue life, so we had to keep an eye on how that particular latch was used. Some of the most severe vibration conditions were created by the Shuttle engine noise right at liftoff.

"the flip side of that is that margins are low ('cause weight is so critical)" Typically a 1.50 FoS was used for structural anlysis of mechanical system hardware, but a lower 1.25 FoS was permitted in many cases if the structural analysis was validated by testing.

"SpaceX reusable rocket "nonsense" ... shuttle showed reusing components is very expensive (and complicates design analysis significantly ... it's no enough to test one firing, now you need to test re-firing)." The US GAO is preparing a report concerning problems with fractures in the main engine turbopump turbine blades used on the SpaceX Falcon 9 rocket. The Shuttle RS-25 engines had a pretty good service record. But as noted, they were not what most people would consider "reuseable". The basic RS-25 engine was designed for something like 55 flights, but some components like the turbopumps were replaced every 5 to 10 flights. Rocketdyne and NASA did a good job of improving reliability/durability of the RS-25 during the Shuttle program, but even at the end of the program it still required around 10 weeks to service the engines between flights.

 

[Ionut Cosmin]

WKTaylor

Thank you for the references, I already found some that were useful.