Super Sonic Cp shift

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morros266

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Hello,

I'm designing a rocket that's a 98mm minimum diameter rocket. The rocket is for the Spaceport America Cup which has us get 8.8lbs to a specific altitude. Our altitude is 30,000 feet. Due to the size, we end up going supersonic for a portion of our flight and this causes the Cp to shift. I'm looking to see if this is going to be a problem for our design and if so how to go about accounting for this. We end up reaching a max speed of about Mach 1.5-1.9. We are also going supersonic for about 10-15 seconds. Our rocket is also only about 10 feet long and weight about 50lbs with the motor. Our fins are going to be made out of Aluminum from Max Q aerospace. We are also using a 6GXL motor that we are designing and building. I've attached our current rocksim design. If anyone can help me, I would appreciate it.View attachment SAC 2017 current design.rkt
 

Bat-mite

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Since you are using RockSim, run some simulations that show CP and CG. Do you ever get less than 1.0 caliber prior to apogee?

Also, keep in mind that the CG also moves forward as the propellant burns out.
 

mikec

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Never rely on Rocksim for supersonic analysis. OpenRocket is OK, as is RASAero; the latter is the best-validated.
 

Bat-mite

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To quote Emporer Franz Josef from Amadeus: "Well, there it is...." :)
 

FredA

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RASAeroII has a slider that shows Cp at whatever speed you want....VERY NICE for this.
 

morros266

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Thanks, I went over to RasAeroII and imported my file. Found that RasAeroII is using the Barrowman Equations while I was using the Rocksim Equations which changed my Cp location by like 10 inches. The shift due to supersonic was only an inch or two since we're only reaching about a max Mach 1.5.
 

FredA

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I'd trust RASAeroII over anything else.
Especially for anything over ~Mach 1.5
 

Nytrunner

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Hello,

I'm designing a rocket that's a 98mm minimum diameter rocket. The rocket is for the Spaceport America Cup which has us get 8.8lbs to a specific altitude. Our altitude is 30,000 feet. Due to the size, we end up going supersonic for a portion of our flight and this causes the Cp to shift. I'm looking to see if this is going to be a problem for our design and if so how to go about accounting for this. We end up reaching a max speed of about Mach 1.5-1.9. We are also going supersonic for about 10-15 seconds. Our rocket is also only about 10 feet long and weight about 50lbs with the motor. Our fins are going to be made out of Aluminum from Max Q aerospace. We are also using a 6GXL motor that we are designing and building. I've attached our current rocksim design. If anyone can help me, I would appreciate it.View attachment 311369
Sounds like another IREC rocket since they've moved to Spaceport America this year.

Last time I read the rules, I thought the payloads had to conform to the 1U/2U/3U etc... cubesat profile which I think is 100mm^3 for the smallest. That sounds like its a tad too large for an MD 98mm rocket.
 

FredA

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I thought the payloads had to conform to the 1U/2U/3U etc... cubesat profile which I think is 100mm^3 for the smallest. That sounds like its a tad too large for an MD 98mm rocket.

That was my first thought too....those CUBE-sats need a 5.7" ID AFAIK.

Hopefully the OP can elaborate.
 
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tfish

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From "Draft" IREC 2017 https://www.soundingrocket.org/uplo...ements_document_20160925__draft_baseline_.pdf

2.3.4 PAYLOAD GEOMETRY
The portion of payload associated components/systems submitted for weighing shall be integrated into one or more structures, whose stowed outer mold line (OML) are described by the CubeSat Standard (eg 1U, 2U, 3U, 3U+, etc). Alternatively, the PocketQube (aka PocketQub) standard may be implemented to facilitate smaller airframe diameters. This standard uses units of "P", which are equivalent to 0.5U (ie 5 cm). The CubeSat-style or PocketQube-style payload(s) may connect to other payload associated components (eg leads to sensors located variously throughout the airframe, deployment mechanisms, etc...) when integrated with the launch vehicle; however, the CubeSat-style assemblies alone will be weighed by competition officials.

I would contact ESRA-IREC to confirm.

Tony
 
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morros266

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From "Draft" IREC 2017 https://www.soundingrocket.org/uplo...ements_document_20160925__draft_baseline_.pdf

2.3.4 PAYLOAD GEOMETRY
The portion of payload associated components/systems submitted for weighing shall be integrated into one or more structures, whose stowed outer mold line (OML) are described by the CubeSat Standard (eg 1U, 2U, 3U, 3U+, etc). Alternatively, the PocketQube (aka PocketQub) standard may be implemented to facilitate smaller airframe diameters. This standard uses units of "P", which are equivalent to 0.5U (ie 5 cm). The CubeSat-style or PocketQube-style payload(s) may connect to other payload associated components (eg leads to sensors located variously throughout the airframe, deployment mechanisms, etc...) when integrated with the launch vehicle; however, the CubeSat-style assemblies alone will be weighed by competition officials.

I would contact ESRA-IREC to confirm.

Tony
It says we can use pocket cubes which are 5x5x5cm
 

Nytrunner

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I stand corrected. I read the draft a couple months ago.
Will you use more than one to meet the 8.8 lb weight?

Good luck in NM!
 

G_T

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Supersonic CP shift is initially rearwards in the transonic range, so stability improves. As speed increases past the transonic range, the CP will progressively shift forward. If you're under around M3 for conventional designs, the CP won't be farther forward than where it starts.

Keep in mind you'll also usually get a CG shift as the motor burns through its propellant.

The biggest issues though IMHO is to keep it sufficiently stable to have it stable off the launch rail with the max expected crosswind for launching. CP shifts forward rather badly as crosswind angle increases. The shorter the launch rail, or the slower the acceleration up the rail, the greater the crosswind angle is for a fixed windspeed.

Example rocket is Moonchild, a sketched out design of mine based on a dual thrust moonburner (more or less; it is a complex core geometry). Details are OT for this thread.

Gerald

Moonchild.png


2-10-2017 8-43-07 PM.png
 
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Nytrunner

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Supersonic CP shift is initially rearwards in the transonic range, so stability improves. As speed increases past the transonic range, the CP will progressively shift forward. If you're under around M3 for conventional designs, the CP won't be farther forward than where it starts.

Keep in mind you'll also usually get a CG shift as the motor burns through its propellant.

The biggest issues though IMHO is to keep it sufficiently stable to have it stable off the launch rail with the max expected crosswind for launching. CP shifts forward rather badly as crosswind angle increases. The shorter the launch rail, or the slower the acceleration up the rail, the greater the crosswind angle is for a fixed windspeed.

Example rocket is Moonchild, a sketched out design of mine based on a dual thrust moonburner (more or less; it is a complex core geometry). Details are OT for this thread.

Gerald
Do they have a rail exit velocity requirement this year?
 

G_T

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I don't know. I'm not involved.

Moonchild is a sketched up Black Rock design I've been considering for a while; one of many. I've looked at lots of single stage designs on paper with high mass fraction high volume loading high L/D moderately high pressure case bonded motors using dense propellants, up to baby P. They go too fast for aluminum construction to have a prayer of surviving. Some were predicted to hit right around mach 5 and not all that high up. The ensuing destruction should be quite spectacular...

Dual thrust moonburner (it's a wee bit more complex than that makes it sound) was a motor geometry I came up with in an attempt to postpone the high velocity until enough altitude was achieved to reduce the aero forces and the heat loading, while still having plenty of speed off the rail and at low altitude to have some assurance it goes where it is pointed. Burnout is predicted (RasAero) for this one at about 44K AGL and it is really moving at that point, or it has already shreaded. It could coast a long way from there if it survived that far. RasAero says it could loft a 20# payload to perhaps 100Kft (airframe would be lengthened to accomodate the payload), and that slows it down enough I think it might survive. All numbers tentative. This is just modeled to the ROM stage. The design has a number of unusual features and I'm tempted, but, work has left me little time for large projects. Heck, I'm just getting ready for the first burn of my EX M hybrid. I started machining that one a year ago or more.

Gerald
 

Buckeye

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The biggest issues though IMHO is to keep it sufficiently stable to have it stable off the launch rail with the max expected crosswind for launching. CP shifts forward rather badly as crosswind angle increases. The shorter the launch rail, or the slower the acceleration up the rail, the greater the crosswind angle is for a fixed windspeed.
This ^

Supersonic shift is sexy and all, but the mundane liftoff with wind is really the most important stability issue for most flights.

Plot CP and CG with some wind in RAII or OR, and you will see marginal to negative stability coming off the rail (albeit briefly).
 
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