Can someone explain what IS happening to this first rocket?

[rharshberger]

Thats assuming the NC was like that at the RSO table. So the NC may have been seated correctly when it was inspected but you can't hold the RSO responsible for what happens to the rocket between inspection and the pad. Also, the RSO may or may not have been "asleep at the wheel." The RSO inspection is sort of a cursory inspection to make sure nothing obvious is wrong. Also, the RSO relies heavily on the information that the flier gives them such as the location of the CP. Can't say I've ever witnessed an RSO work through a Barrowman Equation to verify CP location much less bust out a laptop to run a few sims. I think pinning this on the RSO is unfair. IMHO it's ultimately up to the flier to make sure the relative CP/CG locations are correct.

Looking at the stills at the end of the Mongoose portion of the video it does not appear that the NC was mis-aligned during flight. But the key word there being "appear." The rather sudden instability right off the rail sort of looks to me like the CG was in the wrong spot right off the get-go.

Your right, the RSO inspection is a CURSORY inspection, we are looking for the secure railbuttons, solidly mounted fins, positive motor retention, airframe failure points, and friction fitting of break points (drogue and nosecone). We can find the CG easily enough by simply balancing on our hand (depending on rocket size), recognizing general CP comes with experience and even then its not absolute which is why the CP needs to be marked on the rocket. Everything else on the rocket is internal and therefore invisible to the RSO, the questions we ask are more to make the flier question whether they did the proper steps than anything else when it comes to the recovery, ejection charges etc. If the exterior looks and feels good and the questions are properly answered the rocket will probably get a pad assignment. Some rockets are know to have special requirements....like noseweight (Patriot, V2, etc), with experience the RSO or pre-flight safety inspector can quickly by feel spot any that "dont seem quite right". The Mongoose has a small fin span relative to the airframe length, pushing the CP forward, and depending on what the expected velocities for that motor/rocket combo were the CP could easily have moved even further forward or in front of the CG.

 

[StreuB1]

The rocket would have been subsonic still when the problem occurred. I wouldn't think airflow disturbance from tha far away would cause that problem from the get go.

Aerodynamic blinding begins before things are supersonic.

Stick your hand out the window of your car in the air stream, then drift it behind the side view mirror, then bounce it above and below the transition point in the airstream. The area that is blind to the oncoming airstream does not occur at the intersection point of the airstream and the outer most point of the mirror. It is some distance away from the intersection point due to the compression front that occurs due to the sideview mirror (or any abrupt protrusion into the airstream).

It is also common to have supersonic flows generated on abrupt geometries at subsonic velocities. You will get these supersonic flows being generated over the crown of a camera shroud at subsonic velocities. This would set up a normal shock at subsonic velocities which could blind the fin. The abrupt sharp ending to the shroud will then set up a trailing edge shock or an expansion wave which is what could also blind the fin.

Its best practice to place the shroud close to the CP and place it between the fins, not in-line with them. The closer to the CP, the less of an affect its geometry will be on the CP. This will not affect shocks [flow instabilities in general], which is why you want it between the fins.

 

[OverTheTop]

Thanks for the explanation. I am still of the opinion the rocket was sufficiently slow and the camera shroud was sufficiently far forward (and has a tapered aft end which will help) for this to not be the explanation in this instance. It really wasn't going that fast from what I see.

 

[StreuB1]

No worries! Its just dialog, no need to be right or wrong. No one but the flyer with the datalogs (if there were any) would ever know so at this point, we're all just speculating and having a conversation.

 

[mtnmanak]

Brian's hypothesis about fin blinding is absolutely sound and may very well have been a significant contributing factor to this unfortunate flight.

However, the flight made with the rocket a year earlier on the M2100G gives us some clues as well.

That flight was still fairly "wiggly" for the first second or so, and that green motor burns a lot faster than the Dark Matter motors. The M1075DM has less than half the initial thrust of the M2100G. If it was slightly unstable with the bigger punch of the M2100G, the M1075DM was always going to have trouble getting this rocket to a safe speed very quickly.

The rails in both flights look incredibly short (6' rails maybe?). The Mongoose 98 is only an 8' tall bird (unless he modified it). Assuming the top rail button was somewhere around the middle of the rocket, that means he only had positive control of the rocket on the rail for about 2'-3'. That feels crazy short.

Finally, the nosecone displacement on the M1075DM flight seems to be significant. It is clear in the M2100G flight video that the nosecone was firmly seated in the airframe, so it is difficult to believe it was supposed to fly with the nosecone sticking out that much. I can't imagine why that wouldn't be noticeable on the pad, but it feels like the nosecone issue is another contributor to this flight's outcome.

Combine a motor with a relatively weak initial thrust, a very short rail, possible fin blinding from the camera shroud, a rocket that is already marginally stable by design and a displaced nosecone, it is not surprising it went sideways. The more important discussion is that all of those factors are extremely avoidable. This was not an "experimental" flight (and, to be clear, I am highly supportive of experimental flights conducted under safe conditions - pushing boundaries furthers the hobby/science) - this was a "normal" flight that, seemingly, did not take into account a number of obvious issues.

 

[smstachwick]

I was a bit puzzled that the reaction was one of amusement (laughter) instead of immediate concern. Safety in rocketry takes diligence at every step, and even then things can go wrong or become hazardous because of the limits of our knowledge and experience.

I do not know the fliers at issue here so I will not judge their practices, nor the RSOs or anybody else at those ranges. It’s possible they knew something I don’t.

But you can bet that if I were at any range and I identified a potential safety risk, I’d at least say “that doesn’t look right. Can we get somebody to look at that?” and make sure that whoever put the questionable thing in place got quizzed on it sufficiently and appropriate fixes prescribed if needed.

A safety-focused club will allow for that kind of scrutiny regardless of certification and experience level of flier, range officers, and whistleblower. Nobody should be immune.

If I got blown off solely for being uncertified or inexperienced and my concerns were later proven correct by a hazardous flight, you can bet that I’d follow up on that with the involved parties. I would be less diplomatic and more blunt that time.

If that’s not resolved to my satisfaction, I would not fly with that club while those people were in charge and I may even report their unsafe practices to whoever granted their land use permit (with the recommendation to demand safer practices instead of withdrawing the permit, I don’t want people to lose their launch site if they don’t have to).

Getting along with people on the range is important, but sometimes saying “Hey turd-for-brains, you could have gotten somebody killed!” is what is needed for the all-important maintenance of range safety.

Of course, this all takes good range discipline to discourage deliberate false alarms. It also requires downplaying of hierarchal relations when safety calls for it and restoration of the hierarchy when the crisis is dealt with, and those matters are worthy of threads of their own.

(Oops! It appears I was a few pages behind. Point still stands, you just may have to scroll back to Page 1 to understand what context I had.)

 

[Ez2cDave]

Just saying, until the flier himself posts here we don't know the whole story just yet.

Why don't we contact him ( Steve Jurvetson ) and ask him to post here ?

https://future.ventures/bio

https://www.bizjournals.com/sanjose/news/2017/11/13/steve-jurvetson-steps-away-tesla-spacex-boards.html

Dave F.

 

[DAllen]

Why don't we contact him ( Steve Jurvetson ) and ask him to post here ?

https://future.ventures/bio

https://www.bizjournals.com/sanjose/news/2017/11/13/steve-jurvetson-steps-away-tesla-spacex-boards.html

Dave F.

What is this "we" business? Got a mouse in your pocket?

 

[Ez2cDave]

What is this "we" business? Got a mouse in your pocket?

Sorry,

I wasn't trying to "steal your thunder" . . . You contact him.

Dave F.

 

[DAllen]

Sorry,

I wasn't trying to "steal your thunder" . . . You contact him.

Dave F.

Its pretty silly when someone makes suggestions like that and say "we" when they really mean YOU. I see that in our rocket club all the time and its pretty stupid. Good times.

No, I will not be contacting him. I usually pass on opportunities to do pointless activities suggested to me by randos on the inter webs. But thanks anyways.

 

[Ez2cDave]

I usually pass on opportunities to do pointless activities suggested to me by randos on the inter webs. But thanks anyways.

Actually, it wasn't random . . . You generated the idea, yourself.

So, my suggestions were merely an effort to help you reach your desired goal of "knowing the whole story".

BTW - Didn't I see "we" used in your quote, also ?

Frankly, either way, it makes no difference in the "big picture".

Dave F.

 

[DAllen]

Cool story bro