Drogueless descent rate from high altitude?

[Adrian A]

I'm setting up my sustainer deployment settings for my 38mm 3-stage flight that's expected to go to around 80kft-90kft. The sustainer has a drogueless deployment at apogee, and I expect it to get going pretty fast on the way down before the air density increases again. The Blue Raven prototype altimeter I'm using has a backup setting for main chute deployment that will fire if the descent rate after apogee is excessive. Normally I have a threshold of 200 feet/second for this, but I'm guess I'll exceed that on this flight. Anyone have a drogueless descent rate from >80k that they could share for comparison?

 

[Adrian A]

The closest flight of my own that had a successful apogee deployment was my L record flight to 32k. It had a drogueless descent rate of 129 feet/second at 30,000 feet. At 90,000 feet the terminal velocity would be 4.4x higher, or 560 feet/second. I think I'll set my main backup threshold at maybe 800 feet/second.

 

[mikec]

This velocity is barometrically derived? It would have to be, I think.

 

[Adrian A]

This velocity is barometrically derived? It would have to be, I think.

Yes. It's from differentiating over 0.5 seconds and then it's heavily filtered, but for safety it should be ignored for transonic and above. I think my 90k flight plan will cause nominal descent rates that are high enough to be out of reach for this feature, so I decided to disable it in my flight setup for this 3-stage flight.

 

[mikec]

That seems wise, transonic at low pressure and possibly tumbling seems like a challenging edge case.

 

[TonyL]

Hi Adrian,

FYI, my rocket exceeded the COCOM limits coming in ballistic from 112kft, I would expect up to mach 1 from greater than 80kft.

br/

Tony

 

[GrouchoDuke]

The closest flight of my own that had a successful apogee deployment was my L record flight to 32k. It had a drogueless descent rate of 129 feet/second at 30,000 feet. At 90,000 feet the terminal velocity would be 4.4x higher, or 560 feet/second. I think I'll set my main backup threshold at maybe 800 feet/second.

800 seems pretty solid. I'd expect it to stay subsonic once it's separated, but it could still get pretty fast. Supersonic (or very close to M1.0) feels likely if it's ballistic at 80k+. 800ft/s is about M0.8 at 100k.

Without doing any ballistic sims, my gut feel is you'd easily get up to M0.92-0.95 if it's ballistic up there. M0.8 is likely plenty, but you could perhaps push that higher (0.85-0.9?) if you want the extra comfort.

You could always program an altitude-tiered max vert speed.

Ooooorrrrr, you could set that at 9999ft/s and be done with it. That's not very testy though.

 

[GrouchoDuke]

This is a completely different dataset, but perhaps worth a few hmms. Felix got to 843mph in his jump from ~128k. https://en.wikipedia.org/wiki/Red_Bull_Stratos#:~:text=The jump records Baumgartner attained,of 36,402.6 metres (119,431 ft)

 

[Adrian A]

This is a completely different dataset, but perhaps worth a few hmms. Felix got to 843mph in his jump from ~128k. https://en.wikipedia.org/wiki/Red_Bull_Stratos#:~:text=The jump records Baumgartner attained,of 36,402.6 metres (119,431 ft)

1236 feet/second. Yeah, I think the excessive descent rate/backup main feature should be disabled for flights planning to go above ~50k

 

[Handeman]

You can always use the NASA terminal velocity calculator to get estimates at various altitudes on Earth or Mars
If I plugged the right numbers in, a 4" rocket coming in ballistic at 80K ft. would have a terminal velocity about Mach 2. At sea level, it would be about 310 mph.

 

[GrouchoDuke]

You can always use the NASA terminal velocity calculator to get estimates at various altitudes on Earth or Mars
If I plugged the right numbers in, a 4" rocket coming in ballistic at 80K ft. would have a terminal velocity about Mach 2. At sea level, it would be about 310 mph.

Minor typo over there though if you run the numbers yourself. Their terminal velocity equation part way down the page has "r" in the denominator, but it should be rho.