hobie1dog

Subaholic
TRF Supporter
The Technical questions are the same ones they've had, but they changed some of the Safety ones this month.

cjl

Well-Known Member
I just took the technical test since I was curious (and it's been a *long* time since I took my L2), and I disagree with these two questions a bit:

For a subsonic rocket, what factors most greatly affect the coefficient of drag (Cd)?

- Speed, airframe dimensions, nosecone shape and fin shape.
Feedback: As speed increases, the drag number changes. The length and diameter of the rocket factors into the total surface area. The nose cone shape affects the airflow over the front of the nose cone. The fin shape and fin area factor into the total surface area.

For subsonic flight, the coefficient of drag actually tends to slightly decrease with speed, but overall it's reasonable to just model it as being constant. The drag force increases, but until you get close to the speed of sound, Cd is close to fixed (and in fact that's a large part of why it's useful). Within reasonable reynolds number ranges, and below about mach 0.8 or so, rocket-shaped objects will have a drag coefficient basically only determined by shape and surface finish. The inclusion of "speed" is kind of misleading here, particularly in the context of specifying "subsonic" in the question.

In general terms, the specific impulse of a rocket motor is:

- The total impulse divided by a unit weight of propellant.
Feedback: Specific impulse is a term used to define the efficiency of a rocket propellant and is the total impulse derived from a unit weight of propellant.

The specific impulse of a rocket motor is not the total impulse divided by a unit weight of propellant. It's the total impulse divided by the total weight of propellant in the motor, and is mathematically equivalent to the total impulse the motor would give if it only contained a single unit weight of propellant.

Overall though, it looks good, and it at least seems better than I remember the test being when I took it, though that was long enough ago that I'm not sure I remember the questions all that well to be honest.

Current host of Thumper
TRF Supporter
The safety quiz gets the maximum model rocket launch angle wrong. The correct answer is 30 degrees per the safety code (12-1) and study guide (question 39), but the quiz grades this answer as wrong and shows 20 degrees as correct.

The maximum high power launch angle question accepts the correct answer of 20 degrees.

hobie1dog

Subaholic
TRF Supporter
The safety quiz gets the maximum model rocket launch angle wrong. The correct answer is 30 degrees per the safety code (12-1) and study guide (question 39), but the quiz grades this answer as wrong and shows 20 degrees as correct.

The maximum high power launch angle question accepts the correct answer of 20 degrees.
oops. sorry

Last edited:

Worsaer

Amateur Propulsionist
TRF Supporter
this doesn't say a whole lot for the Tripoli organization. all this emphasis on passing this test and it has wrong answers in it..... kind of defeats the whole purpose of it.
so the answer sheet says that it's 30° and if we put down 30° it's going to come up as the wrong answer and we're going to miss that question? so then do we have to present a copy of the safety code for the person giving the test so that we can argue with them?... or then have to get on the phone with somebody at Tripoli or send emails and argue with them about the correct answer, if you are intent on scoring 100% on the test
Please don’t confuse Model Rockets with High Power.

Section 12 is for Model Rockets

12-2 A Model Rocket may not be launched more than 30° from vertical.

Section 13 is for High Power

13-6
A High Power Rocket must be launched no more than 20° from vertical. Deviations from vertical should only be made to limit wind drift and any deviation must be approved by the RSO.

Worsaer

Amateur Propulsionist
TRF Supporter
Update: My error - I erred when editing the quiz and clicked on the high power answer for the low power question. I apologize for the typo. It’s fixed now.

BBrown

Kloudbuster Prefect
So the online quiz is a brand new feature on our website. The volunteers who put this in place made a simple mistake. It’s being fixed. I find it amazing someone can condemn the entire organization for a feature that was implemented to help members advance their certification level.
Bob Brown
President, Tripoli Rocketry Association

Last edited:

cjl

Well-Known Member
Agreed, Bob, and my intention above was purely in the hope that the questions could be improved/corrected in the future, not to malign Tripoli in any way.

(I know I'm not the primary one you were responding to, but since I was the one to start the "corrections" topic, I just wanted to make sure that was clear)

Replies
12
Views
696
Replies
80
Views
2K
Replies
6
Views
427
Replies
73
Views
4K