A revival of interest in Nuclear Thermal Propulsion?

SpaceNut · 2020-11-08 21:45:40

The numbers for the Vasimr engine does not work out as we have that topic here as well.

edit
I see you found the solar powered topic

The ion engine works as its a heavy mass being accelerated not one that has next to nothing for mass.

The energy for disassociation does not make it any better as you must super cool it once more to make use of it in a regular engine.

tahanson43206 · 2020-11-09 07:33:39

For SpaceNut re #26

Your post contains some intriguing content which could be developed further if you have time.

The difference between propellant used in an ion engine, and that used in a VASIMR engine would be interesting to explore.

Your observation about the need to cool disassociated ions of hydrogen and oxygen from water makes sense. However, space has plenty of "cool", and electricity can be generated as heat flows from a hot location to a cold one (ie, radiators)

Beyond that, however, your comment inspired me to wonder if there is a reason Argon and Helium are preferred for use in ion engines. Both these neutral atoms are ionized before they can be accelerated, which implies (to me at this point) they are heated to the point they cannot hold onto their electrons.

Is there a reason Oxygen and Hydrogen atoms cannot be used as the propellant for an ion system?

If they ** can ** be used, then water becomes a candidate to hold a supply of propellant.

Since water can be disassociated using electrolysis, it may turn out that electrolysis is more efficient as a mechanism than disassociation by heating, and there would presumably be less stress on the reactor, which would be producing simple steam to drive a generator.

I hope that this discussion will be of interest to someone who has the required education and experience.

Edit#1: As a reminder, the benefit of disassociating water while a vessel is in flight is that the output is a propellant that can be accumulated over time, until it is needed for a massive thrust event, such as "docking" with Mars. Any of the ion engine designs are constant thrust designs that CANNOT perform large scale thrust events.

Edit#2: The paper at the link below contains an explanation of why one ion might be chosen over another:
http://adsabs.harvard.edu/full/2000ESASP.465..833K

The paper includes the statement that any ion can be used for propulsion. However, inert gases are more easily ionized, because their (presumably outermost) electrons are more lightly bound, and less energy is required to free them.

In addition, the mass of the ion is a factor in selection. Xenon (atomic number 54) has a mass of 131, which is greater than Argon (18/40).

However (returning to my theme) if a nuclear reactor is providing energy for ion generation, then the more readily available water molecule might be well worth considering as a propellant storage medium.

Question arising from above: Can water molecules be ionized?

Per Google: Molar mass of water is 18.01528 grams/mol

Per Google: Molar mass of Xenon is 131.293 grams/mol

Asking Google the question: how to ionize water molecule

Search Results
Web results
Self-ionization of water - Wikipediaen.wikipedia.org › wiki › Self-ionization_of_water
The self-ionization of water (also autoionization of water, and autodissociation of water) is an ionization reaction in pure water or in an aqueous solution, in which a water molecule, H2O, deprotonates (loses the nucleus of one of its hydrogen atoms) to become a hydroxide ion, OH−.
Equilibrium constant · Isotope effects · Mechanism · Relationship with the ...

Asking Google about ionization:

Ionization energy - Wikipediaen.wikipedia.org › wiki › Ionization_energy
In physics and chemistry, ionization energy (American English spelling) or ionisation energy (British English spelling), is the minimum amount of energy required to remove the most loosely bound electron of an isolated neutral gaseous atom or molecule. ... In chemistry, the unit is the amount of energy required for all of the atoms in a ...

It should be possible to discover the amount of energy required to ionize a water molecule as compared to a Xenon atom.

Since the purpose of this topic is propulsion, I'm hoping an output/result of this inquiry is comparison of thrust of steam (made by combustion of Oxygen and Hydrogen in a chemical process) with the thrust of hydroxide ions propelled by magnetic acceleration in an ion engine.

A Google search: can we use hydroxide ions n an ion engine

yielded a large number of results about various designs for ion engines.

(th)

SpaceNut · 2020-11-09 19:45:34

Thrust is the pressure at which the exhaust exits the nozzle....

Diy Steam Rocket Engine First Thrust Test and Build - YouTube

Rocket Thrust Equations

https://www.physicsforums.com/threads/t … et.745947/

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

tahanson43206 · 2020-11-09 19:55:42

For SpaceNut ... (re #28) ....

Are these equations applicable to the ion engine?

Ions are accelerated by magnetic fields so they are moving far faster than any chemical reaction could drive them.

How might one measure "pressure" in an ion engine?

Mass flow rate would be knowable for an ion engine.

Exit velocity would be knowable.

Is "temperature" a meaningful value for the output of an ion engine?

Having asked those questions, I note that this topic ** is ** about a Nuclear Thermal rocket, which imparts heat to a working fluid.

In a Nuclear Thermal rocket, all the parameters you listed would seem to apply.

(th)

RobertDyck · 2020-11-09 21:06:47

Ion Propulsion Math

tahanson43206 · 2020-11-10 06:43:10

For RobertDyck re #30

Thanks!

http://newmars.com/forums/viewtopic.php … 13#p173813

SearchTerm:IonPropulsionMath
SearchTerm:PropulsionIonMath
SearchTerm:MathIonPropulsion

(th)

tahanson43206 · 2020-11-10 08:21:20

Right on time! Here is an update on development of an advanced thermal rocket

https://www.yahoo.com/finance/news/ther … 00114.html

Caroline Delbert
Mon, November 9, 2020, 6:06 PM EST
This company's ceramic-coated pellet fuel is low enriched, safer, and more stable.
Ultra Safe Nuclear Corporation (USNC) has designed a new thermal nuclear engine it says could carry astronauts to Mars in just three months—and back to Earth in the same amount of time. By using ceramic microcapsules of high assay low enriched uranium (HALEU) fuel, USNC's thermal nuclear engine could cut the trip in half even from optimistic estimates.

“The problem is to produce a nuclear reactor that is light enough and safe enough for use outside the Earth's atmosphere—especially if the spacecraft is carrying a crew,” New Atlas explains.
Thermal nuclear for propulsion is an old idea. While weapons are thermal, other applications have lingered in the experimental stage and then been discarded, but they’ve still been studied off and on for decades. These designs use the astonishing heat generated by a nuclear reaction to push a rocket at speeds approaching the Star Trek realm compared to what we use today. And they contrast with traditional chemical rockets, where chemical propellants like liquid oxygen are used to make something more like a supersized fossil fuel combustion engine.

(th)

GW Johnson · 2020-11-10 09:14:39

If you want to fly now, you build the 1974 NERVA design. It was ready to fly then, it will be ready to fly now, if you simply build it to the design and specs used then.

If you want to fly later, perhaps some years later, then develop and test an improved design. It is well known that such will work. It was known at the time of NERVA that such would work, but it was also known that it required significant development and testing before it would be ready to fly. That requirement before flight has not changed.

If you are smart, and your pockets are deep, do both. Get to flying with the old design, while also doing the development and test work to make the improved design flyable. It's nuclear, it will be expensive. So just deal with it.

If your pockets are not deep enough, you will have to stretch-out the development and testing of the newer design from a few years to many years. Not very smart technically, but absolutely required from a financial viewpoint. But you can also still fly the old design today.

If the politics is against testing nuclear rockets on Earth anymore, then move that operation to the moon, where there is no air or water to pollute, and no neighbors to annoy. (You cannot do this work flying around in space, you need a stable test stand that does not move when you fire the rocket. And that ugly little fact of life will never change! ) There is no better reason to go back to the moon.

All of that's the reality here. I know. I have done rocket development work. Doesn't matter what kind, it's fundamentally all the same, only the price tag varies with the type.

GW

Last edited by GW Johnson (2020-11-10 09:19:07)

Oldfart1939 · 2020-11-10 09:32:51

I agree with GW here; the NERVA design was functional and resulted in an Isp of 900 sec. The Isp would possibly improve as the temperature of the heating element increases, and new alloys or construction techniques could result in a higher value. But increasing temperature should increase exhaust velocity, and hence, Isp.

tahanson43206 · 2020-11-10 09:52:28

It will take demonstration of NERVA (or their equivalent) by a global competitor (such as China to be specific) to overcome US objections to developing the technology. However, my recollection (my memory of readings has faded) is that the original NERVA design was good for only a few hours of operation before it had to be completely rebuilt.

The ceramic pellet design might be more cost effective over the long run.

There is an opportunity for a forum contributor with an interest in the subject and a bit of free time to find out which of the two designs is more advantageous for China to develop.

Russia could have pulled it off in earlier times, but my impression is that Russia has too many other irons in the fire to work on this now.

Edit#1: India could take the lead here .... it has nuclear power and it has a robust space program.

(th)

RobertDyck · 2020-11-10 12:43:47

NERVA specification dated 9/8/70 (is that Sept 8 1970, or 9 Aug 1970?)
Page 34 (PDF page 35)

3.1.2.3.1.1.1 Operating Service Life - The engine shall be capable of operating for a minimum of 600 minutes accumulated in multiple burn cycles up to 60 of varying length up to one hour for normal mode operations and TBD hour for single turbopump operation, at a nominal thrust chamber temperature of 4250°R. The engine shall be capable of the duty cycles as specified in Table IV. The engine shall be capable of completing any single mission as specified in 1.1 (Mission Definitions) under the malfunction conditions specified in 3.1.1.1.1.2 (Malfunction Mode).

Page 10 (PDF page 11)

Specification No. CP-90290A
Section 1. SCOPE I
This part of this specification defines the requirements for the performance, design, and qualification of equipment identified as the NERVA Nuclear Rocket Engine, Contract End Item (CEI) No. 90290 as established by the NERVA Program Requirements Document, SNPO-NPRD-1. This CEI, herein-after referred to as the engine, is used as a source of primary propulsive power for both manned and unmanned space vehicle applications. The engine is designed to operate at a vacuum thrust level of 75,000 lbs and a specific impulse of 825 seconds and shall be man rated. The engine requires externally supplied liquid hydrogen, command signals, and electrical power. Rated thrust is achieved at a nominal thrust chamber pressure of 450 psia and a nominal design thrust chamber temperature of 4250° Rankine and with a nozzle having an expansion ratio of 100:1. Endurance at rated temperature shall be 600 accumulated minutes. The operating time is utilizable in multiple cycles up to 60 with durations of varying lengths up to 60 minutes.
1.1 Mission Definitions - The following missions are used in the definition of NERVA requirements. Payloads shall be maximized consistent with the engine performance requirements of this specification.
(a) Reusable Interorbit Shuttle - To shuttle payloads (manned and unmanned) between a 262 nautical mile earth orbit and a space station in lunar or geosynchronous earth orbit and return for reuse.
(b) Unmanned Deep-Space Injection - To place a large unmanned payload on a deep space trajectory using the reusable nuclear shuttle from 262 nautical mile earth orbit and returning the shuttle vehicle to earth orbit for reuse.
1.2 Launch Vehicle Definition - The engine shall be capable of being launched into earth orbit by an INT-21 (SIC/SII) launch vehicle modified for a nuclear third stage.

Last edited by RobertDyck (2020-11-10 13:02:21)

GW Johnson · 2020-11-10 12:51:16

The US nuclear rocket effort began in the early 1950's, with actual firing tests by about 1959. This culminated in the NERVA design that was flight ready by about 1974. It was killed by the executive order of President Nixon restricting manned spaceflight to Earth orbit. NASA then killed NERVA because "who needs the rocket if we aren't going to go?" (meaning to Mars, not just the moon).

The CIA briefly revived the NERVA idea, looking for an improved version to fling very large objects orbitally and suborbitally, as one-shot (non-reusable) firings. I think that was the genesis of Timberwind, which could only be run once. That was in the 1990's, I believe.

The Russians built and ground-tested a nuclear thermal design in the 1990's as well. They never apparently flew it. At the time, the radiation releases from those tests were mistaken for a "nuclear explosion MHD generator" as the power source for a particle beam weapon. As it turned out, these releases were really just plumes from nuclear rocket tests.

If you want the truth about Project Rover that gave us Phoebus, Kiwi, and finally NERVA, read the book published by one of the few surviving engineers who actually worked on these things. That would be "Nuclear Thermal Propulsion Systems" by David Buden, published by Polaris Books as book 2 of a 4-book series on space nuclear stuff. I got mine when I met Buden at the Mars Society convention in Dallas, Texas, a few years ago. He was in his 80's then.

To the best of my knowledge, there have been only paper studies done since. No experimental work at all.

GW

PS -- Project Pluto was the nuclear thermal ramjet project that was also ground-tested in the 1960's, at the same Jackass Flats, Nevada, test site as the Project Rover nuclear thermal rockets were ground-tested. It was intended for an infinite-endurance Mach 3 (at very low altitudes) nuclear cruise missile. Turns out, the radioactive plume and the strong Mach 3 shock wave so close to the ground would have killed more people on its way to its target than its multi-megaton warhead would have killed at the target. So, fortunately, it was never built and fielded. Nor even flight-tested.

Last edited by GW Johnson (2020-11-10 12:57:21)

SpaceNut · 2021-02-14 20:23:35

Space Nuclear Propulsion Technologies central to future of Mars Exploration

space-nuclear-propulsion-for-human-mars-exploration-marker-hg.jpg

https://www.nap.edu/catalog/25977

https://www.nationalacademies.org/our-w … chnologies

The fundamental challenge facing an NTP system is the ability to heat its propellant to the proper temperature, approximately 2,700 K. Other challenges include the long-term storage of liquid hydrogen in space with minimal loss; the need to rapidly bring an NTP system to full operating temperature, preferably in under one minute; and the need to develop full-scale ground test facilities that can safely capture the NTP exhaust.

Oldfart1939 · 2021-04-11 17:05:16

Seems as though DARPA is interested enough to pony up some cash for a space capable nuclear rector for propulsion:
https://spacenews.com/general-atomics-w … -the-moon/

Quaoar · 2021-04-12 04:22:15

I agree to GW: if we want to fly now, the only way is to build the old NERVA. But if the problem is to develop a more advanced evolution without contaminate the environment, I suggest the vapor core option,

https://www.google.com/url?sa=t&rct=j&q … LYs2231OZ0

where uranium tetrafluoride vapors are sealed inside carbon composite cells, which are cooled by hydrogen propellant. In this way the propellant never touch the fuel, like in the solid-core and in gas-core NTR designs, and the exhaust is completely free of radioactivity. This hybrid approach can reach an Isp up to 1200 s.

Probably it will take less years and dollars to develop a vapor core on Earth than to build a moon base (where on the other hand we can develop a more advanced and performing gas-core).

Last edited by Quaoar (2021-04-12 04:27:08)

Oldfart1939 · 2021-04-12 09:47:24

The timeline in the previously referenced article is: orbital testing by 2025.

The name of the project is DRACO; Demonstration Rocket for Agile Cislunar Operations.

Last edited by Oldfart1939 (2021-04-12 09:50:04)

Calliban · 2021-04-12 10:09:38

A nuclear powered lower stage for a super booster taking off from and landing in the ocean, would be interesting idea. The booster could use water as propellant. Propellant temperature and exhaust velocity are less important in this application, as propulsive efficiency is poor at submach speeds and expansion ratio is poor at sea level. Of more importance is thrust and acceleration. The booster needs to push the upper stage into the stratosphere with enough residual energy to counteract gravity losses as the upper stage accelerates to orbital velocity.

So we need a cheap thrust producer that ascends vertically, reaching an altitude of perhaps 60,000' before separating the upper stage at a velocity of about 1km/s. It then descends and lands roughly where it started from. If the landing is uncontrolled and it breaks up, then the nuclear components remains intact and sink into the ocean. The ocean provides a heat sink for decay heat removal and and radiation shield between flights.

The use of a heat exchanger may in fact be possible in this more limited application. The reactor would most likely be a lead cooled fast reactor using triso fuel. Water could be heated in steam generators to ~1000°C before entering the expansion nozzle. Total delta-V - about 2km/s, some of which would be reserved for controlled landing.

Last edited by Calliban (2021-04-12 10:16:12)

tahanson43206 · 2021-04-12 10:35:48

For Calliban re #42

SearchTerm:Nuclear propulsion first stage from ocean using ocean water as propellant with controlled landing for re-use Post #42 above Calliban
SearchTerm:Reusable nuclear propulsion
SearchTerm:Launch competitive from Earth using nuclear fission from ocean site with reusable design and failsafe option

(th)

Oldfart1939 · 2021-04-12 10:42:15

The proposal is for an upper stage and use as a shuttle-tug between Earth and the Moon. The thought of using a nuclear powered first stage is an anathema to most engineers. The concept mentioned in the proposal is one remaining either in orbit about the Earth or around the Moon, and with robotic refueling in orbit.

Quaoar · 2021-04-12 10:58:43

Calliban wrote:

A nuclear powered lower stage for a super booster taking off from and landing in the ocean, would be interesting idea. The booster could use water as propellant. Propellant temperature and exhaust velocity are less important in this application, as propulsive efficiency is poor at submach speeds and expansion ratio is poor at sea level. Of more importance is thrust and acceleration. The booster needs to push the upper stage into the stratosphere with enough residual energy to counteract gravity losses as the upper stage accelerates to orbital velocity.
So we need a cheap thrust producer that ascends vertically, reaching an altitude of perhaps 60,000' before separating the upper stage at a velocity of about 1km/s. It then descends and lands roughly where it started from. If the landing is uncontrolled and it breaks up, then the nuclear components remains intact and sink into the ocean. The ocean provides a heat sink for decay heat removal and and radiation shield between flights.
The use of a heat exchanger may in fact be possible in this more limited application. The reactor would most likely be a lead cooled fast reactor using triso fuel. Water could be heated in steam generators to ~1000°C before entering the expansion nozzle. Total delta-V - about 2km/s, some of which would be reserved for controlled landing.

First stage needs a strong thrust rather than high exhaust velocity, so chemical propellants like LOX-RP1 or LOX-CH4 are perfect, considering that a hydrocarbon chemical rocket has a thrust/weight ratio far greater than a NTR.
A NTR makes sense for the second stage, which needs acceleration lower than 1 gee (typically 4-5 m/s2), where an exhaust velocity of 9-10 km/s consents a lower mass ratio, even with a heavier NTR and a bigger hydrogen tank, increasing the payload fraction.

And there are also some problems in using a nuclear first stage:

1) in case of incident all the launch facility would be contaminate
2) neutron scattering due to the dense atmosphere would hit the astronauts even if the rockets has a shadow shield

Last edited by Quaoar (2021-04-12 11:05:18)

GW Johnson · 2021-04-12 15:16:35

There is an oft-forgotten characteristic of solid-core nuclear thermal devices that influences very strongly when, and when not, to use them. The startup transient is not as fast as chemical propulsion, but the real problem is the long slow shutdown to preserve core cooling after you cut the nuclear power. That makes this type of propulsion infeasibly inappropriate for short, sharp burn requirements , like course corrections.

No one ever actually built a gas core thermal engine, but there were two design approaches, quite different: (1) the "nuclear light bulb", which had an actual physical wall, or walls, between the core fireball and the propellant flow, and (2) the open cycle device which had core fireball and propellant flow in the same chamber.

If in the open cycle the propellant/uranium flow ratio was 1000:1 or better, that was as good as perfect containment. But you lose a core at every shutdown. And the exhaust stream is rather radioactive, with reaction products. However, specific impulse potential was said to be quite high: up to 2000-2500 s Isp with regenerative cooling only, or 6000+ s Isp with a massive waste heat radiator and active closed-loop cooling. Startup and shutdown transients would be fairly short and sharp. These were rather lightweight designs, not a whole lot heavier than a chemical engine.

The nuclear light bulb designs had exhaust streams mostly free of radioactivity, and a more limited specific impulse. It was said to be about 1300 s Isp. Startup was fairly quick, but shutdown slow (faster than solid core). These were somewhat heavier, but still far lighter than solid core. You did not lose your core overboard at shutdown like open cycle. But it does take time and a lot of energy to recreate the fireball.

As far as I know, multiple solid core designs were built and tested. Names like Kiwi, Phoebus, and NERVA come to mind. NERVA was the flight-ready one. To the best of my knowledge, Timberwind was a paper design, never actually built and tested. Seems like there was another improved solid core design, but I cannot remember the name.

I know the Russians built and tested some solid core designs, at a place I think was named Sary Shagan. Don't know much about them. Should have been at least somewhat similar to NERVA, though.

GW

Last edited by GW Johnson (2021-04-12 15:35:26)

Calliban · 2021-04-12 16:25:19

GW Johnson wrote:

There is an oft-forgotten characteristic of solid-core nuclear thermal devices that influences very strongly when, and when not, to use them. The startup transient is not as fast as chemical propulsion, but the real problem is the long slow shutdown to preserve core cooling after you cut the nuclear power. That makes this type of propulsion infeasibly inappropriate for short, sharp burn requirements , like course corrections.
GW

I would hazard to guess that the relatively slow power buildup during startup, is necessitated by the need to allow delayed neutron emitters to buildup. This is important because delayed neutrons increase average neutron lifetime, making power transients controllable. The average lifetime of a scission neutron is on the order of 1E-3 seconds in a thermal reactor and a hundred times shorter in a fast reactor. Without delayed neutrons, power could increase by a factor of 1000 or more in less than a second. In the nuclear industry, this is called a reactivity excursion fault. Light water reactors often include startup sources to ensure that there are enough external neutrons in the core at low power levels to stretch the average lifetime of a neutron sufficient to allow power transients to occur slowly enough to be controllable. Heavy water reactors like CANDU generate neutrons when heavy water is subject to gamma rays from irradiated fuel. Thanks to delayed neutrons, all nuclear reactors run slightly subcritical. If delayed neutrons were to disappear, the fission chain reaction would stop.

Slow shutdown transients could potentially occur due to delayed neutrons continuing fission. However, it is a legal requirement for nuclear reactors to have sufficient worth in their control rods to counteract the effects of delayed neutrons in achieving safe shutdown. So more likely those long transients result from decay heat.

Quaoar · 2021-04-13 08:06:43

GW Johnson wrote:

There is an oft-forgotten characteristic of solid-core nuclear thermal devices that influences very strongly when, and when not, to use them. The startup transient is not as fast as chemical propulsion, but the real problem is the long slow shutdown to preserve core cooling after you cut the nuclear power. That makes this type of propulsion infeasibly inappropriate for short, sharp burn requirements , like course corrections.
GW

In fact, the never built NTR Copernicus used the RCS rockets for course correction and is forced to store tons of NTO-MMH even if its main NTR rockets have an Isp of 920 s.

Last edited by Quaoar (2021-04-19 11:11:12)

GW Johnson · 2021-04-13 16:47:30

I'm no nuclear engineer, so I know little about what Calliban says in post 47 above. I just know it takes a lot of cooling to extract the excess heat from a heavy object like a solid reactor core. That's just heat transfer physics.

Zubrin really is a nuclear engineer. He would probably agree with me and Calliban, plus he would fully understand what Calliban posted.

What Quaoar posted in post 48 above is a nice confirmation of what I said about slow shutdowns and infeasibility for making short, sharp burns with nuclear thermal.

GW

Quaoar · 2021-04-14 06:59:59

GW Johnson wrote:

The nuclear light bulb designs had exhaust streams mostly free of radioactivity, and a more limited specific impulse. It was said to be about 1300 s Isp. Startup was fairly quick, but shutdown slow (faster than solid core). These were somewhat heavier, but still far lighter than solid core. You did not lose your core overboard at shutdown like open cycle. But it does take time and a lot of energy to recreate the fireball.

GW

NASA reference design has a predicted Isp of 1875 s

https://ntrs.nasa.gov/api/citations/196 … 012569.pdf

It has a chamber pressure of 500 atmospheres: what perplexed me is how to maintain the perfect equilibrium between the pressure of the propellant inside the chamber and the fuel inside the nuclear light-bulbs during the start-up, the thrust phase and the cool-down, when it takes only one atmosphere of difference to have the bulbs exploded or imploded.

Last edited by Quaoar (2021-04-14 07:02:02)

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#26 2020-11-08 21:45:40

Re: A revival of interest in Nuclear Thermal Propulsion?

#27 2020-11-09 07:33:39

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#28 2020-11-09 19:45:34

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#29 2020-11-09 19:55:42

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#30 2020-11-09 21:06:47

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#31 2020-11-10 06:43:10

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#32 2020-11-10 08:21:20

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#33 2020-11-10 09:14:39

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#34 2020-11-10 09:32:51

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#35 2020-11-10 09:52:28

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#36 2020-11-10 12:43:47

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#37 2020-11-10 12:51:16

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#38 2021-02-14 20:23:35

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#39 2021-04-11 17:05:16

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#40 2021-04-12 04:22:15

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#41 2021-04-12 09:47:24

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#42 2021-04-12 10:09:38

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#43 2021-04-12 10:35:48

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#44 2021-04-12 10:42:15

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#45 2021-04-12 10:58:43

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#46 2021-04-12 15:16:35

Re: A revival of interest in Nuclear Thermal Propulsion?

#47 2021-04-12 16:25:19

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#48 2021-04-13 08:06:43

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#49 2021-04-13 16:47:30

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#50 2021-04-14 06:59:59

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