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The TV program, Myth Busters, has exploded several hot water heaters and also set off bottle rockets for our entertainment and to bust myths that float around. I think it would be fun to create a myth for them to test. The myth is this:

Myth - "A hot water heater properly configured can deliver enough rocket thrust to launch itself into space."

For their TV show, Myth Busters disables all safety devices on the heater then heat the water. I don't recall what temperature they reach but at about 335 psi the bottom blows out of the heater launching the shell upward. The pressure should give the temperature. The current height record for an exploding hot water heater is about 550 feet, but that is using the energy in a single impulse on the ground.

I don't know what size heater they use, but it is 18 inches in diameter so I'm guessing that it is 40 gallon. It is electric.

If one were to weld a rocket nozzle to the bottom, how high could the thing go? After the calculation is made, then we can change the myth from "into space" to X-thousand feet altitude. Space of course is 60 miles up and if the shell of the heater weighs 25 pounds, then at the edge of space it has a potential energy of 25 x 60 x 5280 or about 8 million ft-lbs. That sounds like a lot but it is less than the energy of 2.5 kg of TNT. How much energy is contained within 40 gallons of 335 psi water?

Aero wrote:The TV program, Myth Busters, has exploded several hot water heaters and also set off bottle rockets for our entertainment and to bust myths that float around. I think it would be fun to create a myth for them to test. The myth is this:

Myth - "A hot water heater properly configured can deliver enough rocket thrust to launch itself into space."

For their TV show, Myth Busters disables all safety devices on the heater then heat the water. I don't recall what temperature they reach but at about 335 psi the bottom blows out of the heater launching the shell upward. The pressure should give the temperature. The current height record for an exploding hot water heater is about 550 feet, but that is using the energy in a single impulse on the ground.

I don't know what size heater they use, but it is 18 inches in diameter so I'm guessing that it is 40 gallon. It is electric.

If one were to weld a rocket nozzle to the bottom, how high could the thing go? After the calculation is made, then we can change the myth from "into space" to X-thousand feet altitude. Space of course is 60 miles up and if the shell of the heater weighs 25 pounds, then at the edge of space it has a potential energy of 25 x 60 x 5280 or about 8 million ft-lbs. That sounds like a lot but it is less than the energy of 2.5 kg of TNT. How much energy is contained within 40 gallons of 335 psi water?

I'm suspicous of your numbers. With chemical rockets you need ~ 10 times the weight of fuel compared to payload to reach orbit, For steam to have a similar thrust efficiency the temperature would need to be several thousands of degrees C and ~ 1-2 thousands of lb/square inch of pressure. I saw one of the MB episodes. My dim memory was that they were measurig the temperature, probably in degrees F. I don't know what the pressure would have been. Using an approximation, if the WH reached 500 feet, the average speed vs gravity would determine the height obtained. Playing with some numbers, 6 G of acceleration would give an initial speed of 180 f/s, and it would take 6 sec. to decelerate with an average speed of 90 f/s. 90 f/s * 6 seconds= 540 feet height. This amount of acceleration is ~ 1/130th of that needed to reach orbit (ignoring a number of things like atmosphere and thrust used to gain the required height, Earth rotation).

A good nozzel would presumably have increased the height obtained by some modest (?) amount.

Perhaps the most famous steam rocket ever used was Evil Kineval's.

http://www.canosoarus.com/10X1Skycycle/Skycycle01.htm

Dan Tibbets

I think the water heater in that episode went about 500 feet up after blasting through the roof of the "house" (that met California construction codes) they built around it for the test. The energy needed to get through the roof would have to have reduced the amount available for reaching max altitude.

I don't remember if they did a test purely for height, however.

If the heater holds 40 gallons of water, that is 334lbs (I used 8.35lb/gal). A quick Googling gives a weight of 100lb for an AO-GCV40 heater (I'm assuming the weight will be roughly the same across brands/models) that appears to be very similar to the one used in the show. That means roughly 77% fuel, 23% "payload". Not the 10:1 ratio you'd expect for a rocket, but enough to get a decent height depending on how you did it.

D Tibbets wrote:

Aero wrote:The TV program, Myth Busters, has exploded several hot water heaters and also set off bottle rockets for our entertainment and to bust myths that float around. I think it would be fun to create a myth for them to test. The myth is this:

Myth - "A hot water heater properly configured can deliver enough rocket thrust to launch itself into space."

I'm suspicous of your numbers. With chemical rockets you need ~ 10 times the weight of fuel compared to payload to reach orbit.

The myth didn't say orbit. It said space, which is an altitude of 100km up, clearing most of the Earth's ionosphere.

If both were equivalent, SpaceShipOne wouldn't have been a spacecraft at all, just a piece of carbon composite that happens to reach an altitude near space. It did reach space, but as it didn't attain orbital velocity, it plummeted straight down again.

Plus, your 'payload' mass, in this case, is nothing, except the empty water heater can. I'd have a step aside before the thing came back in for a landing, though.

Actually, Dan, they were measuring both the temperature and pressure. Only I don't remember what the temperature was. I know the pressure was 335 psi because they calculated the total force needed to burst the 18 inch diameter bottom out of the heater. It was like 85,000 pounds-force. So 85,000/(pi x r^2) confirms the pressure of 334.0288929 psi.

A good nozzle would enhance the thrust tremendously, I just don't remember the math and one of my ex's got my text books. Note that the myth doesn't say "reach orbit" only to reach space. To reach orbit requires about 90 miles altitude at 18,000 mph velocity. To reach space only takes 60 miles altitude at zero velocity, a much less energetic problem. Plus, under my assumption of 25 pounds dry weight (and no bottom) compared to 40 gallon expendable mass, (320 pounds) the water heater rocket has a mass ratio above 12.

I don't know how high the energy of the hot water would thrust the water heater shell, so I'm hoping someone will do the math. I hope it would go high enough to make the myth interesting enough to be on TV, because it would be fun to watch. And Jamie (or Adam) did sort of toss out a challenge when he said that MythBusters holds the hot water heater altitude record of 550 feet. Surely that is easy to beat.

OK, we'll need to go with the data and hope that the nozzle mass is small. Maybe an overall mass ratio of 8. Or we could use a 50 gallon water heater, they probably come in 18 inch diameters, too.

I'm sure that they did a test launch without a building, and that one reached an estimated 550 feet altitude based on the time of flight. The one that went through the roof (California construction code) fell back and landed on the roof within a few seconds (2 or 3 seconds) so it couldn't have reached much more than 20 -30 feet above the roof.

The 50 gal version of the heater I looked up before is 140lbs (page).

That is still only 2.98:1 ratio.

Aero wrote:I'm sure that they did a test launch without a building, and that one reached an estimated 550 feet altitude based on the time of flight. The one that went through the roof (California construction code) fell back and landed on the roof within a few seconds (2 or 3 seconds) so it couldn't have reached much more than 20 -30 feet above the roof.

Its been a while since I've seen that episode, so I'm not surprised if I'm mis-remembering something about it.

Here is an interesting clip. Note that all the water is dumped in the first few feet.
http://dsc.discovery.com/videos/mythbus ... speed.html

I would say with confidence that the water heater to space idea is impossible, unless maybe you mean launching it to orbit from Saturn's moon Phoebe.

I've investigated one water heater accident that qualified as a rocket of sorts. The thing was about 60 gallons, oil-heated, and it killed a guy. I determined the thing came on and stayed on, developing steam in about 2/3 of its volume, working at mains pressure (maybe 80 psig). When the water boiled down to the level of one of the copper feed pipes, the oil heat (applied outside the tank in this model) was able to melt solder on a connecting pipe at an elbow. The thing then expelled water and steam out the elbow, causing the water heater to spin, break loose, and fly around the basement, just as the guy came in to investigate the strange noises from the pipes. The safety valve sensor had corroded off.

Failing water heaters sometimes explode. This requires a bad safety valve and a stuck thermostat. When the heater bursts, the water in it flashes to steam, just like a steam engine boiler explosion. The energy stored in superheated (pressurized) water allows it to make a constant-pressure expanding-volume phenomenon. As the cloud of steam expands, more water flashes to steam at the original pressure. This is not just a pressure release pop, it is a vicious expansion without much pressure drop until all the water is steam.

They rate hot water heaters in pounds of nitroglycerine of equivalent explosive power, depending on the mains pressure when they rupture. I've seen numbers up to 8 lbs nitro. But this energy storage is insufficient to reach space for an object of that mass,

From about a 200 yard distance, I once saw a thin-walled, galvanized, cylindrical water tank (about 90 gal water heater size), that some dummy was using as a compressed air tank (~100 psi?), lose its bottom disk and punch out through the roof of a building. It reached about 100 ft altitude, then dropped back down through the roof of an adjacent building about 25 ft from the launch site. A man standing near it when it blew fortunately avoided all of the metal and wood shrapnel. It kind of looked like a hovering UFO when it reached apogee, but when it started to drop again I knew antigravity was not involved. You can have ballistics fun even with plain old compressed air.

Tom - A very interesting post. No, not some moon, but the Earth's surface, else MythBusters couldn't bust the myth. But for calculations I propose to ignore drag.

A question - How does nitro compare to TNT in energy? I calculate that the potential energy of a 100 pound heater shell at 60 miles altitude is equivalent to 20.532 pounds of TNT. But interestingly enough, I further calculate that the thermal energy in 40 gallons of 400 degree Fahrenheit water is equivalent to 64.51 pounds of TNT. (I converted 335 psi to temperature using steam tables.)

Yes, water heaters can be very dangerous and when ruptured the water will viciously flash to steam without much pressure drop until all the water is steam. That characteristic is good for a rocket. I found a value of specific impulse for a steam rocket on Wikipedia, Isp = 190 seconds for the pressure range of interest (250 - 500 psi). I also found some rocket equations here -

http://exploration.grc.nasa.gov/educati ... ozzle.html
Or more helpful
http://exploration.grc.nasa.gov/educati ... ecimp.html

Unfortunately, I can't just apply these equations without knowing for sure what I'm doing. (A little knowledge is a dangerous thing.) The answers I get are at least an order of magnitude to good. I'm missing a conversion somewhere, probably a pounds to slugs factor ... That's why I'm asking for help in constructing this myth.

OK, that was it. I was dividing pounds of water by mdot to calculate the total burn time. Its slugs of mass flow which reduced the burn time by a factor of 32.2

This is what I calculate:

I used a 100 pound heater (empty weight) containing 40 gallons of water (320 pounds) at 335 psi. I assumed no air drag, and an ideal nozzle with no extra mass. I set the initial acceleration at 1 gee, using 840 pounds of thrust. I assumed the trajectory was straight up. I did not account for increased acceleration due to mass burn off. The results of the Excel calculations are:
mdot= 0.164759725 slugs/second
burn time = 60.31746032 seconds
burn out velocity = 2719.11 ft/sec or 1853.94 miles/hour
burn out altitude = 164009.88 feet or 31.06 miles.
And since the vertical acceleration was 1 gee, and the deceleration will be 1 gee, the empty water heater should just reach the edge of space when it coasts to a stop.

Of course, drag forces would reduce the performance numbers, but on the other hand increased acceleration as mass is expelled would increase the performance numbers.

These are my numbers, you are free to check them, but I think the myth stands. -
"A hot water heater properly configured can deliver enough rocket thrust to launch itself into space."

You were right , it was pressure that was recorder. See the Utube video of the first episode.

http://www.youtube.com/watch?v=JmJoyuUJj2Q

Concerning the nozzle, at sea level pressures I believe the exaust cone of a rocket is small, like the Shuttle solid boosters. The sidewalls of the water heater served this purpose to a degree, though I suspect a substantial portion of the blast was lateral (it blew the shed apart).

My feble attempt at rocket science starts with a quick and dirty formula I use for ISP:
ISP= square root (temp. X pressure/ molecular weight of the exaust product) For a steam exaust at ~ 300 lb per square inch, and a guess of a temperature of perhaps a generous 1000 degrees F. the ISP would be ~ 120 (I am not sure of the appropiate units). Actually, thats not bad. It is approaching the ISP of hydrazine.
But the 1:10 payload to fuel ratio is for rockets reaching orbit is in the 300-400 ISP range. For a sounding rocket, the needed thrust is much smaller. Lets see, an initial thrust over a short time delivering 100 G acceleration, would have a top speed of 3000 ft/s, an average speed of 1500 ft/s over 100 seconds (decelerating a 1 G/s) = 150,000 ft height . Lets change that to 150 initial G of acceleration- yeilds a height of ( 4500 ft/s /2 (for average velocity) X 150 seconds= ~330,000 ft.
The water heater with a generous ISP of 120 and ~ 320 lb of water would have a total thrust of perhaps 40,000 lbs. If the average weight of the tank and 1/2 the water = 300 lbs, then the acceleration would be 40,000/ 300 = ~130 G.

So, the water heater rocket could approach space if my calculations are accurate. Things not concidered are nozzle efficiency, atm. drag, and the energy lost in converting the superheated water to steam (100 Calories per gram) . With these suspect numbers, I suspect a lot of the thrust was wasted (very low nozzle efficiency).

[EDIT] Actually my guess for the temperature of the water was ~ two times too high, so a more acurate ISP calculation would yield an ISP of ~ 85 s.
[EDIT 2] Actually, I think I should have been using degees Centigrade in the formula- this gives answers closer to the Shuttle main engines =
sq root (2000lbs/SI X 2000 degrees C/~ 18 (MW of water)) = ~ 450 s.. This would give the water heater an ISP of ~ 55 seconds. I think that would give a height of ~ 1650 f/s/2= 825 ft/s average velocity X 55 sec = height of 45,000 feet. Sorry for all the revisions.

http://www.engineeringtoolbox.com/boili ... d_926.html

Dan Tibbets

@ Dan - The side walls of the water heater can't really be called a nozzle. All of the hot water was dumped in an uncontrolled fashion while the water heater body was still in (but exiting) the first level of the test building. You can clearly see that on one of the videos linked previously. The explosion that occurred was the water flashing to steam after (or as) the heater body left. Here is a convincing illustration of what a steam rocket nozzle should do. Look down the page about half way.

http://orbiter-forum.com/showthread.php?p=130295

Thanks for formula. I knew it existed but I couldn't remember the dependency of Isp on molecular weight of the exhaust products. My googleing results tell me that using steam as the reaction mass should give an Isp in the range of 149 to 215 seconds, and that the temperature of water at 335 psi is 427.6 degrees F. Interestingly, I also discovered while googling that, with water available on the moon, using steam as the reaction mass and a nuclear powered water heater may be the most cost effective means to move significant mass from LEO to lunar orbit.