building rockets (+aerospike discussion)

[seti_sam]

Does any one know any good websites, books or advise on how to build a rocket from scratch.

Thanks Sam.
www.geocities.com/seti_sam

[payloadcontroller]

My Socratic reply would be: How much experience in model rocketry do you have currently?

[Retrospector]

It's been years since I've flown a model rocket. I don't have any good fields around here to fly anything such that I could be sure of recovering it. But I'm curious as to what government regulations are in place now regarding launch weight and propellant amount. I remember there was some kind of flap recently about certain powerful model rocket engines that could conceivably be used by terrorists.

Edit: I found a link about propellant limitations. Apparently the new number on propellent mass is going to be 62.5 grams.

http://www.space.com/missionlaunches/rocketry_security_030306.html

[Skywatcher]

does anyone know the highest you can go with a rocket engine? (i.e. the A in A-16, the D in D-4, etc.) the highest i have seen is a F, but it was specially ordered.

[Skyjim]

No  clue - I used to build zinc-sulphur rockets in high school, under a lot of supervision from experts who often worked in the industry.  Seamless chrome-moly tubes, aluminum press-fit, bolted, and sealed bulkheads, and nozzles we machined out of steel bar stock on a rickety old lathe.  The existence of this rocketry club was a major reason I wanted to attend this small private school.

We just packed them with propellant in a carefully monitored fueling pit.  We routinely got 20,000 feet from our two stage rockets.   Our launch area was (and still is, though I've not been out there in years) located north of Edwards AFB, and the airspace had to be closed when we were firing, for obvious reasons.  That wasn't too hard to get done given all the adjacent restricted airspace.

And, yes, sometimes we had spectacular failures, but we also had a couple of reinforced blockhouses near the launch racks and a good-sized observation bunker about 100 yards away.  I recall a friend of mine insisted on using a strap attachment method on his fins instead of welding.  The fins didn't survive the trip out of the launch rack and the thing looked like a pinwheel.  COOL! (Of course, you didn't tell him that you thought it was cool - you made sympathetic noises as a rule...

 Nobody ever got hurt to my knowledge.

[Skyjim]

This aerospike test flight by a local university group was conducted at the same site (the Mojave Test Area, or  MTA) that my school used for our tests...

http://www.csulb.edu/colleges/coe/ae/rockets/

Amazingly, this is the first-ever flight test of a liquid-fueled aerospike engine!  

Nice to see the old blockhouses are intact...

Jim

[greenleaf1]

Heh, we built one in my HS chemistry (or maybe it was physics - don't remember). It wasn't very powerful thank goodness because on its maiden "flight" it tipped over and slammed into the front door of our high school.  Left a nice dent.  Thank goodness it didn't go through a window or something.  Needless to say, we didn't build anymore rockets  

[remcook]

It's very easy to make a water-rocket  

[Mentor]

http://user.aol.com/sspacepyro/instruct/smallEngs.html
http://members.rogers.com/rnakka3/pvcmot10.html
http://www.mbrocketry.com/exp/experimental.htm
http://www.aeroconsystems.com/motors/lr101.htm
http://www.ukrocketman.com/rocketry/hybridscience.shtml
http://www.apogeerockets.com/links/amateurrocketry.html
http://www.sciencegems.com/HSG/RefCalculators4.html#AERO-SPACEC

some links, (aren`t I kind??)

Mentor.

[Skywatcher]

*bows to mentor*

[Retrospector]

Amazingly, this is the first-ever flight test of a liquid-fueled aerospike engine!  

Jim

Great stuff, I love the video clip! Cooling the aerospike engine combustion chamber seems to be a major issue. Were the materials of the engine selected with ablative cooling in mind? Given that it's a liquid-fueled engine, can regenerative cooling be used? Has it been used?

-Tak

[Skyjim]

Yeah, looks like they went for ablative cooling - this small, regenerative cooling is a real fabrication challenge, and materials are better now.  Not quite good enough, though, it appears. All you need is a slight difference in ablation rate across the nozzle section and the shape is going to go asymmetrical on you...

The gas flow in the calculations is always idealized, as are the material characteristics of the ablator.  Throw in some tiny manufacturing inconsistencies and an injector which might not mix with perfect uniformity, though, and you can quickly have an unintentional thrust vector that throws you out of control.

They might be considering regenerative systems now, although they might also just refine their design.  Ablative can work fine once you have an accurate model of the gas flow.  They got through their static testing OK...  Maybe they picked up some particulates in the injector.  Maybe the flow varied enough to change the injector spray pattern as they accelerated out of the rack.  No idea why they had the problem, but this is why you flight test.   It reveals things.  

[remcook]

ok...in short, what is the principle behind the aerospike engines, and what makes it so good?

I remember the killed 'venture star' thing was going to have these aerospikes, but at that time I didn't have internet, nor was I that interestede in space stuff.

[Skyjim]

An aerospike is sort of a rocket nozzle turned inside-out.  By using the surrounding medium as, in effect, a nozzle boundary, it is able to optimized the expansion rate of the hot gasses for maximum thrust at varying altitude.  An aerospike has what amounts to a variable expansion ratio - the relationship between the area at the chamber throat and the nozzle exit.  Conventional nozzles are fixed expansion ratio, so you wind up having to pick where they are going to work their best.  Booster nozzles tend to be shorter, low expansion ratio bells; high altitude or upper-stage nozzles tend to be long with large ratios.  Example:  the Delta II uses a kerosene core engine which was originally the sole power for the first stage of the old Thor missile.  It needed to heave the thing off the pad all by itself, so the expansion ratio was a fairly small 8:1.  When the Delta II derivatives  eventually had a bunch of solid strapons doing most of the lifting off the pad, the expansion ratio was increased to 12:1 to improve efficiency at altitude.  The SSME, which is effectively in vacuum for 75% of its burn time on a shuttle launch, has an expansion ratio of 77:1.

You can make an aerospike linear - sometimes referred to as 2 dimensional - like the X-33 engines, or toroidal/circular - 3D - which is what they flew on this vehicle.  Either one gives the advantage of automatic altitude optimization - which increases propulsion efficiency over the entire flight profile.

Jim  

[Retrospector]

I hadn't thought of the scale problem before. I would guess that regenerative cooling of model rocket motors does involve intricate small scale plumbing, and stuff like particles or just the viscosity of the fluid are more significant at small scale.

Another question comes to mind-I don't know what the performance targets for the model were, but given the limitations of model rocket scale, I wonder if a small vehicle could reach the altitudes and supersonic speeds to fully test the aerospike principle (performance at low and high altitudes).