What to expect at mach 4

PuLsar_11

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Hi all,
I was wondering if anyone out there has any information on what I should be expecting in terms of heating and forces at mach 4. I know at those speeds things start to get weird but I wanted to know what people's experience has told them about construction techniques, etc.
 

rharshberger

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Hi all,
I was wondering if anyone out there has any information on what I should be expecting in terms of heating and forces at mach 4. I know at those speeds things start to get weird but I wanted to know what people's experience has told them about construction techniques, etc.
Very few members of this forum have built rockets that reached Mach 3 (maybe), even fewer (probably none) have reached Mach 4. Fastest I recall seeing someone post about was like Mach 2.5.
 

G_T

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Aerodynamic forces are a function of the square of the speed, generally. M4 has nearly double the aero forces of M3.

Aerodynamic heating is a function of the cube of the speed, generally. M4 has over twice the heating of M3.

If you get to M4, the duration of high heating and high aero loads are much greater as well. Heat soak is much more of an issue for instance.

The aero and heating effects are proportional to atmospheric pressure. Only getting to high speeds at high altitudes improves the odds of survival.

https://www.calculatoratoz.com/en/stagnation-temperature-calculator/Calc-4853 - one of the online calcs for Stagnation Temperature. That is the temperature at locations where the air becomes stagnant - tip of nosecone and fin leading edge for instance.

Gerald
 

DeeRoc29

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Look up butalane (rip bro) and his honey badger thread describing his project that went M3.9.
 
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rfjustin

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Look up butulane (rip bro) and his honey badger thread describing his project that went M3.9.
Deep sigh, RIP indeed... :(

 

G_T

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I must have missed the story/news. I haven't seen him on the forums for a while.
 

G_T

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Aluminum starts getting non-suitable when one is talking about M4, IMHO. Too low a melting temp, and a high thermal conductivity. Heck, it softens at only a few hundred degrees.
 

Off Grid Gecko

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When I was looking into it (not actually done it b/c not enough money) I was considering a simple ablaitive (sawdust base) for the NC. Fins you have to be careful, if they are fiberglass the epoxy will melt out of them and the fabric will shred. I think some in my searching mentioned a metallic leading edge to "catch the heat" and somehow disperse it evenly through the fin, or just metallic fins.
 

AllDigital

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Very few members of this forum have built rockets that reached Mach 3 (maybe), even fewer (probably none) have reached Mach 4. Fastest I recall seeing someone post about was like Mach 2.5.
Reaching it and surviving it are two different things. I've had lots of rockets over pressurize and cato out the nozzle, accelerating the rocket past 200 gees and momentarily passing mach 5. The post mortem on the launch video below was fascinating, because it pointed out all the structural places that would fail when the motor is going 200 gees but the rocket won't. We had motor retention break loose, fins come off, nose collapse, the upper airframe force down the avBay switch ring and shear off the switches, etc. Ultimately, it disintegrated due to drag forces combined with the forward force of the motor.

 

G_T

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Carbon fiber doesn't have a problem with high temperatures, as long as you are not feeding it oxygen or other oxidizers. Many of the available fabrics should be fine IMHO. The problem is the matrix in which they are embedded. High temperature epoxy systems or other higher temperature possibilities have a chance due to low thermal conductivity and tendency to form a char layer. They only have to survive well enough for perhaps 20 seconds and the up part is substantially over. But they will not maintain a smooth surface unless protected by something else. A rough surface likely increases the heat load quite a bit, drag being converted to heat.

Heck, a phenolic and carbon or basalt or even glass tube would take the heat well enough. Higher temperature epoxy (etc) might be fine - Cotronics or competitors.

One can purchase commercially available aerospace sprayable ablative coating material. It isn't cheap, but you can buy it. You can also make your own. It won't be as good, but it might be more than good enough. I think a few people on here have done that. I'd like to see someone do that sometime to a phenolic nozzle...

You need a nosecone that isn't going to implode and you need a body tube that can take some toasting onto which you can solidly mount sufficiently stout fins. Stock nosecones won't cut it.

I've modeled these rockets more than a few times, and balked at the speed down low (I was looking at M4.5). The motors can be done. Long double-taper with a good propellant will do it. A purpose-built rocket doesn't even have to be all that large. Something in the O or P range could do it readily, if the airframe can survive. Assuming fins attached to motor tube.

I've modeled even going to fairly thick wedge fins and it doesn't hurt nearly as much as you might think. I wouldn't recommend skimping on fin beef and would recommend designing them to take a toasting.

I'd definitely NOT use an aluminum nosecone tip, unless it was hollowed and filled with water or some other liquid or low temperature meltable material that can absorb a lot of heat. Pinholes around the bottom of the metal. Produces a heat shield. This sort of thing has been done before, but probably not on any hobby projects. And you might not need it at only M4. M5 or higher though...

I'd assume everything is single use.

Boosted dart might be the easier approach. Then the dart can be overbuilt or of higher temperature materials since it can use some mass. And the booster doesn't have to survive as well, just be fully functional until the dart is away.

I had a fun little idea to design a booster that was deliberately unstable and full composite construction. It would only be made stable by the mass of the dart sticking out front. As soon as the dart separates, the booster would flip sideways and disintegrate from extreme aero loads. No recovery required for the booster, and no booster electronics. Just a composite motor with fins and a phenolic nozzle. Nothing heavy coming down. Composite confetti... I don't think the TRA board would approve such a project, but it would be cool! Heck, the fallout would be mostly within about a thousand feet or so of the pad. The purpose of the shred recovery is to avoid the mass budget for recovery of the booster.

All IMHO of course.

Check out the Loki Dart. M5. Or, replace the mass of the dart with electronics and recovery gear. Still M5.

Gerald
 

Grog6

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An outer skin that could handle the heat, spaced off the nose cone might survive. My tests of creamic bodies for hypervelocity stuff failed due to improper firing of the ceramic. It turned back into powder. :(
 

OverTheTop

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one of the online calcs for Stagnation Temperature. That is the temperature at locations where the air becomes stagnant - tip of nosecone and fin leading edge for instance.
Remember that the leading edges of the fins are raked in relation to the free airstream. The actual airstream normal to the edge (resolve the vector) is less that the rocket velocity. So the velocity you use for calculating Total Temperature on the leading edges is rocket velocity x Cos(sweep angle).
 

G_T

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(It's rather late; I'm tired, and this post rambles. Sorry.)

Unless you consider the particular mach cone angle which has the shock traveling parallel (or just a little off, in the wrong direction) to the leading edge - hopefully only briefly! Then the pressure can be a lot higher than ambient thereby increasing the heating a lot.

Using cosine weep angle approximation looking at normal flow to the leading edge helped in designing reduced drag wings passing through transonic and M1 by quite a bit, This allowed the creation of the first supersonic aircraft. But this approximation only works well for wings of some aspect ratio, not so well at the tips and not at the root. Our fins are pretty stubby. The formula is a simplification, but not quite accurate.

The fins of the early supersonic planes had sufficient sweep that the leading edge stayed behind its own shockwave (so the wing stayed in subsonic flow). But the higher the airspeed, the more swept the shockwave angle. At sufficient airspeed you need either a considerable leading edge sweep angle or need to design such that the leading edge is in advance of the shockwave from nearer the root. Then you might get something that looks like a Nike fin, or possibly the wing of the F104 Starfighter, or the fins of many sounding rockets.


For almost any reasonable fin on a high supersonic rocket, there is going to be some airspeed for which any chosen sweep angle is well off optimal. One wants to choose a sweep that minimizes time spent under such conditions, weighted by atmospheric pressure. IMHO, of course! That's for altitude, not just for speed.

For max speed, just burn as much propellant as fast as you can without CATO. So there is less [integral drag over time] to subtract from top speed. Also use a nozzle with exit diameter near total airframe diameter. That minimizes drag while the motor is burning.

I was looking for some pictures from old supersonic and hypersonic windtunnel tests where the fins (or wings) melted. The melting initiated at the tip of the nosecone, and at the leading edge of the fins right near the root. For some reason I couldn't find the pics tonight. Sorry!


Gerald
 

wclaybaugh2

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The '96 RRS Dart hit about Mach 4.2 at about 11,000 feet; a reduced radar trace is shown at: https://www.rasaero.com/dl_technical_reports.htm (scroll down for the radar data).

The 60 lbm. stainless steel Dart nose was not recovered (it separated to release a streamer) but neither the stainless steel dart fins nor the anodized aluminum booster fins showed any evidence of heat damage. Likewise the Dart's stainless body tube and the booster's anodized aluminum body tube.

Separately, I have been shown images of anodized aluminum fins that reached Mach 5.5 and while the anodizing was stripped off the leading edges (only), there did not appear to be any visible melting or other heating damage.

Bill
 

Off Grid Gecko

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The '96 RRS Dart hit about Mach 4.2 at about 11,000 feet; a reduced radar trace is shown at: https://www.rasaero.com/dl_technical_reports.htm (scroll down for the radar data).

The 60 lbm. stainless steel Dart nose was not recovered (it separated to release a streamer) but neither the stainless steel dart fins nor the anodized aluminum booster fins showed any evidence of heat damage. Likewise the Dart's stainless body tube and the booster's anodized aluminum body tube.

Separately, I have been shown images of anodized aluminum fins that reached Mach 5.5 and while the anodizing was stripped off the leading edges (only), there did not appear to be any visible melting or other heating damage.

Bill
I think often times there's confusion between the temperature near the fins and NC and the heat that actually soaks into them. Most aluminum alloys melt around 660C if I'm not mistaken from memory. This is actually pretty darn hot, a lot hotter than people think. I fear some confusion may also come from C vs F when comparing temps. I'm not an expert on this just pointing out a couple things I noticed in my research.

Also the ability of metals like aluminum and brass to wick heat away from the vital edge and distribute it through a larger mass block could be a benefit rather than a problem. The thinnest part of the metal will melt first, and that will be the leading edge of the fin, so wicking that heat away to the rest of the fin would, I think make for a more resistant part.
 

ECayemberg

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Interesting discussion. My somewhat snarky initial response was based upon the OP having a single post...we know nothing about him/her. Perhaps the OP is a rocketry star, and not a keyboard warrior...but we don't know. My point is that one doesn't typically go from a standstill to Mach 4 without some progressive experience in between. And I'm agreeing with RHarsh that the relatively small pool of those with the school of hard knocks experience aren't often going to do backflips to share all the info needed to get there. The TRF and rocketry communities can definitely be helpful, but you have to show signs that you're ready before the pages of the bible of tribal knowledge are unlocked.

In general, the faster you go the lower to the ground the more problematic. Slower is easier than faster. Thinner air helps with material degradation issues. Composites can work just fine over M3, but you have to take some level of precaution to prevent melting of important parts. Metallic fin cans are *easier* but come with their own tradeoffs. Rules of thumb based upon observation and experience: over ~M2.5 and ~25,000' *things* get interesting. In general, it is nearly impossible to go from entry level to mach 4 without progressing through a learning curve along the way.

Fun comparison between two flights from a few weeks back at Airfest. The rockets flew within 3 percent of the other's altitudes, but the journey to get there was vastly different.
On Friday, I flew an old 3" minimum diameter rocket on a long burn M to 33k'.
On Saturday, I flew a 4" minimum diameter rocket on a fast burning N to 34k'. The 3" bird's aesthetics were untouched...same as preflight. The 4" bird's high temp paint, primer, and some of the ablative material was mach-rashed, as expected. A few more details:

Carbon Slipper (3")
Sconnie 9475-M794 Superballs Moonburner
Max Altitude: 33,343'
Max Velocity: M1.44
Mach Rash: none

Goner (4")
AMW 16,461-N4000 Blue Baboon (an oldie but goodie)
Max Altitude: 34,378'
Max Velocity: M3.24
Mach Rash: plentiful. Reference attached photo.
 

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G_T

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With aluminum, and other metals, it isn't just the melting point that is of issue. As a hardened metal heats up, at a certain point the structural properties start to go downhill. That affects how much extra mass you need to throw at the problem to have an adequate safety margin.

Gerald
 

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