Atmospheric reentry

[Sean Waters]

Re: Atmospheric reentry

 

The other thing to bear in mind - and I'm no rocket scientist - is that conventional space travel relies on hitting quite astonishing speeds and coasting into space, presumably because, whilst there is more air resistance, there is also a lot more to push against for your reaction mass lower in the atmosphere and, of course, once you've used your reaction mass you can ditch what it came in, reducing the mass of the vehicle and therefore the amount of energy needed to propel it further.

 

In fact if you had a mode of flight that did not bother with reaction mass - a lot of superhero flight doesn't - then you don't even need high speed to get into space, just so long as you keep climbing, you'll get there eventually. Space starts about 100km up, apparently, with satellites from that height on up, so presumably once you are out in space you can meaningfully achieve orbit.

 

Interestingly enough, to Hero this post up a bit, the most efficient way to get into orbit is probably with a tiny bit of flight (so you don't fall) and teleport, as teleport does not suffer the halving of distance when traveling upwards that flight does.

 

The most efficient way to get down from orbit is flight, as you can double your downward velocity when diving (should probably be subject to a +30" cap) - at least if realism doesn't concern you. If you are worried about that - or worried your GM might be - then teleport (no relative velocity) is probably the most efficient way down too: you'll be at rest relative to whereever you appear.

[Matt the Bruins]

Re: Atmospheric reentry

 

Right... being able to lift yourself off the ground via personal flight at all means you can provide at least slightly more than 1g of sustained upward thrust, which is all that's needed to reach space given enough time (though you won't technically be in orbit since removal of thrust would cause rapid descent). Modern space vehicles can't maintain that kind of thrust for long periods of time—instead they do a 9 minute burn at between 2 and 3gs so that they reach orbital velocity and from that point are "falling" fast enough that they don't actually descend relative to the curvature of the earth.

 

With 2" of 0 END Flight and a means of life support you could eventually reach outer space, though you might want to coordinate with NASA beforehand since everything in orbit is going to be traveling tens of thousands of mph relative to you. Be a shame to go to that much effort and then get turned into pink mist by a loose bolt from a communications satellite plowing into you.

[ghost-angel]

Re: Atmospheric reentry

 

The shuttle also doesn't enter nose-first and let aerodynamics help it avoid friction... It drops belly first, which is where the heat comes from.

 

If you had enough time you could probably drop in aerodynamically to avoid a lot of heat issues, and circle the globe a bunch just slowing down very gradually. Instead of trying to get down fast.

[SteveZilla]

Re: Atmospheric reentry

 

The Shuttle already has an extremely long reentry path. IIRC, to land in Florida it starts entering the atmosphere around Hawaii.

[SteveZilla]

Re: Atmospheric reentry

 

The shuttle also doesn't enter nose-first and let aerodynamics help it avoid friction... It drops belly first' date=' which is where the heat comes from.[/quote']

 

I've read that most of the heat from reentry comes from the compression of the air, and only a minor amount of it comes from direct friction.

 

But the shuttle doesn't have a good enough glide ratio to be able to reenter nose first and glide all the way down. If it tried that, it wouldn't bleed off enough speed before it sank into the denser atmosphere where it's now excessive speed would either cause too much heat or too much airframe stress.

 

If you had enough time you could probably drop in aerodynamically to avoid a lot of heat issues' date=' and circle the globe a bunch just slowing down very gradually. Instead of trying to get down fast.[/quote']

 

I don't think that is currently doable. The upper atmosphere is very thin, and even at high velocity does not make for a good glide medium -- thus, without a source of constant thrust, the ship would sink into deeper atmosphere while retaining too much of it's orbital speed. The wing configuration that could sustain the stress of that high speed would not be a good wing for gliding.

 

but in fiction, all things are possible!

[timfnj]

Re: Atmospheric reentry

 

... If I can get back into this thread without being tacnuked... the question was poised because I was looking for a way to build a mp with two slots on charges; a)M-scale flight to get from ground to orbit,+ a way to get down again . The character has the ls+the def[30p+ed hardened plus 25% dam reduction p+ed] but I'd prefer that he not soak the damage on the way down.

The sfx of my idea was to link a forcewall to gliding (for aerobraking) but the character is up against a hard limit in the campaign on def + teleportation is used by and would step on other pc's.

[Zeropoint]

Re: Atmospheric reentry

 

there is also a lot more to push against for your reaction mass lower in the atmosphere

 

Major physics fail!

 

Rockets do not push against the atmosphere (if they did, they wouldn't work in space). They push against their exhaust, and according to Newton's Laws, the exhaust pushes against them.

[SteveZilla]

Re: Atmospheric reentry

 

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?

 

It really depends upon how movement in a vacuum is handled in your game and if "excessive" speed causes damage to a character in an atmosphere. By just 5ER rules, breaking free of the atmosphere doesn't convey unlimited acceleration and movement of any velocity in air does not cause damage.

 

For unpowered Reentries:

 

A capsule-like reentry would IMO not involve any movement power -- its just falling in a parabolic trajectory -- and would have some sort of ED Armor as the Heat Shield.

 

An Aerodyne-like reentry (like the shuttle) would involve the Gliding power because while its unpowered on it's decent, it is usually a much shallower reentry angle (it covers much more ground distance than a straight parabolic trajectory could), and the craft can change directions in the atmosphere. It too would have some sort of ED Armor as it's Heat Shield, just built differently from the typical ablative shield used on capsules.

 

For Powered Reentries (for the full duration of the reentry):

 

All bets are off if the character/vehicle has essentially unlimited ability to hover. Because then all orbital speed can be eliminated while supporting oneself against gravity, and then "reentry" into the atmosphere could be at sub-sonic speeds, eliminating the need for any heat protection.

 

The main reason why the real world doesn't have Powered Reentry is that to carry enough chemical fuel up into orbit with you to achieve this kind of landing would make the vehicle so huge and expensive that even low Earth orbit would be prohibitively expensive. Chemical Fuel just doesn't have the energy density to make this approach feasible.

[Sean Waters]

Re: Atmospheric reentry

 

Major physics fail!

 

Rockets do not push against the atmosphere (if they did, they wouldn't work in space). They push against their exhaust, and according to Newton's Laws, the exhaust pushes against them.

 

Like I said, I'm not a rocket scientist, but you do get ground effect and you do get a similar effect from chucking mass at something less solid than the ground, like the lower reaches of the atmosphere: reaction mass can bounce. Rocket gasses expand rapidly and as the atmosphere is not completely elastic they expand in the direction of the rocket as well as in all the others, providing some small extra force. Obviously vacuum is, effectively, completely elastic.

 

If you read the rest of the paragraph I hope it is clear that I do appreciate that the major thrust is simply coming from the preservation of momentum, which is why we call it reaction mass.

[Matt the Bruins]

Re: Atmospheric reentry

 

If the 30 DEF is coming from a Force Wall, I don't think you'd take any damage on the way down... STUN doesn't get through at all unless the BODY exceeds the wall's DEF and collapses it, right? A 30 ED Force Wall should be impervious to the maximum amount of BODY re-entry heat could inflict. (Assuming you're traveling at orbital speed–if you're coming in faster than that, all bets are off!)

[Tom Carman]

Re: Atmospheric reentry

 

Like I said, I'm not a rocket scientist, but you do get ground effect and you do get a similar effect from chucking mass at something less solid than the ground, like the lower reaches of the atmosphere: reaction mass can bounce. Rocket gasses expand rapidly and as the atmosphere is not completely elastic they expand in the direction of the rocket as well as in all the others, providing some small extra force. Obviously vacuum is, effectively, completely elastic.

 

If you read the rest of the paragraph I hope it is clear that I do appreciate that the major thrust is simply coming from the preservation of momentum, which is why we call it reaction mass.

 

Rocket engines are in fact, LESS efficient in the lower atmosphere. The atmosphere provides a back pressure on the nozzle which slows the velocity of the exhaust. For optimum efficiency, the expansion ratio of the nozzle must be matched to its operating pressure environment. Launch-stage engines generally have a lower expansion ratio while high-altitude nozzles have a much longer bell. The shuttle's main engines are a compromise design since they have to run in both environments; consequently they are less efficient at both extremes than engines specifically designed for that regime.