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Starting at about 32:38:

"We sub-cool the oxygen and methane to densify it, so compared to... propellants normally used close to their boiling point in most rockets, in our case we actually load the propellants close to their freezing point, and that can result in a density improvement of up to around 10 to 12 percent, which makes an enormous difference in the actual results of the rocket."

"It also makes the... it gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high pressure turbo pump if you have very cold propellant."

In the video of the recent presentation Making Humans a Multiplanetary Species Elon Musk mentions that in addition to obtaining higher loading densities of the LOX and LCH4 propellants, sub-cooling them decreases the chances of cavitation in the turbo pumps and improves other aspects of propellant feed to the engines.

Question: How (actually) do sub-cooled propellants reduce cavitation within turbo pumps and make feed easier?

In the Wikipedia article sub-section Cavitation#Cavitation_solutions it says:

"To overcome cavitation: Increase suction pressure if possible. Decrease liquid temperature if possible. Throttle back on the discharge valve to decrease flow-rate. Vent gases off the pump casing." (my emphasis)

but there is no citation for that particular sentence. Further along in the same article in Cavitation#Vascular_plants it says:

"Deciduous trees shed leaves in the autumn partly because cavitation increases as temperatures decrease.${}^{32}$" (my emphasis)

edit: As pointed out by @Andy in the comments, there appears to be some problem in the original Wikipedia article here. Since it's a complex biological system with complex chemistry it probably isn't so meaningful here.

which seems to point in the opposite direction.

The article also links to the on-line book CAVITATION AND BUBBLE DYNAMICS by Christopher Earls Brennen which may be helpful here.

note: While I can imagine that a liquid at its boiling point might boil with even a small additional amount of heat, this isn't necessarily the same as true cavitation. What I'm looking for here is a solid technical explanation of How (actually) do sub-cooled propellants reduce cavitation within turbo pumps and make feed easier?

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    $\begingroup$ Cavitation in a pump happens when the pressure of the pumped fluid drops to around its vapor pressure. This is sometimes called the Net Positive Suction Head or Pressure; in the Shuttle program it was the latter, NPSP. NPSP is simply the difference between the pressure in the fluid and its vapor pressure. Not unique to rockets or even cryogens, it happens with boat propellors and water pumps. Here's a writeup www.pumpschool.com/applications/NPSH.pdf $\endgroup$
    – Organic Marble
    Oct 5, 2016 at 4:06
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    $\begingroup$ Something like Figure 1 in this: fluidsengineering.asmedigitalcollection.asme.org/… - full size here: i.stack.imgur.com/l9QnI.png lower temperature makes it more difficult to to drop below the lower vapor pressure - and I suppose higher static pressure in the feed line also makes it harder as well. Any ideas about the 2nd part: "...and it makes it easier to feed a high pressure turbo pump if you have very cold propellant"? $\endgroup$
    – uhoh
    Oct 5, 2016 at 6:14
  • $\begingroup$ @Andy why do you think that? Besides avoiding cavitation, why would colder propellant feed reduce pump wear or excessive vibration? $\endgroup$
    – uhoh
    Oct 5, 2016 at 7:35
  • $\begingroup$ @Andy So you think Elon Musk is just repeating himself? "makes it easier" is same as "gets rid of risk" and there are no other possible benefits and nothing else is made easier? I think there may be something else. Despite the casual appearance, this guy usually chooses his words very carefully and makes his distinctions clear. $\endgroup$
    – uhoh
    Oct 5, 2016 at 7:51
  • $\begingroup$ @Andy Further, you can take a propellor and put it in boiling water for 100 years and not get the cavitation damage you'd get driving it on the end of a submarine. I think sending people away with the message "cavitation is boiling" is not the right thing to do here in SXSE. I can turn out the lights and stare at the boiling water and I'm not going to see sonoluminescence either. Saying they are the same is myopic as well as misleading. $\endgroup$
    – uhoh
    Oct 5, 2016 at 8:02

3 Answers 3

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Cavitation is boiling, in this case it is caused by reduced pressure in the wake of blades. A boiling point is a combination of 2 factors; heat and pressure. If you can't do anything about the reduced pressure you can manipulate the other factor, which is the temperature. A liquid can stay in it's liquid state at a lower pressure if it is colder. I think the cause & effect is clear. You can see the charts on Wikipedia or even calculate it here.

http://www.trimen.pl/witek/calculators/wrzenie.html

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  • $\begingroup$ No it is not clear. Could you explain better please? I boil water quite often, and I'm pretty sure it is not exactly the same thing as cavitation. Can you find a source that demonstrates that cavitation is significantly more difficult when the fluid is below the boiling point? The purpose of posting answers in Stackexchange is not to demonstrate you are smart, it's to give a good enough answer so that the OP and everyone else reading your answer in the future will understand better and really learn something. $\endgroup$
    – uhoh
    Oct 5, 2016 at 3:50
  • $\begingroup$ You also need to address the second part of the question - why are sub-cooled propellants easier to feed to the engine? This appears to be separate from the issue of cavitation in the turbo pump. Back to cavitation: I've linked to this book in the question. Take a look at sections 1.2 and especially 1.5. $\endgroup$
    – uhoh
    Oct 5, 2016 at 3:51
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    $\begingroup$ +1, I think this brief answer does cover the question. Local boiling (formation of vapour) causes cavitation; reducing the temperature reduces the boiling in those parts of the stream. $\endgroup$
    – Andy
    Oct 5, 2016 at 7:00
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    $\begingroup$ @uhoh boiling is harmless because the bubbles rise normally and break at the surface. Cavitation is a problem when local pressures collapse bubbles again causing shock waves in the (incompressible) liquid. Just to clarify, I have upvoted this answer because in my opinion it does explain the effect of temperature, which is what your question was about. It may not be the "best" answer. $\endgroup$
    – Andy
    Oct 5, 2016 at 8:44
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    $\begingroup$ I didn't say anything about which term to use. Your comment is restating the obvious. $\endgroup$
    – Organic Marble
    Oct 10, 2016 at 10:42
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This answer is based mostly on the NPSH link Organic Marble shared (http://www.pumpschool.com/applications/NPSH.pdf) and the various properties you can infer about subcooled LOX.

  • Subcooling lowers vapor pressure, which helps prevent bubble formation.

  • Subcooling densifies the propellant, which

    • allows lower fluid velocities at the same mass-flow rates, which

      • allows lower pump RPMs, reducing the pressure drop in the wake of the impeller, consequently reducing the incidence of vapor bubble formation (and the severity of re-collapse if they do)

      • helps to reduce friction losses and associated NPSH_A

On this part of the quote:

"It also makes the... it gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high pressure turbo pump if you have very cold propellant."

I'm just going to disagree with your interpretation that "despite the casual appearance, this guy usually chooses his words very carefully and makes his distinctions clear." This reads as one thought, which he decided to rephrase on the fly--the cavitation risk is directly connected to the ease of feeding the pump.

As an example-by-contrast, this usenet archive post about high cavitation risk making things hard, from http://yarchive.net/ac/cavitation.html:

Liquid pumps for propane (and refrigerants) are very easy to cavitate, since the vapor/liquid are at or close to equalibrium, and the slightest pressure drop (a pump trying to "suck" liquid) will create huge amounts of flash gas and and pump cavitation. Any thing like undersized pump input lines, or trying to suck liquid uphill or thru fine screens, etc, leads to extreme cavitation and quickly destroys a pump. I once had to build a 3 ton refrigeration unit to subcool liquid 40-50 degrees due to someone's dumbass attack of running liquid refrigerant lines uphill into the pump inlet and it was easier to build the subcooler than to get the piping changed. Now they listen. THe subcooling reduced the pump cavitation.

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    $\begingroup$ @uhoh Your posts about the purpose of Stack Exchange are starting to range from extrapolations of the rules to statements of opinion. I recommend that rather than continuing to create long comments on multiple users' posts that you create Meta discussions or dedicated chatrooms to work out these issues. $\endgroup$
    – called2voyage ♦
    Oct 25, 2016 at 0:45
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage ♦
    Oct 25, 2016 at 0:45
  • $\begingroup$ @called2voyage The extended discussion actually started here, maybe it can be consolidated there somehow? space.stackexchange.com/questions/18486/#comment48607_18573 $\endgroup$
    – uhoh
    Oct 25, 2016 at 0:54
  • $\begingroup$ The longer quote is a 20 year old un-curated blog post by a refrigerator repairman with the message: " Now they listen." $\endgroup$
    – uhoh
    Oct 25, 2016 at 1:44
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    $\begingroup$ @uhoh I can't really consolidate chats, unfortunately. You can choose which room you want to discuss in, or discuss in both if you want. $\endgroup$
    – called2voyage ♦
    Oct 25, 2016 at 17:17
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The quote:

"We sub-cool the oxygen and methane to densify it, so compared to... propellants normally used close to their boiling point in most rockets, in our case we actually load the propellants close to their freezing point, and that can result in a density improvement of up to around 10 to 12 percent, which makes an enormous difference in the actual results of the rocket."

"It also makes the... it gets rid of any cavitation risk for the turbo pumps, and it makes it easier to feed a high pressure turbo pump if you have very cold propellant."

edit: This answer to the question "What fundamentally distinguishes cavitation and boiling as different phenomena?" by a self-described mechanical engineer and former US Navy nuke is particularly helpful!

Background:

Some examples of process which include some material undergoing a phase changing from liquid to gas are evaporation, boiling, and cavitation.

As we usually see it, evaporation takes place at a pre-existing and often stable boundary between a liquid and a gas. Even if the temperatures are low and pressure is high, evaporation will continue slowly until the partial pressure of the material that is the liquid is reached in the gas. So if you have room-temperature water sitting in air and they are both at atmospheric pressure, the water will continue to evaporate until the partial pressure of water in the air reaches the vapor pressure of water at those conditions.

But boiling and cavitation generally happen inside a liquid, not outside or above it, and so they depend on both temperature and absolute pressure on the liquid, not just partial pressure as in the case of evaporation.

In both cases, a bubble of vapor is produced within the liquid. When the vapor pressure has been increased above the local hydrostatic pressure in the liquid by the application of heat, we usually call this boiling, and when the hydrostatic pressure has been lowered below the vapor pressure in a transient or highly localized manner, we usually call it cavitation.

But let's just consider the effects of reduced pressure, and not temperature.

If the hydrostatic pressure of a volume of a liquid is lowered below its vapor pressure in bulk, as in the case of a bubble chamber or even a vacuum flask of water attached to a vacuum pump pulling almost an atmosphere of pressure, the liquid will boil. The bubbles may reach the surface and release the vapor, or over time they may re-condense within the liquid, especially if the pressure is restored.

The actual nucleation and growth of such bubbles a complicated process - liquids have a tensile strength just as solids do, and usually nucleation sites will dominate in the actually formation of the bubble - tearing a hole in the liquid so to speak.

In engineering, the use of the word cavitation is generally restricted to situations where the bulk pressure of the liquid is not by itself responsible for the bubble formation. Instead, the term cavitation generally refers to a situation where there is a highly localized and transient drop in pressure below the vapor pressure. Almost immediately after the bubble is formed, the conditions change and the pressure is above the vapor pressure again. While the process is complex, an important feature is the rapidity with which the bubble collapses under the influence of the higher pressure of the liquid.

The terms boiling and cavitation refer to complex phenomena which are related but different. The distinction is between the evolution of the pressure immediately after the bubble is formed. Conceptually there could be a conitnuum of situations between these two extremes, but that does not mean they are the same thing.

This video shows an example of water being boiled using an immersion heater. The temperature of the heating element is much higher than the water. Between about 01:00 and 02:00 the sounds gets louder and louder, and while a few bubbles can be seen, the process making the sounds is the production and collapse of mostly invisible vapor bubbles. While the temperature at certain points on the heater is higher than the boiling point, as soon as a bubble begins to form, part of the bubble expands into cooler areas of the liquid causing the vapor to quickly condense.

This results in a phenomenon similar to cavitation, but on a somewhat longer timescale so that it will not damage the heating element. Boiling water will not cause cavitation damage.

Cavitation is bad because the rapid and violent collapse of the bubble is a result of the very rapid change in pressure over time and or distance. It is this rapidity and localization of bubble collapse that generally determines if a phenomenon is called boiling or cavitation.

Subcooling:

Subcooling of liquid propellant as introduced into a turbo pump on a rocket engine can reduce the possibility of cavitation. At lower temperatures the vapor pressure is lower, and thus it is less likely that a given turpbo pump speed can produce a pressure below this point. However, subcooling also raises the density significantly - one of the main reasons that it is being adopted. This means that the turbo pump can rotate more slowly to deliver the same mass per unit time. Slower rotation means less cavitation even at a fixed temperature.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – called2voyage ♦
    Oct 25, 2016 at 17:16

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