For Questions That Don't Merit Their Own Thread

[kerbiloid]

48 minutes ago, magnemoe said:

I see two things here, the supernova and the nebula. Chinese documented the supernova, did the Islamic and Japanese describe an nebula or an supernova. 

No. The Japanese and Islamic records are 200 years late, so definitely not from the witnesses, but from some other elder scrolls, or just from head.

The record from the Chinese chronics belongs to their 52nd volume (which is too weird itself, if remember the fate of the medieval long reads, including the Chinese ones), and are in the raw with flying dragon riders and peasant riots having 2Mhuman armies, which is absolute nonsense for the feudal epoch of hundreds-thousands in the best case. So, even if the record is not fake itself, its date is probably of same accuracy as other "facts" from it.

The 1054 officially belongs to the https://en.wikipedia.org/wiki/Liao_dynasty

which is a dynasty of Kithan nomad conquerors, later conquered by Jurchen nomad conquerors, so it's a bit brave to consider the chronicles feeling untouched, when they were being actively modified everywhere even in more calm times.

48 minutes ago, magnemoe said:

Europeans was late and might only discover it after telescopes? 

The supernova is said to be clearly visible (some say, even in the day light), so the telescopes were required to see the nebula (and it was done in late XVIII).

48 minutes ago, magnemoe said:

But why is it tragic is used for calibration, except that the nebula would expand. 

Because they refer to the very doubtful (if not taken from air) date as to a valid timestamp of a reference point.

By dividing its current size (d = 11 ly) by its current expansion speed (1 500 km/s = 1 / 200 c), we can get 1 300 y of its expansion at the constant speed.
But obviously, in the beginning (first 100 y? 200? 300?) it was much faster.

As it was found in late XVIII, we can presume that its expansion is currently more or less linear, with established speed.

So, we can guess that the supernova exploded 800..1200 years ago, in 800..1200 AD, but its exact year and even century is blurry, but put into astrophysical calculations (its expansion model, also used as a model for others), and a reference point to date some historical events.

In any case, they are always saying with so much serious face "it exploded in 1054", like it indeed did.

Edited May 28 by kerbiloid

[RunaDacino]

How much fuel is "wasted" while a rocket is held by launch clamps in real life?

[Terwin]

35 minutes ago, RunaDacino said:

How much fuel is "wasted" while a rocket is held by launch clamps in real life?

Doesn't the quick disconnect mitigate this to some extent?

While I suspect they cannot pump the fuel into the tanks as fast as the engines pump it out, hopefully they can at least reduce the losses due to hold-down and engine stabilization.

[StrandedonEarth]

38 minutes ago, RunaDacino said:

How much fuel is "wasted" while a rocket is held by launch clamps in real life?

Considering that the Saturn V had a TWR of around 1.1 at liftoff, burning some off during startup was pretty much necessary in that case.

[darthgently]

1 hour ago, RunaDacino said:

How much fuel is "wasted" while a rocket is held by launch clamps in real life?

I think if you are holding it down for a full duration all-systems test then 100% would be used.  But you'd need to ask an expert on full duration all-systems tests of orbital class rockets to get the very deep knowledge on this arcane subject.  I believe there is one that one reads these forums on occasion

[Jacke]

10 hours ago, RunaDacino said:

How much fuel is "wasted" while a rocket is held by launch clamps in real life?

It's not wasted propellants.  The time to hold the rocket is based upon making sure the engines ignited correctly and are now burning properly and stably.  If there's an issue detected, the control system on the rocket automatically shuts down the engines.

[mikegarrison]

On 5/28/2024 at 7:39 AM, kerbiloid said:

So, we can guess that the supernova exploded 800..1200 years ago, in 800..1200 AD, but its exact year and even century is blurry, but put into astrophysical calculations (its expansion model, also used as a model for others), and a reference point to date some historical events.

The supernova SN1054 exploded about 7500 years ago. However, the light reached Earth about 1000 years ago.

[kerbiloid]

1 hour ago, mikegarrison said:

The supernova SN1054 exploded about 7500 years ago. However, the light reached Earth about 1000 years ago.

As an astrophysical object for the external observer - yes.

As a colloquially simplified description of the astronomical phenomenon by the local observer, it has rapidly appeared on the sky when its light had reached us them the earthians.

[magnemoe]

On 6/1/2024 at 3:32 AM, Jacke said:

It's not wasted propellants.  The time to hold the rocket is based upon making sure the engines ignited correctly and are now burning properly and stably.  If there's an issue detected, the control system on the rocket automatically shuts down the engines.

SpaceX hold down on Starship is 3 seconds after the launch program I just saw, this this is around standard time. And as you say its to verify that all engines and systems are operating correctly.
And as starship first stage will burn for 2:45 or 165 seconds, its 1.8% burn time before release, now raptors uses some time to spin up so its probably more like 1.5%. 
Even if you don't check engine status like first stage is only solid you still want hold down both to keep the rocket in place and to make sure it don't lift up before twr is high enough. 

[mikegarrison]

Think of it this way:

When a pilot loads fuel onto the plane, the pilot has to include a certain amount of fuel for the taxi to the runway. This is why the certified "ramp weight" of an airplane is actually higher than their certified maximum takeoff weight, because otherwise they would never be able to take off at MTOW unless they were refueled actually on the runway.

If a rocket is designed to lift off with a certain load of fuel, and it is known that the launch procedure requires a few seconds of engine stabilization before liftoff, then you just design the tanks a little bigger to include this pre-launch fuel burn. Yes, ideally you would want to avoid that extra dry mass of the slightly larger tanks, but we don't live in an ideal world.

So the rocket isn't actually lifting off with less fuel than it was designed to lift off with. It's just lifting off with slightly bigger tanks than are strictly needed to hold the flight fuel.

[K^2]

If cross-feed was as easy as it is in KSP, you could just add external tanks that stay with the launch hardware....

[DDE]

1 hour ago, K^2 said:

If cross-feed was as easy as it is in KSP, you could just add external tanks that stay with the launch hardware....

... which, I believe, has even been done, although I'll need to re-read half of Sutton to find the citation.

[kerbiloid]

9 hours ago, mikegarrison said:

If a rocket is designed to lift off with a certain load of fuel, and it is known that the launch procedure requires a few seconds of engine stabilization before liftoff, then you just design the tanks a little bigger to include this pre-launch fuel burn. Yes, ideally you would want to avoid that extra dry mass of the slightly larger tanks, but we don't live in an ideal world.

It's even worse.

The rocket tanks are welded from sheets of standard size, so the tank consists of cylindric rings of same diameter, and one or two heights.
The tank heads have standard shape, thus their volume is fixed for this tank diameter.

Thus:

• any fuel tank just approximates the required volume with some volume excess;
• the fuel:ozidizer tank volumes never match the engine's component ratio;
• in any case you have to fill at least one tank of two not fully, but to some level, to match the engine's component ratio;
• you actually do this with both tanks;
• if you have some excessive propellant, you can burn it out before liftoff, by throttling the engines;
• if you are a Soviet rocketeer, you can just spit out excessive oxidizer as a beautiful orange cloud.

So, for every flight you recalculate the exact mass of the fuel you need from the actual payload mass, and the excessive fuel is just several centimeters higher level of the liquids in the tanks, which is anyway arbitrary.

Edited June 6 by kerbiloid

[RunaDacino]

For interplanetary ejection burns in real life,
1. is this table applicable - obviously with different values? 

2. If it is...

Can someone please explain the physics of it? It goes against what I know of the Oberth Effect.

[Terwin]

3 hours ago, RunaDacino said:

For interplanetary ejection burns in real life,
1. is this table applicable - obviously with different values? 

2. If it is...

Can someone please explain the physics of it? It goes against what I know of the Oberth Effect.

This sounds like a balancing act between 'starting' velocity(aka orbital altitude) and the oberth effect.

If you assume that the initial velocity is 'free' then there is a point where additional 'free' velocity is less advantageous than the oberth effect for the acceleration needed to achieve the new trajectory.

On the other hand, if you are starting from the ground, there is no 'free' starting velocity other than the rotational velocity of the surface, which is pretty minor, making 'not hitting the ground or enough atmosphere to matter, but just by a hair'  the ideal orbital altitude so as to maximize the oberth effect.

 

[NFUN]

Do fermions mediate pairing of bosons?

[Kerwood Floyd]

3 hours ago, NFUN said:

Do fermions mediate pairing of bosons?

This sounds like the physicists' version of "Do bears [poop] in the woods?"  

 

[Jacke]

5 hours ago, NFUN said:

Do fermions mediate pairing of bosons?

It's the other way around.  The exchange particles of quantum fields are I believe all bosons.  In certain circumstances, pairs of fermions can act together as a boson.

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

[K^2]

1 hour ago, Jacke said:

The exchange particles of quantum fields are I believe all bosons.

Forces can be carried by any particle, so long as they interact with the given charge. Gauge bosons are simply guaranteed to be always there for a given symmetry group (and therefore given conserved charge). The main limitation is that rest mass of the particle limits its range, which is why massless photons and gravitons have an "infinite" range (only affected by the inverse square law) while massive bosons of the weak interactions have a very short range. At very short range, fermion exchanges are a factor. Nuclear forces can, in some models, be described as a virtual pion exchanges, where each pion is a quark-antiquark pair.

So while it's often said for simplicity that gauge bosons are the mediating particles for a given force, in practice things are a lot muddier.

The simple thing to keep in mind, though, is that the quantum picture of the force exchange is just sending out parcels of momentum along with a particle. The classical analogy is two people on sliding platforms throwing a ball back and forward. It will cause the platforms to accelerate from each other, as each person catches the ball, then reverses its direction, resulting in a recoil. The time-averaged interaction is identical to the two people pushing away from each other. The "fun" thing about virtual particles is that they can move in direction not matching their momentum, which allows an attractive force instead of a repulsive, for example. The details of how that works and why that limits the interaction range for particles with non-zero rest mass has to do with mass shells and the math of the propagators (i.e. Green's functions) and that gets mathy, but the bottom line is that the specific nature of the particle isn't important. So long as you can emit a particle that carries momentum, you can have an interaction force. Pairing up fermions into bosons is still helpful here too, though, which is why it's the pion exchange that is involved in nuclear forces and not individual quarks, but that has to do with fermion vs boson statistics and gets relevant in decay processes, and we're getting into the weeds again.

[magnemoe]

On 6/6/2024 at 4:24 AM, K^2 said:

If cross-feed was as easy as it is in KSP, you could just add external tanks that stay with the launch hardware....

The problems with large orbital rockets is that the liquid flow is multiple tons / second at high pressure, think an small hydroelectrically plant, not fueling an airplane. For an modest moon lander you could use drop tanks as the fuel flow is more like an large aircraft. Look at the pipes on the Saturn 5. The mass flow make shutting down engines hard. 

[K^2]

3 hours ago, magnemoe said:

The problems with large orbital rockets is that the liquid flow is multiple tons / second at high pressure, think an small hydroelectrically plant, not fueling an airplane. For an modest moon lander you could use drop tanks as the fuel flow is more like an large aircraft. Look at the pipes on the Saturn 5. The mass flow make shutting down engines hard. 

Yeah, I'm vaguely aware of the problems. People generally underestimate how bad the hammering can get when you have to get that much fluid moving that fast. So it's not just the plumbing itself, but all the valve operations that become incredibly complex. Shuttle tank separation is a fun topic to read about, and there they had time for a multi-stage process, separating oxidizer and fuel lines separately, and there was no need to re-route - the engine was killed as part of the process. Doing this while an engine is running and timed to a clamp release? I don't know where to even start. You'd need multiple pumps, diverters, pyrotechnics, and it might still take longer than the standard clamp hold time at the switchover, making the whole operation moot.

[kerbiloid]

Meanwhile somewhere are quietly weeping the Breeze and Frigate rocket stages.

https://en.wikipedia.org/wiki/Briz_(rocket_stage)

http://www.khrunichev.ru/main.php?id=49

https://russpace.ucoz.ru/index/razgonnyj_blok_briz_m/0-66

Spoiler

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

Spoiler

P.S.
Funny fact: the inner part of Breeze (that 2.5 m) is/was the post-boost vehicle of the UR-100N ICBM and/or LV based on it, i.e. the jettisonnable torus tank was added later, to put it on Proton.

[DDE]

34 minutes ago, kerbiloid said:

Funny fact: the inner part of Breeze (that 2.5 m) is/was the post-boost vehicle of the UR-100N ICBM and/or LV based on it, i.e. the jettisonnable torus tank was added later, to put it on Proton.

Isn't it the Naryad ASAT, though?

[kerbiloid]

1 hour ago, DDE said:

Isn't it the Naryad ASAT, though?

Iirc, yes. The UR-100N is a universal thing. One of the IS/US launch vehicles, together with UR-200 and R-36.

[kerbiloid]

Previous time - Notre Dame de Paris, this time - Versailles.

Time to update the scenery?