9 ms·

And then on top of it all, throttling ability. People don't grasp what an unbelievably complex engineering problem that is. It's at least an addition of diffic

And then on top of it all, throttling ability. People don't grasp what an unbelievably complex engineering problem that is. It's at least an addition of difficulty at the same magnitude as building a steady-state 63000 hp turbine pump in the first place. Mechanically implementing it in the inherently steady-state design rocket of most rocket cycles. Having variable controls able to work at those pressures. Testing structural dynamics for a range of harmonic conditions instead of one. And do all of that with materials that need to tolerate temperatures going from cryogenic to white hot, without allowing thermal expansion to affect the mechanical tolerances of parts running at thousands of RPMs. And now you have varying flow rates and negative pressures in the lines coming from the external tanks, so have to design such that cryogenic liquids (that normally would require immense positive pressure to keep liquid) don't spontaneously boil or cavitate or cause a shock-like wave (think water hammer turning off your bath faucet) under changing negative pressures. It's really difficult even for seasoned engineers to grasp the scale of difficulty involved.

LeifCarrotson 5y ago
Appreciation of engineers for the difficulty of an assignment like this seems bimodal. On the one hand, you have https://xkcd.com/793/ https://xkcd.com/793/ responses: > You're trying to predict the behavior of <complicated system>? Just model it as a <simple object>, and then add some secondary terms to account for <complications I just thought of>. Easy, right? So, why does <your field> need a whole journal, anyway? Just pressurize the tanks, and meter the flow with some valves. Easy, right? They move to the other side of the distribution after a little more thought, when they they realize it's simply infeasible to put thousands of horsepower in a pump that size, and declare the whole endeavor completely nonsensical and impossible. Ran through this on a recent project involving an automated sewing machine. At first, it seems ludicrous that you could tie knots thousands of times per second. Oh wait, it's a single motor and old cam-driven tech from the 1800s, available off the shelf for a couple hundred dollars?
- avmich 5y ago
  They also move to the "simple" side as well. The phrase "the rocket science is not a rocket science" has reasons to exist.
codeulike 5y ago
It's really difficult even for seasoned engineers to grasp the scale of difficulty involved. Its rocket science
- InitialLastName 5y ago
  A joke you hear unceasingly when you take tours of engineering schools with a heavy aerospace focus.
  - gerdesj 5y ago
    That's nice but I hope my calcs on the geotechnics of the ground that your school or house is situated on are correct. Obviously, I didn't do the calcs myself and probably no calcs were done at all either. I'm sure everything will be fine. Don't forget the foundations!
- sebazzz 5y ago
  According to Elon it is engineering actually, because science discovers the existing whereby engineering _engineers_ something that isn't now.
- Taniwha 5y ago
  To be fair much of rocket science is essentially extreme plumbing
  - avmich 5y ago
    True :) . Turbopumps are finicky, but at least heat and cryogenics are separated with the turbine shaft. In combustion chambers, you have both mechanical stress from high pressures (and thin walled constructions) and thermal stress - difference of a few thousands K, so chambers aren't that simple either. Still - now we have enough knowledge to repeatedly design flyable rockets from scratch, different teams, periods of time, countries. A lot of work still remains - and the plumbing is a good manifestation for that.
    - 5y ago
kaliszad 5y ago
Yes, it is difficult and it is stunning. On the other hand, in theory, you can go to an engineering school and learn the calculations and design involved in making sure it works. Also, it has been done like 80 years ago with the V2 with at best mechanical calculators, without most of the materials we can use today, without 3D printing, without simulations etc.
- amelius 5y ago
  Honestly, I think building micrometer/nanometer scale stuff like a nanopore sequencer is a lot more impressive. That's the true "rocket science" for me.
zaarn 5y ago
To be fair, that turbo pump and throttling control only has to work for about 5 minutes, that does make it a bit simpler by just engineering it to work for 15 minutes without breaking (safety margin included then).
- regularfry 5y ago
  Assuming single-use. Reusable rocket engines are different beasts.
  - zaarn 5y ago
    Engineer it for 30 minutes and swap it regularly (SpaceX swaps turbines and pumps often enough because they do break down after only one or three launches)
- NamTaf 5y ago
  You're perhaps overstating how easy it is target a certain design life when constrained by other parameters. It's entirely possible that targetting '15 minutes' is actually meaningless for many of the parts because all of the other constraints are far tighter. Once you're through the stress of getting up and running, then chugging along in the steady-state is often less stressful (for something non-ablative, and of course start-stop cycles are another matter entirely). To give a far less glamourous example, I am a mech eng who works in rail. We needed some more orecars to complement an existing fleet that only had about 10 years' operational life left in them. Thus, they wanted me to design for a shorter life than the 25-30 year standard that we target, to save cost. However, trying to thin out the structure so it only had 10-15 years' of fatigue life in it meant that it fell well short of the proof load requirements needed to stop it ripping in half in a worst-case shock load. Put differently, the constraint around peak loads effectively baked about 20 years of operational life into the structure, and in turn made it difficult to save money on a shorter-life design. It would not surprise me if many of the non-ablative parts in a rocket are in fact fairly durable without the stop-start cycles. So whilst a launch may only take 5-8 minutes, I could totally believe that a 15-20 minute launch wouldn't demand heaps more from the parts. Of course, this absolutely doesn't translate to ablative parts, or items that undergo stop-start cycles. The latter of course is where the devil is for reusable equipment. Depending on the failure modes in question for turbo pumps and throttling controls, those may or may not apply.

LeifCarrotson 5y ago

Appreciation of engineers for the difficulty of an assignment like this seems bimodal. On the one hand, you have https://xkcd.com/793/ https://xkcd.com/793/ responses: > You're trying to predict the behavior of <complicated system>? Just model it as a <simple object>, and then add some secondary terms to account for <complications I just thought of>. Easy, right? So, why does <your field> need a whole journal, anyway? Just pressurize the tanks, and meter the flow with some valves. Easy, right? They move to the other side of the distribution after a little more thought, when they they realize it's simply infeasible to put thousands of horsepower in a pump that size, and declare the whole endeavor completely nonsensical and impossible. Ran through this on a recent project involving an automated sewing machine. At first, it seems ludicrous that you could tie knots thousands of times per second. Oh wait, it's a single motor and old cam-driven tech from the 1800s, available off the shelf for a couple hundred dollars?

avmich 5y ago
They also move to the "simple" side as well. The phrase "the rocket science is not a rocket science" has reasons to exist.

codeulike 5y ago

It's really difficult even for seasoned engineers to grasp the scale of difficulty involved. Its rocket science

InitialLastName 5y ago
A joke you hear unceasingly when you take tours of engineering schools with a heavy aerospace focus.
- gerdesj 5y ago
  That's nice but I hope my calcs on the geotechnics of the ground that your school or house is situated on are correct. Obviously, I didn't do the calcs myself and probably no calcs were done at all either. I'm sure everything will be fine. Don't forget the foundations!
sebazzz 5y ago
According to Elon it is engineering actually, because science discovers the existing whereby engineering _engineers_ something that isn't now.
Taniwha 5y ago
To be fair much of rocket science is essentially extreme plumbing
- avmich 5y ago
  True :) . Turbopumps are finicky, but at least heat and cryogenics are separated with the turbine shaft. In combustion chambers, you have both mechanical stress from high pressures (and thin walled constructions) and thermal stress - difference of a few thousands K, so chambers aren't that simple either. Still - now we have enough knowledge to repeatedly design flyable rockets from scratch, different teams, periods of time, countries. A lot of work still remains - and the plumbing is a good manifestation for that.
  - 5y ago

kaliszad 5y ago

Yes, it is difficult and it is stunning. On the other hand, in theory, you can go to an engineering school and learn the calculations and design involved in making sure it works. Also, it has been done like 80 years ago with the V2 with at best mechanical calculators, without most of the materials we can use today, without 3D printing, without simulations etc.

amelius 5y ago
Honestly, I think building micrometer/nanometer scale stuff like a nanopore sequencer is a lot more impressive. That's the true "rocket science" for me.

zaarn 5y ago

To be fair, that turbo pump and throttling control only has to work for about 5 minutes, that does make it a bit simpler by just engineering it to work for 15 minutes without breaking (safety margin included then).

regularfry 5y ago
Assuming single-use. Reusable rocket engines are different beasts.
- zaarn 5y ago
  Engineer it for 30 minutes and swap it regularly (SpaceX swaps turbines and pumps often enough because they do break down after only one or three launches)
NamTaf 5y ago
You're perhaps overstating how easy it is target a certain design life when constrained by other parameters. It's entirely possible that targetting '15 minutes' is actually meaningless for many of the parts because all of the other constraints are far tighter. Once you're through the stress of getting up and running, then chugging along in the steady-state is often less stressful (for something non-ablative, and of course start-stop cycles are another matter entirely). To give a far less glamourous example, I am a mech eng who works in rail. We needed some more orecars to complement an existing fleet that only had about 10 years' operational life left in them. Thus, they wanted me to design for a shorter life than the 25-30 year standard that we target, to save cost. However, trying to thin out the structure so it only had 10-15 years' of fatigue life in it meant that it fell well short of the proof load requirements needed to stop it ripping in half in a worst-case shock load. Put differently, the constraint around peak loads effectively baked about 20 years of operational life into the structure, and in turn made it difficult to save money on a shorter-life design. It would not surprise me if many of the non-ablative parts in a rocket are in fact fairly durable without the stop-start cycles. So whilst a launch may only take 5-8 minutes, I could totally believe that a 15-20 minute launch wouldn't demand heaps more from the parts. Of course, this absolutely doesn't translate to ablative parts, or items that undergo stop-start cycles. The latter of course is where the devil is for reusable equipment. Depending on the failure modes in question for turbo pumps and throttling controls, those may or may not apply.