Atmospheric pressure effects on transition to and from orbit and other tidbits

[Spartan159]

What are they? I am stealing the procedure from DGP's Starship Operator's manual to calculate normal space travel times. Step 4 multiplies distance to orbit by the atmosphere UWP number (only if 2+) Min. 1 to simulate the need to restrict speed while traveling through an atmosphere, in order to avoid dangerous heat buildup on the hull. I would like to tie this to actual pressure rather than the UWP code.

Side effects to be considered would include heat shielding and streamlining (Or lack thereof).

For the record I am using MgT 2E but stealing from any other Traveller source I can get away with.

I am also curious as to delays imposed in taking off from or landing at a Starport. I am thinking Takeoff is fairly quick (Get outta here, ya bums!) but landing involves waiting your turn depending on Starport size and pad/runway availability. I have been looking at GURPS Starports as well as MgT 1E Starports, but I'm still wading through all the information.

I look forward to input on these subjects.

 

[Spinward Flow]

Takeoff is fast once you're given clearance to take off ... much like at any airport ... but obtaining that clearance isn't necessarily an instantaneous matter. If you've ever endured/survived airline travel, you might be familiar with the concept of aircraft lining up to take turns to use the runway for takeoff. I've personally been in aircraft that have sat on the taxiway to the run up area for the better part of an hour before clearance to takeoff was granted, after which the aircraft moved onto the runway, throttled up the engines and thundered down the runway to achieve liftoff velocity and start climbing to cruising altitude.

So the largest time sink isn't the duration it takes to accelerate down the runway, but rather the time it takes to wait your turn to be cleared to use the runway.

VTOL is less constrained in that regard, since you aren't sharing a single runway for takeoffs and landings, but even a VTOL still needs to respect traffic patterns to avoid collisions.

In other words, best to pad out your estimates around takeoffs and landings so as to account for delays taxiing to and from berths to takeoff/landing runways.

 

[Grav_Moped]

Aerodynamic drag isn't an issue for long if you're going straight up.

Wikipedia: Max Q

The max q condition is the point when an aerospace vehicle's atmospheric flight reaches maximum dynamic pressure. This is a significant factor in the design of such vehicles because the aerodynamic structural load on them is proportional to dynamic pressure. This may impose limits on the vehicle's flight envelope.

During a normal Space Shuttle launch, for example, max q value of 0.32 atmospheres occurred at an altitude of approximately 11 km (36,000 ft) about one minute after launch.

During a typical Apollo mission, the max q (also just over 0.3 atmospheres) occurred between 13 and 14 kilometres (43,000–46,000 ft) of altitude;[4][5] approximately the same values occur for the SpaceX Falcon 9.

 

[whartung]

Are the differences in time meaningful?

The typical starship trip is 1-2 days + the week of jump. Does the climb out the atmosphere take hours depending on density? Or just minutes?

 

[Spinward Flow]

Are the differences in time meaningful?

The typical starship trip is 1-2 days + the week of jump. Does the climb out the atmosphere take hours depending on density? Or just minutes?

To put it politely, they're meaningful in the immediacy of the moment, but over a 10 day/240 hour time window the differences are relatively minor. The only time it wouldn't be minor is if surface gravity is very close to (or exceeds) the 1G maneuver drive performance, in which case it can become incredibly important ... until you just hand wave that all away and say it takes "about an hour" to go from the planetary surface to a stable orbital insertion at 10,000km altitude (standard orbit, according to LBB2). You then depart your 10,000km orbital trajectory for the 100 diameter distant jump point of choice, giving you a wide latitude/longtitude of possible destination points to jump from.

 

[Grav_Moped]

I worked it out for an air/raft (with 0.1G more than gravity neutralization) on a size 8 atm 6 world (i.e., Earth), and atmospheric drag adds about an hour to its time-to-L.E.O., if that.

In the context of even a starship's run to Jump Limit from a small planet, it's statistical noise.

 

[kilemall]

That RL asteroid probe reenetered safely at approximately 1G, although the decel would be killer on sophonts. I would think we could do multiples with higher TLs.

I've looked into this whole question extensively for quite different reasons- what is the standoff distance missiles can be fired as ortillery and be functionally used without having too much 'hang time' decel to get through atmo to reduce point defense time. It's that atmosphere brick wall that drives the consideration. Played with different numbers, the answer so far seems to be you have to be REAL CLOSE for bombardment especially for standard and up.

I also want to work up damage rules for ships going through reentry controlled or otherwise. Armor, streamline state, and of course speed and atmosphere should matter. I tend towards the simple, so how about this-

G speed x (atmo code x .1) x hullsize/(100 x armorvalue) (ignore 0) = number of tons hull damage sustained by reentry, EVERY time.

Then it becomes an economic tradeoff vs. desperation/hero moment. Adjust the hullsize/100 multiplier down in case of loss of control, partially streamlined or unstreamlined state.

I would tend to largely ignore this formula for launches except for the fastest of ships. It's the landings that hurt.

This formula implies that armored missiles would be a better bet for ortillery, much shorter burn time/more armor to allow at least a few 1000 kms off.

As for traffic patterns, atmo is a factor in terms of how many ships can get through incoming reentry lanes, speed vs. damage, I'd say most of your starport sky is taken up by landing so launches get slowed down by the reentries just for fewer 'slots'.

But other factors should be the starport code and population code increasing the busyness plus starport traffic avoiding risk to population centers and thus the total time. This would be cut by the world's size, a larger planet means more real estate in the sky for ship ingress/egress, and world TL.

So QND formula for this-

Given the old DMs for starport TL A +6, B +4, C +2, D +0
-
(Starport value x Pop Code x Atmo Code) - TL /World Code Size = Average minutes getting clearance.

Treat asteroids as world size A, and vacuum as atmo 1.

Your view may be that a higher starport value means greater facility to handle starship/small craft traffic, in which case you could try this version-

(Pop code + Atmo code)² /(Starport value + World code size) = Average minutes getting clearance.


I don't pretend to think these formulas are finished products and certainly have not run them through every scenario, just more illustrative of the sort of factors and their relationships that I think feed into answering the questions.

 

[Grav_Moped]

That RL asteroid probe reenetered safely at approximately 1G, although the decel would be killer on sophonts. I would think we could do multiples with higher TLs.

Currently, manned re-entry is typically at 4-5Gs (with the re-entry capsule generating lift and extending the descent time), though 8Gs (straight ballistic trajectory) is survivable. The latter is not a recommended procedure, though, and serious injuries are likely. (Soyuz TMA-11 mission, October 2007 -- service module failed to separate from the re-entry capsule)

 

[Grav_Moped]

Currently, manned re-entry is typically at 4-5Gs (with the re-entry capsule generating lift and extending the descent time), though 8Gs (straight ballistic trajectory) is survivable. The latter is not a recommended procedure, though, and serious injuries are likely. (Soyuz TMA-11 mission, October 2007 -- service module failed to separate from the re-entry capsule)

.... but note that these are tailsitter-oriented craft with appropriate acceleration couches.

A belly-lander Free Trader trying this is going to have an unpleasant day. Might even end up roasting the grav lifters, which will somewhat complicate the landing.

 

[Spartan159]

Thanks for all the information as I had no real idea. MT had times in minutes, whereas T5 was in hours.

 

[kilemall]

Currently, manned re-entry is typically at 4-5Gs (with the re-entry capsule generating lift and extending the descent time), though 8Gs (straight ballistic trajectory) is survivable. The latter is not a recommended procedure, though, and serious injuries are likely. (Soyuz TMA-11 mission, October 2007 -- service module failed to separate from the re-entry capsule)

I don't think that figure is right.

I'm using the CT Traveller term for G accel as shorthand for vee too, 1G= 10 meter per second.

So 1G at velocity is 36,000 kmh. Maybe should use the notation 1V??

ICBMs don't even get that fast, most ICBMs and space missions have been reentering at .7G at best, although the newer ones are probably getting their higher reentry speed from incorporating that probe design.

Ballistic missile - Wikipedia

en.wikipedia.org

Fastest reentry speed of any manmade object is the Stardust probe, 1.24V by Traveller standards (granted, may be military tests that have done better). It HAD to be able to do high speed reentry because of the combinations of the object it was coming from and available approaches back to Earth. I theorize that the uptick we are seeing in missile speeds is due largely to the engineering that showed this was possible.

Stardust (spacecraft) - Wikipedia

en.wikipedia.org

I assume our higher tech Traveller future can do better, or the whole problem will be sidestepped by slow gravitic reentry.

 

[whartung]

I've looked into this whole question extensively for quite different reasons- what is the standoff distance missiles can be fired as ortillery and be functionally used without having too much 'hang time' decel to get through atmo to reduce point defense time. It's that atmosphere brick wall that drives the consideration.

Well the simple ("simple") solution is drop them over the horizon, and cruise them in, thus hang time isn't really relevant.

For whatever reason, Chadwick didn't really explore development of hypersonic weapons, which naturally can exacerbate the problem even more.

The dark side is that with light speed weapons, hang time is pretty irrelevant as well, unless the missile can get inside the detection/reaction loop (which is unlikely from orbit, hang time or no). And if the missile is within the loop, the launcher likely isn't.

Of course, there's orbital kinetic penetrators, which are just hard to kill. And cheap. Super cheap. Hard part is getting them in to orbit, and, well, that's a solved problem in Traveller.

 

[Grav_Moped]

I don't think that figure is right.

It's right, but all of those deceleration Gs are from aerobraking, and the peak Gs are relatively brief.
It's also describing braking from orbital velocity, while starships have plenty of delta-V to bring their vector down to something far more manageable before hitting atmosphere.

If you're aerobrakng that hard in a Free Trader, something has gone horrifically wrong.

 

[Condottiere]

I would think your manoeuvre drive just gave out, or was damaged below local gravity.

Though come to think of it, you do need an atmosphere for that event to occur.

 

[kilemall]

It's right, but all of those deceleration Gs are from aerobraking, and the peak Gs are relatively brief.
It's also describing braking from orbital velocity, while starships have plenty of delta-V to bring their vector down to something far more manageable before hitting atmosphere.

If you're aerobrakng that hard in a Free Trader, something has gone horrifically wrong.

Umm, terms.

I believe you are referring to the relative physical effects of Gs in the sense of what's experienced onboard.

I'm using the term 1G in Traveller acceleration terms over 1000s, which I'm relabeling 1V for delta vee and clarity. So we can both be 'right', just within our contexts.

 

[kilemall]

I would think your manoeuvre drive just gave out, or was damaged below local gravity.

Though come to think of it, you do need an atmosphere for that event to occur.

I'm thinking in terms of hot reentry for battle damage and also rapid assaults of the sort hotshot players like to try.

One example are those starship trooper tubes on the Kininur, I envision those nowadays as like ortillery version bay missiles, it's ok if they are ruined for reuse they just have to last the one trip with their 'payload'.

Hot reentry could also be a thing when there are few time slots available for ships to sedately land at an air/raft speed, only way to make the shipping and passenger contract going surface to surface is to accept a little hull damage. Adds up after months, we'll fix it when we get yearly maintenance, hold together old girl till we get there.

 

[kilemall]

Well the simple ("simple") solution is drop them over the horizon, and cruise them in, thus hang time isn't really relevant.

For whatever reason, Chadwick didn't really explore development of hypersonic weapons, which naturally can exacerbate the problem even more.

The dark side is that with light speed weapons, hang time is pretty irrelevant as well, unless the missile can get inside the detection/reaction loop (which is unlikely from orbit, hang time or no). And if the missile is within the loop, the launcher likely isn't.

Of course, there's orbital kinetic penetrators, which are just hard to kill. And cheap. Super cheap. Hard part is getting them in to orbit, and, well, that's a solved problem in Traveller.

Not sure OTH is gonna work that much better, other then it might help in limited opposition situations. Still going to pay some time getting help to your forces on target, and at least will need an FO with eyes on target and linking comms to render an accurate coordinate as opposed to direct target acquisition.

The point of my gedanken experiment in firing a missile is to try and stand off as much as possible, not be hugging the atmo, and get help down fast. It's really tough. Sand probably matters more then we are used to.

Kinetic penetrators would be cheap, that's where putting mass drivers onto starships for that could be a thing, still has to be built well enough to survive high speed reentry and maintain speed and relatively little drift to retain their kinetic speed and hit a target. Still means the penetrator can only go so fast but needs to go fast enough for one of several shots to hit.

I suspect reflec/armored facilities and vehicles are a thing so maintaining a physical bombardment capability would still be important.

 

[Grav_Moped]

Umm, terms.

I believe you are referring to the relative physical effects of Gs in the sense of what's experienced onboard.

I'm using the term 1G in Traveller acceleration terms over 1000s, which I'm relabeling 1V for delta vee and clarity. So we can both be 'right', just within our contexts.

Exactly. You're talking about re-entry velocity, but phrasing it differently. I'm talking about the effective (de-)acceleration due to atmospheric drag at re-entry velocity (and lower, as kinetic energy gets transformed into shock waves and thermal energy).

Re-entry velocity for modern-day manned capsules is slightly lower than orbital velocity at Low Earth Orbit, which is 7.8km/sec. That's .78 of the vector from one LBB2 2nd Ed. space combat turn at 1G, which is I think what you're calling 1V.

 

[kilemall]

Exactly. You're talking about re-entry velocity, but phrasing it differently. I'm talking about the effective (de-)acceleration due to atmospheric drag at re-entry velocity (and lower, as kinetic energy gets transformed into shock waves and thermal energy).

Re-entry velocity for modern-day manned capsules is slightly lower than orbital velocity at Low Earth Orbit, which is 7.8km/sec. That's .78 of the vector from one LBB2 2nd Ed. space combat turn at 1G, which is I think what you're calling 1V.

Exactly.

So if we are going to do something simple like Starship Troopers from the K BC, much less missile fire from above for Our Heroes, we're going to need to quantify what if anything can survive 1V+ reentry speeds and how much damage does that cost.

 

[Grav_Moped]

Exactly.

So if we are going to do something simple like Starship Troopers from the K BC, much less missile fire from above for Our Heroes, we're going to need to quantify what if anything can survive 1V+ reentry speeds and how much damage does that cost.

Look at ICBM re-entry profiles, then add ultra-tech handwavium.