300fps Apogee Velocity

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s3th

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Hello,
I am currently building a hybrid HPR and I am a little bit worried about the apogee velocity. Due to some safety issues there could be a small possibility of launching at 75° above horizon. That is pretty flat and I calculated a "worst case" scenario depending on angle and wind ... 273fps in apogee. I am a rookie if it goes to such high apogee speeds and I would like to ask you guys if you have any experience with such challenges. (its not a problem - for engineers there are no problems, just challenges!)

Short info:
OD
6 inches
Length
12ft
Thrust
2000lb for 10s (N2O + HDPE)
Apogee
16500ft
Weight
40lb (during descent)

I considered the usage of a two stage-system with a smaller drogue-chute (3-4ft) and a larger main paracute (deployment at 1500ft - no problem with that one). For the given "worst-case" scenario the apogee shock-load based on "Parachute Recovery Systems" (T.W. Knacke) will be around 1000lb during deployment. So I was thinking of a Mach-II chute (3ft) from rocketman as a drogue, but it is pretty heavy (approx. 2lb.) and I need so save some more weight (maybe its also overrated ... I am not supersonic during deployment). So how about an ProXP from rocketman or something else? Fruitychutes? Angle? Nobody provides max. deployment velocities.

Thanks for you help!!

s3th
 

Steve Shannon

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I have a couple of the Rocketman Mach parachutes. They are stout and they are heavy. I believe they would survive the deployment speed you're asking about, but will your rocket?

Launching at 15 degrees away from vertical seems like the real problem. Is that because of the shape of the launch site? To be honest, I've never seen a launch that far off vertical end well. Every year we get someone who insists on doing so because they're afraid that their rocket will drift too far once it's under chute. So, they tilt the rail into the wind and launch, thinking they're compensating for the drift by launching upwind. Those that we haven't caught and corrected have ended up with cruise missiles when their rockets have weathercocking.


Steve Shannon
 

mikec

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Hello,
Due to some safety issues there could be a small possibility of launching at 75° above horizon.
Given the typical thrust curve of a hybrid this is a recipe for disaster, just don't do it.

This is a huge hybrid (2000 lbs for 10 seconds is 89000 N-s or a Q motor) for a self-described "rookie".
 

StanO

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yea, that's just a bad idea all over.
Ditto!

Not a lot of experience, but if rocket is designed with proper static margin (1+ for RockSim and 2+ for RASAero) rocket should fly straight. Then you can use a smaller drogue and you will get a smaller dispersion area. I haven't used it yet but here's a link to alternative software called RocketShell, https://www4.vmi.edu/faculty/squirejc/Rockshell/.
 

s3th

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Apologize gentleman, bad typo - 200lb thrust -> approx. 10000Ns total impuls (puhhh 2000lb of thrust would be ways over top)

Thank you for the first comments. The structure should handle the load, everthing calculated by hand (CLT) and double checked by ANSYS (models verified by coupon-tests with flight material). Shock-loads up to 1500lb with a safety-factor of 2 (so around 3000lb).

So the challenge is just the drogue-chute.

s3th
 

Steve Shannon

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Apologize gentleman, bad typo - 200lb thrust -> approx. 10000Ns total impuls (puhhh 2000lb of thrust would be ways over top)

Thank you for the first comments. The structure should handle the load, everthing calculated by hand (CLT) and double checked by ANSYS (models verified by coupon-tests with flight material). Shock-loads up to 1500lb with a safety-factor of 2 (so around 3000lb).

So the challenge is just the drogue-chute.

s3th
So your shear pins for your main will withstand having your drogue come out at velocity?
Your body tube won't zipper?
I understand you think the hardware will hold together.

You can always fly drogueless.
 

Steve Shannon

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So your shear pins for your main will withstand having your drogue come out at velocity?
Your body tube won't zipper?
I understand you think the hardware will hold together.

You can always fly drogueless.
P.S. 1500 lbs for the hardware is only about a 40 gee loading. Chutes coming out at high velocity frequently cause much higher than that.
 

s3th

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thx Steve for your input!
Well I will not use share-pins for the second stage. I plan to use two 32g CO2 cartridges to pop-off the nose-cone to deploy the drogue-chute and a Tender-Descender to pull out the main chute from the same compartment. The load bearing will be heavy bulkhead made of CFRP in sandwich structure design.

P.S. 1500 lbs for the hardware is only about a 40 gee loading. Chutes coming out at high velocity frequently cause much higher than that.
I don´t get this one. What do you mean with only about a 40 gee loading?!

thx

s3th
 

mikec

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You can always fly drogueless.
You can always fly with a drogue, and if it shreds, then you'll revert to flying drogueless. :)

If I were RSOing your rocket, I'm not sure I would allow you to launch 15 degrees off-vertical. Having a high horizontal velocity at apogee by accident is one thing, doing it intentionally is something else.
 

DavidMcCann

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16,000' apogee, at 15 degrees off vert? You're running a serious risk of leaving the waiver, and having one hell of long recovery
 

Steve Shannon

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thx Steve for your input!
Well I will not use share-pins for the second stage. I plan to use two 32g CO2 cartridges to pop-off the nose-cone to deploy the drogue-chute and a Tender-Descender to pull out the main chute from the same compartment. The load bearing will be heavy bulkhead made of CFRP in sandwich structure design.



I don´t get this one. What do you mean with only about a 40 gee loading?!

thx

s3th
40 times the acceleration of gravity.

Pardon me for asking but what is your experience level? Are you certified for HPR? Where do you fly?
 

s3th

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If I were RSOing your rocket, I'm not sure I would allow you to launch 15 degrees off-vertical. Having a high horizontal velocity at apogee by accident is one thing, doing it intentionally is something else.
To get this straight: I don´t want to launch 15° off-vertical, it would be stupid! But this is a given requirement by the launchsite management and I need to prepare the rocket and the paperwork to show this to a reviewboard: the rocket is capable to handle this requirement. I will do anything to launch around 5° off vertical. But this is not in my responsiblity. I think they are worried about their nice GSE all around. Additionally I started the evaluation of other launchsites, ... and I am not talking about a "not used corn field" somewhere in the backcountry. I am looking for a real launch site for high-altitude ballistic flights.


40 times the acceleration of gravity.
Pardon me for asking but what is your experience level? Are you certified for HPR? Where do you fly?
Got it, I was just confused about the "gee" - 40g -
Thank you for your concerns, I totally agree with you: safety first! Experience in deploying a drogue-chute with 300fps? Zero - that is the reason why I am here to learn from you guys. Experience in building rockets of all sizes: approx. 20 years - I am not a member of Tripolis or NAR. The launchsite will be a managed facility with designated areas for high-altitude ballistic flights (restricted airspace, restricted area)


s3th
 

Steve Shannon

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Correct. I use "gee" or "gees" in order to differentiate the acceleration of earth's gravity from the other use of "g" which is the S.I. abbreviation for gram.
NFPA 1127 is the basis of both NAR's and Tripoli's safety codes. It limits launches to within 20 degrees of vertical, so you're within that.
Launch site owners' requests do tend to trump other considerations, except for the conditions of FAA Certificates of Authorization (waivers). When those two cannot be simultaneously satisfied you just don't launch. Our FAA waiver requires that all launches intended to surpass 25k AGL be vertical.
The best way for you to accomplish what you need would be launch as close to vertical as possible, even if that means 15 degrees, but I would suggest either drogueless or use a slider and taped shock cord on a small drogue to delay the shock of opening. I've done a rocket of that same mass drogueless. You want to make sure that your various rocket parts don't slam into each other when your main opens. On your main use a slider and tape loops in the shock cord also to absorb more energy. It can be done, but you may need to consider your drogue chute as expendable.
You seem to show some disdain for "not-used corn fields", somehow differentiating them from "real launch sites" yet you request advice from those of us who have done the work to legally develop "not-used corn fields" and similar sites into waivered, insured, launch sites that allow flights to 50,000 feet.
Good luck.


Steve Shannon
 

s3th

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Thank you Steve for the advise. The usage of slider/tape to delay paracute opening for shock-load reduction is currently under testing.

I think you misunderstood me a little bit. It was not my intention to be disdain in any way! I´ve checked your Big Sky launch site - that is awesome, I would really appreciate something similar in a range of 1000miles .... unfortunatly this is not possible (lots of law regulations - I am working on that issue for years and I could tell you stories you will not belive, really, you will not belive). And as you guys always make clear that safety should be the #1 priority (I totally agree) it was my intention to make clear that even the launch site is special selected for safety reasons. I hope you are not upset - I just wanted to show you, that I am taking that very serious - like you all do - because I had the feeling you think I don´t. (I have sent you a p.m.)

Anyway I appreciate any recommondation: sommercial-off-the-shelf drogue-chutes, lightweight, and strong ...

s3th
 

mccordmw

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Checking the math here...

200 lb avg. thrust (890 N) and 10,000 Ns total thrust = burn time of ~11.23s.
A CTI M795 is in that general ballpark with 10,133 Ns total thrust and a burn time of 12.8s.
So propellant weight will be about 4.9 kg (10.8 lb)
If your descent mass is 40 lb, your pad weight will be about 51 lb?

That's a thrust-to-weight of ~3.5:1. That's really really pushing it! Especially if you're 15 degrees off vertical. Any slight winds will weathercock you into a cruise missile for sure. How many stability calibers do you have?

If there are any winds at all, I'd scrub that launch. I'm usually very cautious, so I'd never launch something like that. I prefer 6:1 TTW ratios minimum.
 

mccordmw

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More rough numbers here:

Velocity off the rail by ThrustCurve will be about 58 fps (39 mph). The usual safe margin is 4:1 wind speed, so you shouldn't exceed winds of 9.8 mph, but that's for a typical launch where the rail is 5 degrees angled away from the crowd. I've heard that assumptions are to cut that wind speed in half for every 5 degrees, so 10 degrees = 5 mph (rounded), and 15 degrees = 2.5 mph max wind speed.

Better launch that in dead calm skies.

Assuming a ~6" airframe to hold a 98 mm motor mount
Max velocity will be 720 fps (490 mph or mach 0.64)
Apogee around 10,300 ft (but likely lower due to the launch angle)
 

mccordmw

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Can anyone recall the angle vs wind speed rule of thumb? I don't have a source for it, but I've written down:

4:1 launch speed vs wind with rail at 5 deg.
Divide max wind by 2 for each additional 5 deg.

So much tribal knowledge in this hobby.
 

s3th

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Again ... the 15° off-vertical is a "worst-case" I need to deal with under certain cirumstances ... I don´t want to, I don´t need to ... if I find a workaround!

Back to the math...
ok average thrust is approx. 1000N with 10s burn time - flight engine is not on the testbed - engineering engine showed good performance in that way.
Propallent weight is 5kg N2O + 1,5kg HDPE + 300g N2 feed gas ...
Pad weight is around 400lbs. and fixed to the ground.
Well that thrust to weight ratio is still a pain-in-the-*** for me ... as I only reach Ma=1.1 (theoretically) during flight so there is not much space for miscalcuation.
Current stability is 3 caliber (I know a little bit high) as the acceleration is not so good during the first 0,5s.
You think that is too much thrust for that light rocket? Well I have seen even lower TTW ... and they all performed quite good. What are your concerns?

... launch termination will be at 5m/s based on current calculations.

s3th
 

FredA

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Pad weight is around 400lbs.

You're never going to lift 400 pounds with 1kN thrust.

I know you meant the pad itself weighs 400 pounds -- who cares?
What is the weight of the ROCKET, wet?
Is it over 40 pounds? If so, you are in trouble.....
 

s3th

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Hello mccordmw,

thank you for that little hint ... I see we need to go a litte bit further into detail. ... sorry I haven´t seen your other posts befor posting my previous one!

It was not my intention to go that much in detail... as I only wanted some experience in deployment velocities for COTS drogue chutes ... but if you guys express your concerns, and I like to be criticized - you can only get better if someone tells you the truth about your work - I will support you with more information. I just don´t dig it out, if I want to know something about a chute .... :eek:

Maybe someone should move this ... its not longer about recovery, isn´t it?

The guidance rail has a lenght of 30ft. (10m) - 6ft for the launch-lug distance - we got a guidance length of 24ft. (8m)
If I don´t care about the mass-loss within the first 0,2s we can read this:

s in (s)F in (N)m in (kg)
0,00524923
0,0256723
0,0478323
0,06120723
0,08149323
0,1162123
0,12163723
0,14161023

Based on some over the thumb calculations and some xls interation calculations I think the TTW and the rail-exit velocity should be ok leaving the rail (in vertical 90°) to this point. For detailed information I need to look for the files ... what do you think?

Anyways ... how about the drogue-chute issue?:rolleyes:


s3th
 

AlphaHybrids

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Your ISP for that hybrid seems very low - 156s from the numbers you provided. That is less than nitrous oxide as a monopropellant. The university that I work with who enters the IREC/ESRA competition had hybrid last year that was getting ~215s for ISP from a hybrid.

I don't know what pressure you are operating at, but I'm guessing if you are using a feed gas you are trying to boost the combustion chamber pressure. Nitrous is self pressurizing, so most applications you don't need to add a pressurant to it.

Edward
 

s3th

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ahh, here we go - couldn´t upload thrust curve in the last post .... lift-off weight is 50lbs. (23kg) thrust.png
 

s3th

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Your ISP for that hybrid seems very low - 156s from the numbers you provided. That is less than nitrous oxide as a monopropellant. The university that I work with who enters the IREC/ESRA competition had hybrid last year that was getting ~215s for ISP from a hybrid.
Hi Edward, how did you calculate the I_sp?
 

mikec

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To get this straight: I don´t want to launch 15° off-vertical, it would be stupid! But this is a given requirement by the launchsite management... I am looking for a real launch site for high-altitude ballistic flights.
So you're not a TRA or NAR member and you're looking to launch a big high-power hybrid. Exactly which launch sites are you considering? Is this in the US?

I only know of a couple of US launch sites that would touch this. Your altitude is too high for FAR and Black Rock is a no-go without TRA.
 

s3th

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Hi mikec,
no it is not in the US, don't worry there is no chance that this one will drop on your head. :)
 

s3th

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I'm gonna bail out of the actual launch discussion, but heres some options-
Thx David!
Do you have any experience with the ProXP? I am asking because I think the Mach-II is quite heavy. I wrote several emails to Ky ... but never got any responds.


s3th
 

DavidMcCann

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Thx David!
Do you have any experience with the ProXP? I am asking because I think the Mach-II is quite heavy. I wrote several emails to Ky ... but never got any responds.
I've got several of their standard chutes, no flights with data recorded hit at high speed, but I've had delays go long on smaller rockets, in the 3 pound range with a 3 foot rocket man chute. The design in general lends itself to a gradual opening.

This one opened shortly after burnout, and the rocket was still going fairly quick-



another-






In my experience, they tend to open "softly" not just a big slam. Of course, the rest of the recovery system needs to be up to the task, but I'd think these chutes can absolutely handle the speeds you're looking at, if sized and mounted properly.
 
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AlphaHybrids

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Hi Edward, how did you calculate the I_sp?
One way to calculate Isp is to take the total impulse divided by unit weight of propellant

I converted everything to imperial units.

10,000 Ns = 2248 lbf
5.0kg oxidizer + 1.5kg HDPE fuel = 6.5kg propellant = 14.33 lb

2248/14.33 = 156s

Edward
 
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