For Questions That Don't Merit Their Own Thread

[Serenity]

Is there a video or a post explaining how a rocket is kept in the launchpad and what holds it right before the engines fire and eventually what happens at those moments?

I am interested more in general but if its like for a specific rocket its okay eitherway.

Edited June 5, 2021 by Serenity

[Shpaget]

Explosive bolts. They blow themselves up when the time to go up comes up.

[AHHans]

21 minutes ago, Shpaget said:

Explosive bolts. They blow themselves up when the time to go up comes up.

And if you are the Space Shuttle then the SRBs have enough thrust to rip apart some (or all?) of the bolts if they are too slow to let go.
(And AFAIK the mass of the whole shuttle was essentially supported by the SRBs already on the ground.)

But I'm sure there are also lots of other mechanical contraptions that release once the force goes from "umbilical supporting the rocket" to "rocket pulling up on umbilical". I don't know if Scott Manley ever made a video especially about this, but that would be the place where I would start looking.

[wumpus]

5 hours ago, AHHans said:

And if you are the Space Shuttle then the SRBs have enough thrust to rip apart some (or all?) of the bolts if they are too slow to let go.
(And AFAIK the mass of the whole shuttle was essentially supported by the SRBs already on the ground.)

The SRBs contributed ~80% of the thrust (the diameter of the SRBs were carefully tapered to have a "pre-planned throttle schedule", while I don't think the SSMEs can throttle at all.  Kind of the reverse of KSP), so they essentially supported the shuttle until they were jettisoned.

[SpaceFace545]

2 minutes ago, wumpus said:

I don't think the SSMEs can throttle at all

They throttle between 67% and 109%.

[Serenity]

Thanks for the replies, i tried finding videos but not luck, maybe i am looking too specific and its somewhere in a different titled video.

I am so curious to see up close how these bolts hold the rockets, i can't recall seeing any images or videos of that, i mean generally not the moment they launching.

Edited June 6, 2021 by Serenity

[kerbiloid]

Google, images, "pyrotechnic fastener"

[Serenity]

1 hour ago, kerbiloid said:

Google, images, "pyrotechnic fastener"

Thank you that got me closer to what i was looking for.

A fast search with that and already found a cool video.

 

[AHHans]

17 minutes ago, Serenity said:

A fast search with that and already found a cool video.

Damn! I love the Space Shuttle! It is a horrible design in way too many ways, but it just looks cool!

[Serenity]

15 minutes ago, AHHans said:

Damn! I love the Space Shuttle! It is a horrible design in way too many ways, but it just looks cool!

Yeah i agree, i think science needs to be also cool and not be 100% perfect, like not everything has to make sense, some things are there to inspire us and give us

the motivation to make more, SpaceX is doing it for sure but there are so many people oh this doesn't make sense, this has no such purpose, this is not designed for that.

Edited June 6, 2021 by Serenity

[ARS]

So... Today I got 2 things to ask: First, is there any limit on the size of the moon a planet can have? Is it possible that the moon of a planet is larger than the planet it orbits? For instance, if the planet has heavier mass despite it's smaller in terms of size than the moon? And second, is it possible for a moon's orbital period and characteristic to perpetually place it on 'eclipse' position on the planet (in other words, the moon is always between the star and the planet, so there's always a total solar eclipse on illuminated side of that planet)

[DDE]

1 hour ago, ARS said:

Is it possible that the moon of a planet is larger than the planet it orbits? For instance, if the planet has heavier mass despite it's smaller in terms of size than the moon?

No. Size is utterly irrelevant for gravity-related issues. At some point the moon is so heavy the barycenter is outside the planet's radius, e.g. Charon,

Spoiler

but the heavier of the two bodies in such a binary is, by definition, the parent.

1 hour ago, ARS said:

And second, is it possible for a moon's orbital period and characteristic to perpetually place it on 'eclipse' position on the planet (in other words, the moon is always between the star and the planet, so there's always a total solar eclipse on illuminated side of that planet)

That would be the L1 point. It's naturally unstable, it's not what we would traditionally consider an Earth orbit - most spacecraft end up in a halo or tadpole orbit around said point

Spoiler

finding any sort of a major moon (big enough to cause an eclipse) there would be... suspicious.

Spoiler

 

[Admiral Fluffy]

Would it be possible to land a rover in the polar regions of Venus, and have it survive longer than it would otherwise? Because logic dictates that the polar areas would be colder.

[Spacescifi]

20 minutes ago, Admiral Fluffy said:

Would it be possible to land a rover in the polar regions of Venus, and have it survive longer than it would otherwise? Because logic dictates that the polar areas would be colder.

 

What do you know about Venus?

 

Did you know Venus is blanketed by clouds that rain acid? Everywhere? Even the poles?

 

I cannot say I know the definitive answer, but given that, I do not think it would make a lot of difference.

 

Venus is so hot precisely because of it's cloud blanket that absorbs and retains so much solar heat.

[DDE]

19 minutes ago, Admiral Fluffy said:

Would it be possible to land a rover in the polar regions of Venus, and have it survive longer than it would otherwise? Because logic dictates that the polar areas would be colder.

Venus's souposhere is a major problem: it's so thick there's no real difference between day and night. It, and the non-existent axial tilt, mean Venus have no seasons.

As a consequence, I don't expect a meaningful temperature difference.

[AHHans]

3 hours ago, ARS said:

First, is there any limit on the size of the moon a planet can have?

Yes. If the size of the moon is so large that it extends to the L1 point then it will shed material to the parent body. That's something that actually happens in binary stars that get close enough to merge eventually (Roche lobe overflow). Under the "right" circumstances this can cause a Type Ia supernova.

The other questions were already answered by @DDE

 

[Snark]

1 hour ago, Admiral Fluffy said:

Would it be possible to land a rover in the polar regions of Venus, and have it survive longer than it would otherwise? Because logic dictates that the polar areas would be colder.

Basically no.  As @DDE points out, there's not much temperature variation over the surface.  Looks like altitude is more of a factor than latitude.  Consider this:

https://www.esa.int/ESA_Multimedia/Images/2006/12/Temperature_maps_of_Venus_surface

...it's not full-globe coverage, but you can see that the entire temperature span of their scale is only 20 degrees C, and the closest spot to the pole that they show is actually where one of the "hot" spots is.

[magnemoe]

On 6/5/2021 at 6:30 PM, SpaceFace545 said:

They throttle between 67% and 109%.

That was the shuttle main engines who could throttle as you say.  Solid fuel engines can not throttle at all but they tend to be designed with an trust profile who reduces trust over time as they become lighter. 
Some SRB typically ICBM can stop trust with blow out pannels who work a lot like flight termination systems but more controlled. 
This is very useful on an ICBM as you might want to attack an target much closer than maximum range, but is pointless on an orbital rocket with an liquid fueled upper stage there the spare performance simply leaves some fuel in upper stage as margin.

[K^2]

10 hours ago, magnemoe said:

That was the shuttle main engines who could throttle as you say.  Solid fuel engines can not throttle

wumpus actually said SSMEs in their post, so it sounds like they meant main engines, but maybe that was a mistake, because I don't know why somebody would think that SSMEs couldn't throttle.

[wumpus]

9 hours ago, K^2 said:

wumpus actually said SSMEs in their post, so it sounds like they meant main engines, but maybe that was a mistake, because I don't know why somebody would think that SSMEs couldn't throttle.

Because it is completely atypical (67% is extraordinarily deep for a pumped engine.  And the thing also has all the complexity of a full flow engine).  But certainly for the mission (lighting at sea level and proceeding nearly to orbital velocity) throttling should help.  The really irritating thing about the SSME is that it couldn't relight (and they didn't feel safe lighting it in flight) when it was pretty  clear that would make a lot more sense for SLS.

On 6/5/2021 at 6:53 AM, AHHans said:

And if you are the Space Shuttle then the SRBs have enough thrust to rip apart some (or all?) of the bolts if they are too slow to let go.
(And AFAIK the mass of the whole shuttle was essentially supported by the SRBs already on the ground.)

Pretty sure they would light the SSME (because they could turn it off, and it wouldn't rip any bolts while on) first, then the SRBs.  Hopefully it could survive one SRB burning away on the pad with the other a dud, but I suspect that's the "name elementary schools after you" condition that Chris Hadfield was afraid of.

[AHHans]

21 minutes ago, wumpus said:

Pretty sure they would light the SSME (because they could turn it off, and it wouldn't rip any bolts while on) first, then the SRBs.

Well, yes. If that isn't already general knowledge, then it is also visible at 3:05 in the video that @Serenity linked yesterday: you can see two SSME light in the upper right corner of the film well before the SRBs light.
[...]
And now that I re-watched that: in the next clip - where the SSME are in the upper left corner - you can see that the SSMEs start to push the orbiter up before the SRBs light.

44 minutes ago, wumpus said:

Hopefully it could survive one SRB burning away on the pad with the other a dud, but I suspect that's the "name elementary schools after you" condition that Chris Hadfield was afraid of.

I couldn't find anything I'd consider a reliable source, but this and this internet "discussions" both came to the same conclusion that it was more feasible to engineer the SRBs so that they don't fail to ignite rather than figuring out how to stop or hold down one of them.

[Kerwood Floyd]

Can anyone help me? Sometime in the last week or so, I saw a picture of a computerized control panel for an auxiliary diesel generator.  As I remember it, the diesel was on a submarine and the picture was posted in one of the threads on the Science & Spaceflight sub-forum. But now I can't find it. Can anyone point me to it?  Thanks

 

[K^2]

1 hour ago, wumpus said:

Because it is completely atypical (67% is extraordinarily deep for a pumped engine.  And the thing also has all the complexity of a full flow engine).  But certainly for the mission (lighting at sea level and proceeding nearly to orbital velocity) throttling should help.  The really irritating thing about the SSME is that it couldn't relight (and they didn't feel safe lighting it in flight) when it was pretty  clear that would make a lot more sense for SLS.

That deep of a throttle is, indeed, very atypical, along with many other atypical features of SSMEs. But any liquid fuel engine will have some throttling capability. It'd be more effort to build it as purely on/off than having ability to tune the fuel and oxidizer flows individually, and that just naturally gives you a throttle range. Which is why I was surprised to hear you expecting that particular engine not to throttle at all. It would be perfectly reasonable to suspect a shallow throttle for that engine, but when you're riding up with a solid booster, even a shallow throttle can make a big difference in being able to correct for small deviations from nominal in booster thrust and weather conditions.

That said, I don't want to make it sound like this should all be obvious as day to everyone. I found the statement odd and surprising, but maybe that says more about me than anyone else.

 

On a slight tangent, as far as the reasons they wanted a deep throttle, which certainly would have been a major design challenge, I suspect it has to do with inability to relight engines. I strongly suspect that shuttle throttles way down close to the booster burn-out to reduce the G forces and/or aerodynamic forces. If they could relight the engines, they'd probably opt to just kill one of the three instead and re-light it after booster separation. Would it have been easier to design shallow throttle engines that you can re-light? Possibly, and these would probably be two competing designs at some point, and somebody somewhere probably thought that deep throttle would be more versatile of the two.

The fact that thrust goes down to 2/3 also feels almost intentional here. Almost like they weren't sure they'll be able to get that deep throttle, and the rest of the planning was done under assumption that they might switch to engine shut-down instead. But this one's total speculation. Might be just coincidence.

[SunlitZelkova]

How is lunar base resupply supposed to work without reusable landers? In the case of the Artemis program, all of the cargo landers are non-reusable (excluding Starship).

What is the distance you would need to land away from the base and other landers to avoid causing damage? For the purposes of this question, in a regular LM.

[kerbiloid]

Depends on how deep they are going to go in the lunar base project. So, on how much the ISRU is envolved.
Basically, from the lunar ice.

And this depends on how much they are going to gain from the invested funds. Currently it looks like not much, until they get able to develop a whole industry.

But the reusability is not required for the simple supplies delivery to a small outpost base.

 

3 hours ago, SunlitZelkova said:

What is the distance you would need to land away from the base and other landers to avoid causing damage? For the purposes of this question, in a regular LM.

Another planet, and preferrably with atmosphere.
As the orbital speed is ~1.7 km/s, escape speed 2.4 km/s, while the exhaust speed ~3.5 km/s. So, a shower of small ejecta from landers can reach LEO in one direction and leave the Hill sphere of the Earth in the opposite one.

And as there is no air drag between the Moon and the LEO, the LEO object can be hit at ~3 km/s speed by a stone bullet from the Moon.

So, only crater walls and concrete plates can prevent the shrapnel bursts and protect everything around.