tater

ULA launch and discussion thread

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6 hours ago, sevenperforce said:

I have a sudden desire to spend entirely too much time mathing....

 

but then you'd need a butter interceptor...

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6 hours ago, Nightside said:

but then you'd need a butter interceptor...

I'm used to an air popper anyway.

Am I doing this? I am probably doing this.

Kernels need to hit 180 C external temperature in order for the boiling moisture inside to reach a pressure high enough to pop. At 180 degrees, the pressure inside is 10.2 atmospheres, but in the high atmosphere the pressure is essentially zero. So the kernel will pop sooner, at 9.2 atmospheres, at a temperature of around 173 degrees.

Someone stop me.....

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2 minutes ago, sevenperforce said:

Someone stop me.....

They need to reach the target temp in a certain time frame, presumably. That would constrain entry velocities (too slow or too fast and they just burn).

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10 hours ago, tater said:

They need to reach the target temp in a certain time frame, presumably. That would constrain entry velocities (too slow or too fast and they just burn).

There are a ridiculously high number of variables, actually, which makes me suspect that it DOES have a valid solution. If there were fewer variables then it's possible there wouldn't be one at all.

The ultravelocity path length of an entering projectile is independent of its speed (to a first order) due to Newton's approximation for kinetic penetration. The only variables are cross-sectional density and entry angle. That means you can select your path length independent of your velocity, which may prove helpful.

The way I see it, there are two possible solution families. In one, the entry speed is high and the angle is low, so the kernel pops while still in very thin air and is rapidly slowed thereafter; in the other, entry speed is lower but the angle is higher, so the kernel just barely reaches popping temperature. Risk of first family is charred or disintegrated popcorn; risk of second family is unpopped kernels.
 

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The tweet also talks about the ability to control release altitude, so you could shield for a while, then release. When launches are $10/kg would make a funny microsat.

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19 minutes ago, tater said:

The tweet also talks about the ability to control release altitude, so you could shield for a while, then release. When launches are $10/kg would make a funny microsat.

I wonder if you could use blue corn. The pigmentation might make them absorb enough sunlight to pop in vacuum, and the white popped kernels would have a high enough albedo to not cook any more. Then they could float-fluff down.

In theory, compressive heating should be a straightforward function of air density (which is a function of altitude), velocity, and cross-sectional projectile density, but I'm not finding any simple equations. Munroe did a similar exercise here.

I was trying to model it as a baked-popcorn effect, but since heating will be primarily radiative, it might actually be better to look at microwave settings.

This article is about hot-plate or oven-cooked popcorn....

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There's a table here which is very unscientific BUT may be useful:

Quote

 Set microwave on full power setting timer based on table below for best results: 1000 watt 1:45 to 2:00; 700 watt 2:15 to 3:15; 650 watt 3:00 to 3:30; 500 watt 4:30 to 5:00.

Watts are joules per second, so you can do a straightforward calculation based on this person's (unintended) experiments:

  • 1000W: 105-120 kJ
  • 700W: 95-137 kJ
  • 650W: 117-137 kJ
  • 500W: 135-150 kJ

The lone comment adds, "Just a note, 1:28 - 1:40 for a 1200 watt microwave like I have," corresponding to 106-120 kJ. This gives us a neat little table:

Energy-absorption-of-85-grams-microwave-

I got the mass from here, which may not be the same popcorn type but is close enough.

I'm going to pick 112 kJ as a decent median value. Upper bound is likely to be too high anyway. That's 1.32 kJ per gram. According to this teacher's guide, a popcorn kernel masses about 0.105g, corresponding to 139 J per kernel. I also note that this teacher's guide also quotes the popping pressure at 9.2 atm, which is exactly what I calculated above based solely on measured internal popping temperature.

According to this site, commercial popping corn is dried to an internal moisture level of 13.5%. I assume that means 13.5% water by weight, which corresponds to 0.0142g of water per kernel. Water has a higher heat capacity than dried starch so it is going to suck up all the heat from the microwave. The enthalpy of vaporization of water is 40.65 kJ/mol or 2.27 kJ/g, so it takes 32 Joules to push the moisture into a supercritical state in addition to the energy it takes to heat it up. The specific heat of water is 4.184 Joules per gram-Kelvin, so with 139 - 32 = 107 J, the temperature change is going to be up to 1800 degrees Kelvin on top of room temperature (291 K), or about 2100 K.

Of course that's vastly higher than what it really is due to heating inefficiency, but we're not trying to calculate the internal popping temperature (we already know that). Rather, we are trying to figure out what sort of all-around heating levels are actually required to get energy transfer to that internal temperature. 

2100 K is 3780 Rankine. Conveniently, NASA has informed us that this stagnation temperature is reached at around Mach 6.5 between 37,000 and 85,000 feet.

The excess-velocity path length of a popcorn kernel is easily estimable from Newton's approximation based on atmospheric density.  The density of air is around 6.5 lbs/ft3 or 0.000104 g/mm3 at 37k ft and around 0.0000114 g/mm^3 at 85k ft. To reach terminal velocity, the kernel needs to push 0.105g of air out of the way, corresponding to 1009 cubic mm of air at 37k feet and 9211 cubic mm at 85k feet. This paper puts the mean diameter of a popcorn kernel at around 5 mm, corresponding to a cross-sectional area of 19.63 mm2. So the path length is 5.1 cm at 37k feet and 46.9 cm at 85k feet.

Acceleration is linear, not constant, but assuming constant average acceleration for the sake of simplifying the calculation and a starting velocity of 2.23 km/s, we get something like 540,000 gees at 85k feet. So the "low altitude medium speed" family of solutions is definitely not going to work. 

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Posted (edited)

Technically this launch is by ULA... so without further ado :

EDIT : OK that wasn't a launch. I didn't read enough...
 

Edited by YNM

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Three issues with the Atlas V that will be used to launch Perseverance caused a cumulative 13 day delay.

7/17 (original) --> 7/20 (crane problem) --> 7/22 (PHSF contamination)  --> 7/30 (WDR issue)

The launch date is now July 30, and the launch window tentatively closes on August 15.

(I wrote about it here)

Edited by EchoLima
Forgot about the crane issue

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3 hours ago, EchoLima said:

7/17 (original) --> 7/20 (crane problem) --> 7/22 (PHSF contamination)  --> 7/30 (WDR issue)

804698589055a40e91e517a329f1dc0d.gif

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18 minutes ago, XB-70A said:

804698589055a40e91e517a329f1dc0d.gif

Let's hope the wait ends soon before the mission gets renamed Mars 2022...

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13 minutes ago, EchoLima said:

renamed Mars 2022...

My curiosity is triggered. Will they also give the opportunity to chose another name for the rover? Just asking, as I found that Perseverance doesn't really match the spirit behind those missions.

Edited by XB-70A

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10 minutes ago, RCgothic said:

This is unusual because ULA's Unique Selling Point is Always Launch On Time.

Not always. I remember a recent previous launch that was delayed a few months

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14 minutes ago, linuxgurugamer said:

Not always. I remember a recent previous launch that was delayed a few months

I assume your referring to NROL-71 from January 2019 which was launched on Delta 4 Heavy, a particularly temperamental launch vehicle.

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2 hours ago, V_Bomb2001 said:

I assume your referring to NROL-71 from January 2019 which was launched on Delta 4 Heavy, a particularly temperamental launch vehicle.

Probably.  It was exceptional in the number of delays, was very happy to see it finally go up

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