For Questions That Don't Merit Their Own Thread

[kerbiloid]

On 2/12/2024 at 8:08 PM, TheSaint said:

Because if Three Mile Island never happened, then nobody is going around saying, "We prevented Three Mile Island from happening." Because it never happened.

The real curse of the time machine travellers.

You jump into future. You prevent a catastrophe. The catastrophe has never happened. They say "what catastrophe? you are nothing!", if not arrest you for tresspassing the perimeter.

People remember only failed attempts.

13 hours ago, ARS said:

bombing from higher altitude than that does no increase in penetration capability? (since the terminal velocity remains the same

That's why they add solid boosters to the penetrator bombs.

Edited February 14 by kerbiloid

[DDE]

3 hours ago, kerbiloid said:

That's why they add solid boosters to the penetrator bombs.

I had a huge post about it, but decided to trim it.

The original high-altitude BetAB-500 didn't have a booster even though WWII BetABs did. The booster reappears on the BetAB-500ShP, because it was equipped with a ballute for low-altitude deployment. There's no advantage in stated penetration.

[kerbiloid]

I meant https://en.wikipedia.org/wiki/Matra_Durandal  for example.

[DDE]

6 hours ago, kerbiloid said:

I meant https://en.wikipedia.org/wiki/Matra_Durandal  for example.

Quote

Designed to be dropped from low altitudes, the bomb's fall is slowed by a parachute.

 

[kerbiloid]

2 hours ago, DDE said:

 

After slowing by the parachute (to prevent the plane damage and to increase the accuracy by making trajectory less horizontal), is accelerated by the booster.

[ARS]

Could submarine share targeting data with each other's sonar? Like when two submarines operate together to hunt enemy sub and they know each other's position, if for example, sub A activate sonar pings (and sub B knows that sub A is about to do active pinging), could sub B use A's echo to locate the enemy sub?

[darthgently]

2 hours ago, ARS said:

Could submarine share targeting data with each other's sonar? Like when two submarines operate together to hunt enemy sub and they know each other's position, if for example, sub A activate sonar pings (and sub B knows that sub A is about to do active pinging), could sub B use A's echo to locate the enemy sub?

Military sub's sonar operators already have the skills and equipment to make use of passive sonar techniques which take advantage of known sound sources other than the sub's own ping.  I gather it is largely an art with a good dose of science

[TheSaint]

9 hours ago, ARS said:

Could submarine share targeting data with each other's sonar? Like when two submarines operate together to hunt enemy sub and they know each other's position, if for example, sub A activate sonar pings (and sub B knows that sub A is about to do active pinging), could sub B use A's echo to locate the enemy sub?

I don't know how far it has gone, since I've been out of the submarine force for thirty years, but when I was in I know there were discussions about using drone subs to carry active sonar away from the main sub. So you would launch a drone sub out of your torpedo tube (swimout launch, obviously, since an impulse launch would give your location away), drive it a good distance away from the sub, and then the drone would start pinging active sonar which the parent sub would use to pick up contacts.

As for two subs communicating underwater, the only way, currently, to communicate would be with acoustic communications, like Gertrude or JANUS. The problem with those is that they can be detected by passive sonar, so by transmitting information between two friendly submarines they would be giving their positions away to enemy submarines. In a science fiction scenario you could be communicating using a tight-beam blue laser communicator, but that isn't reality yet.

[ARS]

2 hours ago, TheSaint said:

I don't know how far it has gone, since I've been out of the submarine force for thirty years, but when I was in I know there were discussions about using drone subs to carry active sonar away from the main sub. So you would launch a drone sub out of your torpedo tube (swimout launch, obviously, since an impulse launch would give your location away), drive it a good distance away from the sub, and then the drone would start pinging active sonar which the parent sub would use to pick up contacts.

As for two subs communicating underwater, the only way, currently, to communicate would be with acoustic communications, like Gertrude or JANUS. The problem with those is that they can be detected by passive sonar, so by transmitting information between two friendly submarines they would be giving their positions away to enemy submarines. In a science fiction scenario you could be communicating using a tight-beam blue laser communicator, but that isn't reality yet.

Interesting. Does using it different (technical-wise) compared to using regular active sonar? Considering the sonar source and receiver is not in the same sub?

[TheSaint]

19 minutes ago, ARS said:

Interesting. Does using it different (technical-wise) compared to using regular active sonar? Considering the sonar source and receiver is not in the same sub?

That I don't really know. I was a nuke, not a sonarman.

[DDE]

On 2/19/2024 at 4:44 PM, ARS said:

Could submarine share targeting data with each other's sonar? Like when two submarines operate together to hunt enemy sub and they know each other's position, if for example, sub A activate sonar pings (and sub B knows that sub A is about to do active pinging), could sub B use A's echo to locate the enemy sub?

There are two strong examples already. One, the towed sonar array is almost a mile behind the sub and so already this poses a noticeable challenge in signal handling... that has evidently been surmounted. Two, some of the sonobuoys are active emitted-only, so again, they can use an off-board emitter.

Additionally, underwater digital communication by sound is a thing, so it's not difficult to create a sensor-sharing datalink.

[kerbiloid]

Thousands of sound columns at the bottom, repeating as a chorus any message they hear.

Try to understand, where is the real sub.

[DDE]

Look at me

I'm a Foxhound now

https://aviationweek.com/shownews/singapore-airshow/boeing-boasts-near-mach-3-top-speed-f-15ex

[tomf]

Where are upper stages launched into editorial orbits disposed of? The plane change delta v for a launch from kourou for example is going to be too large to allow disposal in the usual south Pacific graveyard. Do the stages aim for the equatorial Pacific and hope not too hit anything?

Edited February 26 by tomf
Wow that was bad spelling

[kerbiloid]

Kraken eats them on passing above the 0° / 0° coordinates due to the not fixed real number rounding issue.

 

 

[AckSed]

Could someone check my assumptions here? I'm trying to work out Photonic Laser Thrusters (PDF) (really quick explanation: form an optical cavity in between source and mirror with gain medium to bounce laser back and forth thousands of times, creating useful thrust at mirror) and how they respond to added mass.

The 1 gigawatt laser is turned off when it is 30,000 km away from the beaming station, due to attenuation. I am assuming the beam is firing from the Moon and the receiving craft is in orbit around the Moon, so it is travelling at ~1600 m/s.

The PDF gives the example of a future craft with 1-ton total mass, 50-metre mirror and 500kg payload, has an acceleration of 32.27 m/s² and is within range of the beaming station for 1.2 hours, or 4320 seconds. It exerts a force of 3227N, and final velocity is 141km/s.

With a 10-ton total mass, the same mirror, and 9.5 tons of payload, that acceleration is reduced by a factor of ten to 3.227 m/s², because a = F / m.

Here's the assumption: I can apply the same force for longer on a greater mass, to gain the same final velocity once it passes beyond range of the laser applying motive force, correct? Obviously forces when leaving orbit are more complex as they have to take a vector, but to a naive approximation, is this correct?

[darthgently]

Makes sense to me.  I'm wondering about having the laser and a big fission reactor mounted on a chemical rocket that launched from the Moon so as to keep the sail in range and chased the sail as the laser craft accelerated until eventually the sail grew too distant.  The rocket and laser acceleration on the sail should be additive, short of relativistic speeds, right?

[AckSed]

It should, but there's no free lunch in that case. Sure, you're pushing further, but you now have to get the beaming station back into place, and even with a small, efficient chemical rocket that's a lot of fuel. Allowing it to be pushed back into position by the laser (because both sides of the cavity are bouncing photons back and forth) halves the force.

This is what solar sails would excel at. Either acting as a statite at L1/L2/L5, providing opposing force and maybe solar power for the beam, or used for the return trip. Diffractive sails are a fascinating variation that provide "a component of force perpendicular to the sun line, thereby allowing navigation without sacrificing the amount of solar power on the sail."

You could use boring solar-powered Hall-Effect Thrusters too. If it's staying in one place, there's no drop in power.

Edited February 28 by AckSed

[DDE]

Wake up, everyone! New orbital drop pod style has just... dropped.

Now, it's clear someone merely focused on creating a quick-disembark pod that doesn't produce much "clutter" on the ground, and so the character just rises out of the grand standing upright like a damned Myrmidon.

However, thing is, that's a legit penetrator-style lander right there, and you don't see that in fiction very often, if at all. Two sets of penetrators have flown on failed Martian probes. However, literature still seems to be generated for them in 2020s on both sides of the pond.

https://luna1.diviner.ucla.edu/~dap/pubs/109.pdf

https://www.energia.ru/ktt/archive/2022/02-2022/103-117.pdf (exactly what is a hyperspeed penetrator?)

Let's be charitable and assume the character isn't simply standing inside the pod. Is there any benefit to lithobraking like that, beside reducing the retroburn dV (and thus, potentially, AA exposure when it's a factor)? All the papers I see mostly cite low size/cost and instant digging-in of your geological instruments.

[K^2]

I think we'd need to attach some numbers to it, and it's not trivial. The first question I have is how fast the capsule can hit the ground in order to stop without burring itself far below the surface, because there's a limit to that. Both from the type of surface you hit and from the material properties of the capsule.

The second question I'd have is if we do find a sweet spot for the speed at which the capsule buries itself most of the way in, but leaves an exit on the surface, how big of a crater is that going to dig? Which, again, might depend on the type of soil you're hitting.

Even without putting numbers to it, I think it's pretty clear that the method is far from universal. Hitting a swamp might be comparable to hitting water, and hitting hard rock is unlikely to give you any significant penetration without going far beyond what any theoretical material you could use for capsule can withstand. (We're not even talking about the pilot here.) But if we kind of ignore the fact that it's a game where you need to be able to plunk the exit point anywhere, and go with a more realistic, "Yeah, we're going to be picky about the landing site," something like sand or soil might be in the right ballpark. That said, the entry velocity will be moderate, far below orbital, meaning we will need to rely on another method of braking. It could still be entirely passive, such as aerobraking, or rely on the retrorockets of some sort. Either way, having a number for velocity on impact might give us an idea of whether it gives you AA evasion benefits or not.

I'll try to do a quick scan through literature, because the naive thoughts I had on how to estimate the impact speed for given penetration depths are not giving me anything reasonable. There might be good models for soil as loose particles that should be appropriate here.

[kerbiloid]

15 hours ago, DDE said:

what is a hyperspeed penetrator?

Him:

Spoiler

 

15 hours ago, DDE said:

quick-disembark pod

It:

Spoiler

 

 

[JoeSchmuckatelli]

Would a false roof (shade roof) make a difference in the cost to air condition (cool) a large warehouse like building (ESA French Guyana facility)? 

I know that the challenge is the ambient air temperature - which shading won't improve - but it could reduce thermal heating. 

I clearly don't know the answer - but the fact that I don't see large buildings with false / shade roofs does suggest the answer. 

 

Anyone know Fer Shure? 

[Terwin]

Adding insulation on the inside is probably cheaper and can be just as effective, if not more effective if you need it.

Structural components are very expensive and require maintenance, but insulation is cheap and only requires care if it gets wet in most cases.

 

[darthgently]

Can anyone here ID the make and model of this craft?  It occurred to me to ask one the bigger pools of flight nerds around (typed amiably)

 

 

[K^2]

On 3/6/2024 at 10:29 AM, darthgently said:

Can anyone here ID the make and model of this craft?

MD-88