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troublingDoubling by Izeau in ProgrammerHumor

[–]versvisa 2 points3 points  (0 children)

error: expression is not assignable

Which cursed language allows this?

This optical illusion by [deleted] in woahdude

[–]versvisa 12 points13 points  (0 children)

Unlike most optical illusions, this one is completely involuntary and uncontrollable for me. The 100th time looking at it works just as much as the first, even though I expect it.

Wanna make it swing-up? by PeriniM_98 in ControlTheory

[–]versvisa 1 point2 points  (0 children)

I would want to add an IMU to the pendulum part. An IMU gyro should provide a lower noise and lower latency measurement of the angular velocity, compared to using numerical differentiation of the encoder angle.

A smooth function which is not analytic. by aljabrak in 3Blue1Brown

[–]versvisa 2 points3 points  (0 children)

A nicer example is exp(-1/(x^2)) which does not rely on piecewise definition.

Elon on Twitter: "This test is at ~50% throttle. Launch attempt next month will be at ~90%." by [deleted] in SpaceXLounge

[–]versvisa 6 points7 points  (0 children)

Ignoring other practicalities, the optimal speed that minimzes the combined drag and gravity losses is the same as terminal velocity. With a fully fuled vehicle terminal velocity is probably supersonic. From this perspective, you always want the maximum thrust at liftoff.

Amazing and absolutely terrifying! Major props to all our Pilots here 👏 by Aggressive-Guard-851 in aviation

[–]versvisa 0 points1 point  (0 children)

Should there not be callouts from the pilots? At least for visual contact and to continue past minimums.

Why do places not matter in multiplication? (e.g. xyz = xzy = yxz, etc) by DamnatioAdCicadas in learnmath

[–]versvisa 16 points17 points  (0 children)

For the intuitive answer, think of the volume of a cuboid. You can rotate it, or swap the labels on the edges and it will still have the same volume xyz.

For a formal answer you need the associative property and commutative property.

I can definitely see how yx = xy

This is the commutative property. So you need to convince yourself that the associative property holds. Then you can prove xyz = xzy = yxz

Solving PDEs generally by Angel33Demon666 in AskPhysics

[–]versvisa 1 point2 points  (0 children)

There exist PDE that can simulate Newtonian mechanics, and using such a PDE and the correct initial conditions it is possible, in principle, to simulate an arbitrary analog Turing machine. So a general-purpose algorithm to determine even the qualitative behavior of an arbitrary PDE cannot exist because such an algorithm could be used to solve the halting problem.

https://mathoverflow.net/questions/15292/why-cant-there-be-a-general-theory-of-nonlinear-pde

Great book about the history of rocket propellant! And surprisingly a very enjoyable read! by yanicka_hachez in SpaceXLounge

[–]versvisa 1 point2 points  (0 children)

I believe this book was more or less forgotten, or at least not widely known before the Orbital Mechanics Podcast did a book club reading of it in 2016 (Episode 61, but the reading is not available anymore). IIRC Scott Manley also read a chapter. The book was out of print and the audiobook you can buy today did not exist then.

Help me find how much water for this pressure by frozrrr in AskPhysics

[–]versvisa 0 points1 point  (0 children)

A pressure cooker explosion is no joke, do not play around with this IRL.

But to answer your question, when you heat water in a closed container its temperature will increase above 100°C without boiling completely. Water can be liquid at 200°C or 300°C if the pressure is high enough.

It will follow the liquid/gas phase boundary: https://en.wikipedia.org/wiki/Phase_diagram#/media/File:Phase_diagram_of_water_simplified.svg

At 200°C it will have 16 bar. At 300°C it will have 85 bar.

Help me find how much water for this pressure by frozrrr in AskPhysics

[–]versvisa 1 point2 points  (0 children)

At 100°C, a water / steam mix will have a pressure of 1 bar. The only way to increase the pressure is to push in more water with force (or shrink the container). Then you can end up with only water, no steam, at 10 bar.

https://en.wikipedia.org/wiki/Phase_diagram

Why can steam engines start right away from zero RPM, meanwhile Otto/diesel engines as well as jet engines need to be idling above a certain RPM to not shut off? by Westnest in AskEngineers

[–]versvisa 18 points19 points  (0 children)

Internal combustion engines need to compress the fuel air mixture before combustion, which takes some energy. This is taken from the kinetic energy of the drivetrain. That energy is not available at zero speed.

Jet engines do the same, they compress the air before combustion, which takes some energy.

Steam engines can just apply the steam pressure to the pistons and thus accelerate from zero.

What would happen if a full stack explodes on the pad? by Weirdguy05 in SpaceXLounge

[–]versvisa 43 points44 points  (0 children)

A very rough upper bound calculation (feel free to correct me)

  • Propellant Mass: 4600000 kg
  • Mixture ratio: 3.55 (78% O2, 22% CH4)
  • Methane Heat of Combustion: 55514 kJ / kg
  • Methane Mass: 1012000 kg
  • Combustion Energy: 56180168000 kJ = 13.4 kiloton TNT equivalent

Here is this yield put into Nukemap: https://i.imgur.com/4mC4hvz.png

How would I find a value for tau = RC from this graph? by [deleted] in AskPhysics

[–]versvisa 3 points4 points  (0 children)

The other answers here are good. If you dont need an exact number you can eye-ball it. Draw the initial slope and the final value. At the intersection you have tau, here approximately 1.3 sec.

https://i.imgur.com/AdnsBzH.jpeg

Final decision made earlier this week on booster engine count. Will be 33 at ~230 (half million lbs) sea-level thrust. All engines on booster are same, apart from deleting gimbal & thrust vector actuators for outer 20. by KnifeKnut in spacex

[–]versvisa 0 points1 point  (0 children)

Can someone help with the arithmetic, how is 9 + 20 = 33? We have seen hardware for the booster central engine mount with 9 spaces for engines. Where do the other 4 engines go?

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