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We used OpenRocket for designing our rocket for UK youth rocketry competition UKROC[1]. It is great for getting a 'spherical cow in a vaccuum' idea of likely a

We used OpenRocket for designing our rocket for UK youth rocketry competition UKROC[1]. It is great for getting a 'spherical cow in a vaccuum' idea of likely altitude with different motors, centre of pressure, center of mass etc. But it obviously doesn't take account of detailed aerodynamics etc and we found the maximum altitude estimates were about 15% too high. But it was still very useful. [1] UKROC is an amazing competition for UK school kids. And there are equivalent competitions in the US, France and Japan, with an International competition for the 4 country winners. If you know any kids interested in engineering I recommend you look into it. https://www.ukroc.com/ https://www.ukroc.com/

plugger 7mo ago
> we found the maximum altitude estimates were about 15% too high. this often happens when the wrong rocket finish is selected. Everyone chooses a polished finish when in actuality they've just sprayed the thing with paint. Skin drag is real.
- hermitcrab 7mo ago
  Possibly. I don't remember the details. Also, it isn't a full aerodynamic simulation. Plus the hobby motors are not manufactured to NASA standards. So you can't expect it to be too accurate.
- sandworm101 7mo ago
  They also seem to be treating rockets as rigid bodies. Flex of parts is real at these accelerations but the calculations get into supercomputer territory very quickly. All the math is questionable if, at any given moment, your rocket isnt actually straight. Look into the physics of archery, which has similar accellerations albeit of a narrower tube. The choice between a heavier/stronger arrow v a lighter/flexible shaft is the entire game.
  - hermitcrab 7mo ago
    If your rocket is flexing significantly, then it probably won't be a rocket for very long!
    - sandworm101 7mo ago
      Shuttle was held on the launch pad after ignition for a couple seconds because of flex. The stack initially flexed out of alignment when the main engines were lit, the stack being asymetrical. Only once the nose drifted back to vertical would the srbs ignite and the stack begin to move.
      - hermitcrab 7mo ago
        OpenRocket is intended for amateur rocketry, not NASA space shuttles. Amateur rocketry people try very hard to make their rockets symmetrical. Hobby rockets have to be stiff enough not to banana. I don't think it is figured into model rocketry beyond that.
  - plugger 7mo ago
    Not really applicable here imho. Most high power high performance rockets aren't cardboard. The majority are either fibreglass or carbon fibre. Flex doesn't really become a concern with carbon fibre until you approach Mach 3. I say this because I've built two rockets of an identical design philosophy; one for 54mm motors and one for 75mm motors. The 54mm motor rocket did M2.64 and I could fly it again today if I wanted. The 75mm rekitted itself at M3. It failed where I'd not reinforced the hand rolled airframe that I was flying. 4 wraps of 2x2 200gsm twill CF. Same number I used on the 54mm fine. For both of these rockets the vast majority of the airframe is directly over a aluminum motor casing that is a rigid body. These things are grossly over-engineered when compared to the pressures/forces they see.
simgt 7mo ago
I can't find the French competition, ironic given that the front page of ukroc is full of French gov officials... Could you share the name if you have it?
- teraro 7mo ago
  The biggest one is C'space: https://cnes-edu-cspace.com/cspace-2025-en/ https://cnes-edu-cspace.com/cspace-2025-en/ There is also the European Rocketry Challenge in Portugal: https://euroc.pt/ https://euroc.pt/
  - notahacker 7mo ago
    And for older students in the UK there's https://race2space.org.uk/ https://race2space.org.uk/
  - lencastre 7mo ago
    uau uau uau uau… uau obrigado!
- hermitcrab 7mo ago
  The French and Japanese competitions seems to be rather low-key compared to the UK and US competitions. The winner of the French schools is mentioned here: https://www.safran-group.com/news/have-you-heard-about-rocketry-challenge-international-final-took-place-paris-air-show-2023-06-23 https://www.safran-group.com/news/have-you-heard-about-rocke... Maybe you can contact the school and find out more?
GordonS 7mo ago
Heh, UK tinkerer here :) How hard is it to get started for amateurs? Are rules/regulations problematic?
- hermitcrab 7mo ago
  Not hard. Buy an Estes kit. Go along to one of the regional meetings, fly your rocket and talk to people there - they are very friendly. The UK regulations are fairly sensible and they won't let you do anything stupid at one the regional meets. Just start with something small (A/B/C motor) and work your way up from there. Don't try to start with a K motor, 2 stage or liquid propellant and you'll be fine. Where are you based?
  - GordonS 7mo ago
    Thanks, I'm in NE Scotland.
    - hermitcrab 7mo ago
      See: https://www.sara.rocketry.org.uk/ https://www.sara.rocketry.org.uk/ https://ukra.org.uk/links/uk-clubs/ https://ukra.org.uk/links/uk-clubs/ https://irw.rocketry.org.uk/ https://irw.rocketry.org.uk/

plugger 7mo ago

> we found the maximum altitude estimates were about 15% too high. this often happens when the wrong rocket finish is selected. Everyone chooses a polished finish when in actuality they've just sprayed the thing with paint. Skin drag is real.

hermitcrab 7mo ago
Possibly. I don't remember the details. Also, it isn't a full aerodynamic simulation. Plus the hobby motors are not manufactured to NASA standards. So you can't expect it to be too accurate.
sandworm101 7mo ago
They also seem to be treating rockets as rigid bodies. Flex of parts is real at these accelerations but the calculations get into supercomputer territory very quickly. All the math is questionable if, at any given moment, your rocket isnt actually straight. Look into the physics of archery, which has similar accellerations albeit of a narrower tube. The choice between a heavier/stronger arrow v a lighter/flexible shaft is the entire game.
- hermitcrab 7mo ago
  If your rocket is flexing significantly, then it probably won't be a rocket for very long!
  - sandworm101 7mo ago
    Shuttle was held on the launch pad after ignition for a couple seconds because of flex. The stack initially flexed out of alignment when the main engines were lit, the stack being asymetrical. Only once the nose drifted back to vertical would the srbs ignite and the stack begin to move.
    - hermitcrab 7mo ago
      OpenRocket is intended for amateur rocketry, not NASA space shuttles. Amateur rocketry people try very hard to make their rockets symmetrical. Hobby rockets have to be stiff enough not to banana. I don't think it is figured into model rocketry beyond that.
- plugger 7mo ago
  Not really applicable here imho. Most high power high performance rockets aren't cardboard. The majority are either fibreglass or carbon fibre. Flex doesn't really become a concern with carbon fibre until you approach Mach 3. I say this because I've built two rockets of an identical design philosophy; one for 54mm motors and one for 75mm motors. The 54mm motor rocket did M2.64 and I could fly it again today if I wanted. The 75mm rekitted itself at M3. It failed where I'd not reinforced the hand rolled airframe that I was flying. 4 wraps of 2x2 200gsm twill CF. Same number I used on the 54mm fine. For both of these rockets the vast majority of the airframe is directly over a aluminum motor casing that is a rigid body. These things are grossly over-engineered when compared to the pressures/forces they see.

simgt 7mo ago

I can't find the French competition, ironic given that the front page of ukroc is full of French gov officials... Could you share the name if you have it?

teraro 7mo ago
The biggest one is C'space: https://cnes-edu-cspace.com/cspace-2025-en/ https://cnes-edu-cspace.com/cspace-2025-en/ There is also the European Rocketry Challenge in Portugal: https://euroc.pt/ https://euroc.pt/
- notahacker 7mo ago
  And for older students in the UK there's https://race2space.org.uk/ https://race2space.org.uk/
- lencastre 7mo ago
  uau uau uau uau… uau obrigado!
hermitcrab 7mo ago
The French and Japanese competitions seems to be rather low-key compared to the UK and US competitions. The winner of the French schools is mentioned here: https://www.safran-group.com/news/have-you-heard-about-rocketry-challenge-international-final-took-place-paris-air-show-2023-06-23 https://www.safran-group.com/news/have-you-heard-about-rocke... Maybe you can contact the school and find out more?

GordonS 7mo ago

Heh, UK tinkerer here :) How hard is it to get started for amateurs? Are rules/regulations problematic?

hermitcrab 7mo ago
Not hard. Buy an Estes kit. Go along to one of the regional meetings, fly your rocket and talk to people there - they are very friendly. The UK regulations are fairly sensible and they won't let you do anything stupid at one the regional meets. Just start with something small (A/B/C motor) and work your way up from there. Don't try to start with a K motor, 2 stage or liquid propellant and you'll be fine. Where are you based?
- GordonS 7mo ago
  Thanks, I'm in NE Scotland.
  - hermitcrab 7mo ago
    See: https://www.sara.rocketry.org.uk/ https://www.sara.rocketry.org.uk/ https://ukra.org.uk/links/uk-clubs/ https://ukra.org.uk/links/uk-clubs/ https://irw.rocketry.org.uk/ https://irw.rocketry.org.uk/