[Ships] Atmospheric Speeds?

[GJD]

Since the lower boundary of the atmosphere is ill-defined, the pressure level of 10 bars, at an altitude of about 90 km below the 1 bar pressure level with a temperature of around 340 K, is commonly treated as the base of the troposphere

Thanks GJD, nice article - but the quote above refers to a 'designation' of the end of the troposphere and the source abstract is about the Galileo probe - so I think the pressure is too low at those temps to 'liquefy hydrogen'. Google-ing didn't turn up much for me, except (for Saturn)

Under the conditions found within the planet, hydrogen behaves as a liquid rather than a gas at pressures above about one kilobar, corresponding to a depth of 1,000 km (600 miles) below the clouds; there the temperature is roughly 1,000 K (1,340 °F, 730 °C).

Given the temps here the problem as I understand it (or don’t :? ) is that its not really 'liquid' so much as 'liquid-like' (supercritical liquid) - so no surface tension and its compressible like a gas, but otherwise acts like a solvent. My layman’s knowledge is useless here and the stuff I googled is confusing and contradictory.. (no nice equations/tables)

Granted, the change is gradual and far from clearly defined at any point, or, come to that, observed, but the point is you don't NEED it to have kilobar pressures or thousands of Kelivin to get normal, liquid hydrogen. The phase change will happen within the mid reaches of the troposphere in localised areas.

However, why bother with all that tedious mucking about with kilo Kelvin temperatures and tens or hundreds of atmospheres to skim for liquid hydrogen? If the magic fuel processor can crack seawater to hydrogen, then filtering, chilling and compressing gaseous hydrogen to Lhyd in the tanks should be a piece of cake. Just do it up higher where the atmosphere is still H2 and all its its naughty hydrocarbon, ice, ammonia and methane buddies, but rarefied.

G.

 

[Commander Drax]

Coming back to aerodynamic speeds a quick and dirty rule of the thumb is

Unstreamlined max 300 kph
Streamlined max 1000 kph
Airframed max 1300 kph per g of M-drive rating.

This ties in with previous materials such as those in MegaTraveller where atmospheric speeds were detailed. Cruising speeds are 75% of the vessels maximum speed.

Not scientific or proven but it will do.

 

[Kilgs]

Coming back to aerodynamic speeds a quick and dirty rule of the thumb is

Unstreamlined max 300 kph
Streamlined max 1000 kph
Airframed max 1300 kph per g of M-drive rating.

This ties in with previous materials such as those in MegaTraveller where atmospheric speeds were detailed. Cruising speeds are 75% of the vessels maximum speed.

Not scientific or proven but it will do.

That looks the numbers that the SSD came up with somehow. Good enough for me.

Thanks!

 

[BP]

Kinda hard to get to orbit on any reasonably massed/sized planet (one with an atmo) with those speeds :shock:

While each planet is different - earth's 'minimal 100 km' orbit requires something like 28,000 kph.

(quessing kph is kilometers per hour in above or ~0.540 knots, so these figures look like typical small prop aircraft (160 knots) and jet airline speeds (540 knots) and maybe fighters).

For Traveller, I'd think something different would be in order for spaceships since all must be capable of transatmo...

Now where did I put that thinking cap...

 

[GypsyComet]

Kinda hard to get to orbit on any reasonably massed/sized planet (one with an atmo) with those speeds :shock:

While each planet is different - earth's 'minimal 100 km' orbit requires something like 28,000 kph.

(quessing kph is kilometers per hour in above or ~0.540 knots, so these figures look like typical small prop aircraft (160 knots) and jet airline speeds (540 knots) and maybe fighters).

For Traveller, I'd think something different would be in order for spaceships since all must be capable of transatmo...

Now where did I put that thinking cap...

You are confusing the modern usage of escape velocity with Traveller's sustainable accelerations.

If I had four days of constant thrust and an air and pressure habitat available, I could reach the International Space Station at freeway speeds or less.

Escape velocity is for vessels that don't have constant and sustained thrust available. The big old Saturn V had all of what, 5 minutes of primary burn? Ten? It had to be able to push the remainder of the vessel up to escape velocity so that it would *keep going* once the thrust stopped.

 

[Ishmael]

This ties in with previous materials such as those in MegaTraveller where atmospheric speeds were detailed. Cruising speeds are 75% of the vessels maximum speed.

MT did not detail atmospheric flight at all.
Basing 'cruise speed' on "top vacuum speed" <?> given an amount of thrust?
In a std atmosphere, a streamlined ship/grav vehicle was limited to 1000kph which is about mach .8 . In otherwords, in MT, streamlined ships cannot go faster than the speed of sound. You have to have an airframe to do that.
Unstreamlined ships CANNOT enter an atmosphere at all.
FFS1 allows a streamlined ship to reach only mach .9, but you need an airframe to go faster than that according to those rules too.....
No wonder ships are helpless during GG skimming!

in my mind, airframe simply means having the ability to augment controls via aerodynamic means, not being extra extra streamlined. An raindrop is streamlined, but not an airframe while a wwi Voisin bomber is an airframe but not streamlined.

Pretty wacky stuff, imho.

That's pretty much the reason I worked things out the way I did.
I wanted something that makes some sort of sense.
The extra math doesn't bother me because its something to be done in a spreadsheet.

 

[BP]

Traveller starships have to be capable of reaching escape velocity - wether under constant or impulse acceleration...

How many commercial jetliners (constant thrust) can you name that have reached orbit?

-and 4 days to reach a station might bore one's players :lol:

 

[rust]

How many commercial jetliners (constant thrust) can you name that have reached orbit?

A commercial jetliner needs a certain atmosphere density, both to provide
oxygen for the engines and lift through its wings, so this is not a very good
example - a type of engine that would work without oxygen could change
the picture.

 

[hdan]

They don't have to be capable of reaching escape velocity in the atmosphere though.

I imagine that most of them "float" up as high as they can before really firing up the M-drives, much like current space launches go vertical until they gain enough altitude to get past the worst of the air resistance.

 

[BP]

How many commercial jetliners (constant thrust) can you name that have reached orbit?

... this is not a very good
example - a type of engine that would work without oxygen could change
the picture.

Quite true - my bad - darn my hasty posts!

Was (poorly) attempting to point out that 1000 km/h (~Mach 0.8 below stratosphere) is way too low a limit for a streamlined spaceship that can withstand the heat (plasma) and stress of atmospheric entry - supersonic to hypersonic is more like something that would be transatmospheric...
full orbital speed is not needed in atmo - but fairly high speeds are needed to go into orbit in a reasonable amount of time...

To use GypsyComet's 'big old Saturn V' as an example (a streamlined configuration of ~400 dton - if I did my math right ) - after first stage burn of less than 3 minutes it is 68 km in altitude and travelling at ~8,600 km/hr - its still in the atmo (mesosphere I believe) and still has a ways to go for even a minimal orbit... and this is pretty low Traveller tech

The topic of this post is the speed of spaceships in atmo... (I think :? - but I've proven myself wrong many a time :roll: )

 

[GypsyComet]

Traveller starships have to be capable of reaching escape velocity - wether under constant or impulse acceleration...

How many commercial jetliners (constant thrust) can you name that have reached orbit?

speed + direction = velocity

Since they don't have constant thrust OR lift above a certain elevation, none. If their lift and drive wasn't dependent on atmosphere, and their doors weren't likely to blow out much above where they DO reach, then yes, an airliner could reach orbit.

ALL you have to do to reach orbital elevations is beat gravity. With no thrust, that means going so fast that gravity won't stop you in time. THAT is what escape velocity *means*.

But if you have constantly available lift, it only needs to beat gravity by a little, because you are *ALWAYS WINNING* the fight with gravity.

Now, if you have too little additional thrust you might be subject to wind resistance (ie. friction), but that's a different question.

It is, however, the controlling question for the original question in this topic. Friction heating is what keeps a non-airframe from going Mach 6 in atmosphere. Your drives might be able to get you going that fast, but you'll arrive blackened, bent, and possibly dead.

 

[GypsyComet]

To use GypsyComet's 'big old Saturn V' as an example (a streamlined configuration of ~400 dton - if I did my math right ) - after first stage burn of less than 3 minutes it is 68 km in altitude and travelling at ~8,600 km/hr - its still in the atmo (mesosphere I believe) and still has a ways to go for even a minimal orbit... and this is pretty low Traveller tech

TL6-7, in fact. By earlier editions standards, an "Airframe", since its top speed is set more by the motor than by atmospheric friction.

Escape velocity at the surface of Earth is 11.2 km/s. The higher up you get the lower escape velocity gets.

The Apollo missions didn't need to *quite* hit true escape velocity, since they weren't escaping Earth's gravity completely, just getting out to where the Moon would grab them.

MT did not detail atmospheric flight at all.

Yes it did. Not everything with a hull configuration in MT is a spacecraft. There was also COACC, but a lot of people bounce off of the design sequence in COACC, so it tends to not register in memories of MT.

 

[BP]

... Friction heating is what keeps a non-airframe from going Mach 6 in atmosphere. Your drives might be able to get you going that fast, but you'll arrive blackened, bent, and possibly dead.

Actually today's spacecraft are quite capable of handling the friction (plasma) - they lack the propulsion systems. With Traveller propulsion systems Lift is not neccessary - and they already have to withstand extreme heat..

 

[Ishmael]

X-43a has already gone faster in atmosphere

mach 9.8 at about 110,000 ft
hotspot temp was around 3700F as I understand
carbon fibre thermal protection kept the underlying metal cool enough to not melt

based on googling around, tungsten has a melting point of 3400C ( about 6100F ), so it may be possible to go faster still

imtu, I may consider saying one-shot re-entry at even higher speeds can be done with ablat armor surfaces which are replaced after re-entry

hmmmm what energy density (joules/cm^2?) that creates temps similar to max speed of re-entry?
use that to find penetration for various kinds of armor as if the ship is being attacked with a plasma gun against the hull's armor...

this discussion has an impact on drop troops too.

 

[BP]

.. TL6-7, in fact. By earlier editions standards, an "Airframe", since its top speed is set more by the motor than by atmospheric friction.

Not sure what you mean here – I’m a CT only guy so I may be missing something (edit: okay I'm missing a lot - but that's another topic :roll = since Saturn V isn’t my definition of a lifting body and I don’t think it would make such a great glider I wouldn’t consider it an ‘Airframe’

..Escape velocity at the surface of Earth is 11.2 km/s. The higher up you get the lower escape velocity gets.

Given.. (my physics stopped at QCD ), but you still got to get there!

My points (poorly presented.. )
- Traveller ships orbit and de-orbit (and presumably in less than days ..)
- This means high velocity, high heat and high dyamic stress thresholds
- 1000 km/h seems low (Mach 6 is > 7,000 km/h at Earth sea level and 15 deg C)

 

[BP]

... use that to find penetration for various kinds of armor as if the ship is being attacked with a plasma gun against the hull's armor...

Funny that - I had some vague thoughts (ok all my thoughts are vague) on using the plasma weapon pressures to determine relative pressure limits on armored hulls...

 

[BP]

X-43a has already gone faster in atmosphere
....

Cool! Didn't know that - assume it is scramjet based... guess I should google more (I did google Saturn V above to try to get volume and atmo speeds - but I ended up calc-ing volume - my google skills seem to be slipping every day)

Kinda high - but if it is shaped spacecraft like (mostly lifting body) then it would be representative of a space craft (like a free trader...)

Starships not relying on inlet gases should be able to use high heat materials we have today - tungsten and hi-temp metal alloys are needed on engines for shaping/high tensile strengths, etc.

 

[Kilgs]

Traveller starships have to be capable of reaching escape velocity - wether under constant or impulse acceleration...

Not with anti-gravity, I don't think. I doubt an air raft is capable of reaching escape velocity on engine power alone. Doesn't AG just simply cancel an escape velocity? It's not "thrust" so much as "negation".

That was always my interpretation.

 

[BP]

Starships

 

[GypsyComet]

Starships

The rules that apply to one apply to all, once life support and gravitic drives are involved. The hull streamlining classes are also universal, so they factor into the top speed regardless of size.