Atmospheric reentry

[timfnj]

Getting up to orbital heights seems easy; getting down again is giving me galloping fits. Force wall linked to gliding/flight on a continuing charge? any ideas?

[ghost-angel]

Re: Atmospheric reentry

 

Safe Environment: Heat & Gliding would probably cover it for most games.

[Sean Waters]

Re: Atmospheric reentry

 

LS: Breathing vacuum might also help

 

Basically you need to cover the lack of air, the cold of space, the massively increased solar radiation, the heat of re-entry and the 'smacking into the ground' aspects and I think you've got it covered. The last bit either needs impressive defences or gliding/flight or a triggered TP with 'no relative velocity'.

[CptPatriot]

Re: Atmospheric reentry

 

According to Star Hero, page 284:

Vehicles entering an atmosphere from space experience heat as the result of friction generated by moving into/through air. Entering an atmosphere causes a ship to suffer 5 BODY damage per Phase; therefore, 5 DEF (or other defenses providing equivalent Resistant Defense) protects the ship from that effect.

[Sean Waters]

Re: Atmospheric reentry

 

There you have it, folks!

 

I might quibble with the damage (but then I always do) and I've never really understood why entering the atmosphere should cause any more friction than flying through it for a character. Most vehicles get th efriction thing because they are moving so damn fast in relation to the atmosphere, but there is no reason for something just dropping to earth at a more sedate pace to encounter any heat problems at all.

[Tech]

Re: Atmospheric reentry

 

According to Star Hero' date=' page 284:[indent']Vehicles entering an atmosphere from space experience heat as the result of friction generated by moving into/through air. Entering an atmosphere causes a ship to suffer 5 BODY damage per Phase; therefore, 5 DEF (or other defenses providing equivalent Resistant Defense) protects the ship from that effect.[/indent]

I might quibble with the damage (but then I always do) and I've never really understood why entering the atmosphere should cause any more friction than flying through it for a character. Most vehicles get th efriction thing because they are moving so damn fast in relation to the atmosphere' date=' but there is no reason for something just dropping to earth at a more sedate pace to encounter any heat problems at all.[/quote']

 

 

Player: I'm carefully re-entering the atmosphere.

GM: You take 5 Body damage.

Player: But I'm only going 20 mph!

GM: Sorry, take your damage. The book says so.

 

[Sean Waters]

Re: Atmospheric reentry

 

Don't even joke. I know people who would do that and not think it funny at all. Does seem weird though that a light kevlar longcoat, gloves, boots and balaclava and you can survive the heat of atmospheric reentry.

[Marcus Impudite]

Re: Atmospheric reentry

 

Wait a minute, so you're telling me that a character wearing a 5 DEF armored spacesuit could concievably survive atmospheric reentry? (provided, of course, they have a parachute or some other means to safely decend once inside the atmosphere)

[Nolgroth]

Re: Atmospheric reentry

 

Wait a minute' date=' so you're telling me that a character wearing a 5 DEF armored spacesuit could concievably survive atmospheric reentry? (provided, of course, they have a parachute or some other means to safely decend once inside the atmosphere)[/quote']Where was that link to the orbital skydiving article? Oh yes, here it is.

[Marcus Impudite]

Re: Atmospheric reentry

 

Where was that link to the orbital skydiving article? Oh yes' date=' here it is.

 

Wow.

 

Note to self: maybe I should renew that subscription afterall...

[Matt the Bruins]

Re: Atmospheric reentry

 

Wait a minute, so you're telling me that a character wearing a 5 DEF armored spacesuit could concievably survive atmospheric reentry? (provided, of course, they have a parachute or some other means to safely decend once inside the atmosphere)

 

The shuttle reaches about 1700°C during re-entry, which would be midway between the melting point of steel (4d6 K heat damage) and the top temperatures of a blast furnace (6d6 K heat damage). Based on that you should need at least 18 DEF to make it down safely, and 30 would give a lot more peace of mind.

 

The write-up on Life Support says Safe Environment: Intense Heat won't protect against being dunked in lava, and that's about 2/3 the temperature we're talking about. I wouldn't let that do the trick, although maybe the version in Galactic Champions protects against more extreme conditions.

[prestidigitator]

Re: Atmospheric reentry

 

Then again, the kind of heat the shuttle takes on during re-entry wouldn't even be generated if you were using a few inches of Hero System Gliding, so if we really want to get down to the nitty-gritty....

[Tom Carman]

Re: Atmospheric reentry

 

Then again' date=' the kind of heat the shuttle takes on during re-entry wouldn't even be generated if you were using a few inches of Hero System Gliding, so if we really want to get down to the nitty-gritty.... [/quote']

 

Thanks to the design decision to save a bit of money by carrying propellants in an expendable external tank, the shuttle is a rather dense vehicle. This is also what made those fragile super-heat-resistant tiles a requirement. If the tanks had been internal, the shuttle would be larger and somewhat more expensive, but also more robust and not need such extreme thermal protection.

[SteveZilla]

Re: Atmospheric reentry

 

Where was that link to the orbital skydiving article? Oh yes' date=' here it is.

 

Joseph Kittinger skydived from the edge of space back in the 1960's. He have the benefit of starting at rest relative to the ground instead of hurtling into the atmosphere at orbital speeds. While falling, he did break the sound barrier (which is slower at high altitude), but never Mach 2. A controlled rentry from orbit means speeds around Mach 25 -- just a tad bit faster.

[SteveZilla]

Re: Atmospheric reentry

 

Thanks to the design decision to save a bit of money by carrying propellants in an expendable external tank' date=' the shuttle is a rather dense vehicle. This is also what made those fragile super-heat-resistant tiles a requirement. If the tanks had been internal, the shuttle would be larger and somewhat more expensive, but also more robust and not need such extreme thermal protection.[/quote']

 

Actually, I would expect the shuttle to be less dense than Mercury, Gemini, and Apollo command capsules because of the huge cargo bay. It was resuability that made the silica foam tiles a requirement, not excessive heat. Reentry of the command capsules generated much higher heat than the shuttle does upon it's reentry.

 

From: http://en.wikipedia.org/wiki/Atmospheric_reentry

 

Thermal soak TPS is intended to shield mainly against heat load and not against a high peak heat flux (a long duration heat pulse of low intensity is assumed for the TPS design). The Space Shuttle orbit vehicle was designed with a reusable heat shield based upon a thermal soak TPS.

 

It should be emphasized that the tradeoff for TPS reusability is an inability to withstand a high heat flux, e.g. a Space Shuttle TPS would not be practical as a primary thermal protection for lunar return. A Space Shuttle's underside is coated with thousands of tiles made of silica foam, which are intended to survive multiple reentries with only minor repairs between missions.

[Tom Carman]

Re: Atmospheric reentry

 

Actually, I would expect the shuttle to be less dense than Mercury, Gemini, and Apollo command capsules because of the huge cargo bay. It was resuability that made the silica foam tiles a requirement, not excessive heat. Reentry of the command capsules generated much higher heat than the shuttle does upon it's reentry.

 

From: http://en.wikipedia.org/wiki/Atmospheric_reentry

 

I would consider all of those to be dense. Command capsules were simply crew-return craft, without engines or fuel tanks or even much life support. The shuttle may be a little less dense, but not really comparable to a complete (with tanks) spacecraft. Such a vehicle, with more drag-area for its mass, would slow down at higher altitude in less-dense atmosphere.

[SteveZilla]

Re: Atmospheric reentry

 

I would consider all of those to be dense. Command capsules were simply crew-return craft' date=' without engines or fuel tanks or even much life support. The shuttle may be a little less dense, but not really comparable to a complete (with tanks) spacecraft. Such a vehicle, with more drag-area for its mass, would slow down at higher altitude in less-dense atmosphere.[/quote']

 

IIRC, it turns out that using a refurbishable crew-capsule that not only doesn't contain the main fuel tanks, but also doesn't enclose the payload being delivered is the least expensive way to get man & material into orbit. The crew capsule would need to use an ablative type shield like for Apollo, but there's no reason why that couldn't be replaced during the refurbishment prior to the next launch.

 

Drag compared to overall mass is IMO more a function of how it's designed than just it's size. After all, the Square-Cube law says that as surface area doubles, volume (and likely mass) quadruples.

[Wyrm Ouroboros]

Re: Atmospheric reentry

 

Still doesn't matter. Orbital speeds are major; hitting more than a few atoms per cubic meter when you're doing 18,000 mph means major heat generation, and whatever’s necessary to avoid it. Avoiding the generation of that heat is the first and easiest way to avoid the heat. Shuttles, capsules, meteors, earth-killing asteroids, the Hulk -- none of them have any way of controlling their descent, so they need a way to deal with the generated heat, thus heat shields, tiles, boiling rock, and 20 DEF.

 

If you have powered Flight -- not Gliding, because Gliding is just ‘falling with style’, and you need air to use it (can’t glide in a vacuum, y’know?) -- you don’t have a problem. Move at zero speed relative to ground, control your descent into the atmosphere, and you’ll be saying, "Heat? What heat??"

[SteveZilla]

Re: Atmospheric reentry

 

Still doesn't matter. Orbital speeds are major; hitting more than a few atoms per cubic meter when you're doing 18' date='000 mph means major heat generation, and whatever’s necessary to avoid it. Avoiding the [i']generation[/i] of that heat is the first and easiest way to avoid the heat. Shuttles, capsules, meteors, earth-killing asteroids, the Hulk -- none of them have any way of controlling their descent, so they need a way to deal with the generated heat, thus heat shields, tiles, boiling rock, and 20 DEF.

 

If you have powered Flight -- not Gliding, because Gliding is just ‘falling with style’, and you need air to use it (can’t glide in a vacuum, y’know?) -- you don’t have a problem. Move at zero speed relative to ground, control your descent into the atmosphere, and you’ll be saying, "Heat? What heat??"

 

A "Zero Speed Reentry From Orbit*" is not doable with current technology -- unless you want to build a full Saturn V sized rocket just to go to low earth orbit & back.

 

*Meaning that the ship thrusts to remove all orbital speed and then falls under normal gravity into the atmosphere at a "slow" speed.

[Fabricati]

Re: Atmospheric reentry

 

Yes, but the physics work out, so if someone conceivably matched rotation speed with the Earth via a Flight power, it's more than conceivable. It'd probably require some major MegaScale and/or noncombat doubling, but I could see it happening.

 

Just saying.

[prestidigitator]

Re: Atmospheric reentry

 

Hmm. There was an experimental sub-orbital craft that exited and entered the atmosphere at close to normal cruise speeds for ordinary aircraft I believe. Can't remember many details. I'll have to do some searches. Though it didn't have to reach orbital energy levels, it sounded like a nifty project.

[SteveZilla]

Re: Atmospheric reentry

 

Hmm. There was an experimental sub-orbital craft that exited and entered the atmosphere at close to normal cruise speeds for ordinary aircraft I believe. Can't remember many details. I'll have to do some searches. Though it didn't have to reach orbital energy levels' date=' it sounded like a nifty project.[/quote']

 

Key phrase: sub-orbital. Meaning not to orbital speed.

 

But was it the hypothetical trans-sonic airplane that "skips" along the upper atmosphere during it's cruise?

[Sean Waters]

Re: Atmospheric reentry

 

The speed of various objects entering the atmosphere is kinda a function of what they are doing re-entering the atmosphere. A meteor wil be travelling through space at a hell of a clap, so will hit the atmosphere as if it is hitting a wall. Orbital vehicles - well - orbit - which means that, even if stationary with relation to a point on the earth below, they are moving at a huge speed in fact and the atmosphere they enter may not be moving in the same direction, let alone be at relative zero speed.

 

OTOH if Hulk did a mighty leap, popped out into space and fell back immediately, he would have no problem with heat as he is at relative rest compared tot eh atmosphere.

 

Now we may not have the tech to acheive a re-entry at low speed relative tot eh atmosphere, but in a superhero game, all it would take is someone who can 'see' the air and has enough NCM flight to move along with it.

 

Looking at atmospheric reentry heat damage, the Star Hero 5 points per phase is a bit simplistic. Most things heat up i.e. transmit heat through themselves if they do not melt or ablate away. Heat energy remains in what it affects, so the continued application of heat does more damage than multiple 'instant' applications would suggest.

 

I'd say that heat damage should start at 1DC eKilling and rise at 1DC per phase of re-entry to a maximum level that is determined by the mass (or possibly density - have to think about that) of the object, possibly modified by any aerodynamic considerations. I'd think something like the space shuttle should probably be taking around 15-20DCs of eKill before it slows enough to start cooling, but that is just eyeballing it.

 

I might build an ablative heat shield as relatively low resistant defence +additional BODY only useable to resist atmospheric re-entry heat damage (not an efficient build, but probably reasonably accurate). Or resistant energy defence with one continuing charge. That could work.

[McCoy]

Re: Atmospheric reentry

 

There you have it, folks!

 

I might quibble with the damage (but then I always do) and I've never really understood why entering the atmosphere should cause any more friction than flying through it for a character. Most vehicles get th efriction thing because they are moving so damn fast in relation to the atmosphere, but there is no reason for something just dropping to earth at a more sedate pace to encounter any heat problems at all.

 

Still doesn't matter. Orbital speeds are major; hitting more than a few atoms per cubic meter when you're doing 18' date='000 mph means major heat generation, and whatever’s necessary to avoid it. Avoiding the [i']generation[/i] of that heat is the first and easiest way to avoid the heat. Shuttles, capsules, meteors, earth-killing asteroids, the Hulk -- none of them have any way of controlling their descent, so they need a way to deal with the generated heat, thus heat shields, tiles, boiling rock, and 20 DEF.

 

If you have powered Flight -- not Gliding, because Gliding is just ‘falling with style’, and you need air to use it (can’t glide in a vacuum, y’know?) -- you don’t have a problem. Move at zero speed relative to ground, control your descent into the atmosphere, and you’ll be saying, "Heat? What heat??"

What he said. The friction is generated because of atmospheric braking, using air resistance to shed excess speed. Current technology, spacecraft are as aerodynamic as possible to avoid air resistance during launch, non-aerodynamic to optimize air resistance during re-entry. IIRC, you have a couple hundred miles to go from >Mach 25 to

 

If you have some sort of non-newtonian powered flight, like many superheros, a controlled reentry with no aerobraking should not generate excessive heat.

[prestidigitator]

Re: Atmospheric reentry

 

This was it.

 

http://www.thespacereview.com/article/73/1