Technical Liftoff Question

[kilemall]

Since this seems to be the catchall forum for ship issues, thought I would post my query here.

Been thinking through the liftoff portion of player ships taking off, and the parameters of speed and windows for things like authorities shooting them down sort of thing.

If one is to take the vector vs. gravity thing literally, an earth sized and density planet is invoking approximately 1-G of 'vector' down as gravity, negating the ability of 1-G ships to take off.

Alright, let's say our ship cannot do a 1:1 vertical but has to build up speed flying perpendicularly, get up to orbit then leave the planet's grav well. I am also assuming up to .5G in combined thruster maxed to help clear the pad and local obstacles.

If we go with the escape velocity rating of Earth at orbit, 10.6 kms, that's going to be at least 1000+ seconds, not counting time lost getting to the 8 kms to get in orbit in the first place.

Free Trader Beowulf is not pulling any escape Mos Eisley scenes anytime soon.

High-G ships are another matter, but if I understand how this would play out, even those will initially be moving as slowly as a Saturn V liftoff, although not for the fuel/mass reasons, just the slow constant accel on the low end part of the curve.

So, what should the actual physics of takeoff be for say a 1-G ship vs. a 4-G one, including planetary orbits for escape velocity?

No grav modules please, they are expensive add-ons that just take up space and weight after you get out of orbit. I do have them, but for special environmental limited areas or people with more wealth that want a pleasant landing and will pay for it. Straight up thrust please.

 

[whartung]

The problem is not acceleration, it's lift.

Your generic free trader ship design has no natural lift. In fact, none of the common designs have natural lift, at least not as pictured.

So something else must provide the lift that gets the ship off the ground. Some design sequences have "airfoil" designs, but the classic Book 2 ships are merely Streamlined (meaning they don't fall apart in an atmosphere as they accelerate).

The acceleration can inevitably provided enough velocity to achieve orbit, but not if the ship is dragging its belly in the dirt the entire way.

To me, it's always been some implied gravitics that provides the lift (or neutralizes the local gravity, providing buoyancy). Then it's simple math to achieve escape velocity (whatever that may be). Point the ship "up" and start accelerating.

This is how a "1G" ship can get off of a "1.2G" heavy gravity planet, for example. It also explains how ships can get off of planets with this atmospheres (where a normal airfoil or lifting body may simply not provide enough lift -- an Earth airplane has smaller wings than a Martian one, for example).

As for the higher G drives, it's linear (v = a * t) -- a 4G drive is 4 times as fast as a 1G drive, and thus will achieve escape velocity 4 times faster.

 

[Condottiere]

There's an order in Mongoose that allows you to add an extra thrust factor, so in practice, you could lift off at two gees.

 

[Brian Smaller]

Contra-Gravity technology is the first big bit of handwavium technology in Traveller (coming in at Tech 8-9 if I remember correctly). I think without it unless you have reaction mass burners you just can't have Traveller type starships as they are presented. Hard to belly land a scout or trader at any of those typical starport landing pads without contra-grav - let alone take off.

 

[martletsquire]

Truly. Grav and counter-grav are an implied and necessary technology, else most ships would be either grounded or space-to-space only.

But, consider anti-grav plates as part of the technology package that gives you inertial dampers and ship gravity, well, the three go hand in hand, don't they.

It is all handwavium, but necessary handwavium.

It has always been my understanding that the anti-grav level equaled the M drive level. So, a ship equipped with M1 can lift off of a 1G to about 1.7G planet (grav provides 1G of boost, the rest is the maneuver drive minus basic drag and stress maximums of a ship.) A M4 ship can lift on grav plates up to a 4G planet, with the rest of the power provided by the M drive.

Lifting may be slow, think of going 100-200km/h up to space, but it gets you there.

Check out the Andy Griffiths show "Salvage 1". It gives a good idea of slow acceleration versus fast acceleration (the typical rocket is a fast examination example).

Hope this helps.

 

[aramis]

Truly. Grav and counter-grav are an implied and necessary technology, else most ships would be either grounded or space-to-space only.

MT solves this a different way - noting that Gravitic drives can be pushed to 400% thrust for short periods. (explicit in Starship Operator's Manual; implicit in that the time to orbit calcs all use a constant 1G acceleration above local.)

 

[kilemall]

I was looking more for the following conditions-

1) no antigrav except as a fairly expensive add-on, if we look at the per ton lift of the standard gravitic craft, looks to me like it works out to be something like 100,000 Cr per ton (the remaining 1/3 is fuel, controls, hull, crew/cargo mounting, etc.). Pretty consistent across the air/raft and GCarrier, the GSpeeder would appear to have some serious performance boost for an extra cost plus greatly reduced cargo/crew capacity.

I'm sure I could dig out Striker or learn one of the newer build systems, but just on that comparative note a 100-ton scout's 1-G lifters would cost 10Mcr. Even if they 'scale up' and become more efficient per ton the larger the lifter, even an additional 1-2Mcr racks up a significant monthly burden for the typical starship loan.

In specialized apps like luxury passenger craft, high speed in and out of downports with a far steeper drop from orbit, smuggling with stealth skins/no reentry signature or precision assault/spec ops craft this would be a differentiating feature worth paying for.

Also, against the design ethos of waste not want not for space, seems to me most starship economics should work against having a second maneuver specialized system for landing and takeoff.

So, all thruster all the time as that is useful in ground and space 100% of the time, unless it is a desirable add-on, in which case I would mandate HG ship design and the next step up in maneuver drive cost/space to cover it, no extra power plant unless the designer/player wants to be able to G-lift AND accel with main engines.

2) What exactly the speed would be pulling out at 1-G and 4-G, understand the linear difference, but are we talking literally pointing the nose at an angle with the ventral thrusters and 10 mps constant accel, or slower due to drag and gravity looking to force us into an arc?

3) I know many of you may not be following my IMTU thread, but I already have the lift issue covered with the conformal lift body feature of the deflector system. It wouldn't generate the plasma until higher hypersonic speeds are reached or we have a higher TL with power to burn to feed in the plasma, so no good for the critical initial ground-to-air phase, but will help once we pick up speed. SO I'm not worried once we get up to that speed, more how many minutes is Beowulf hanging out there in starport artillery range before it gets away?

Makes the old getaway more like a Firefly escape, 'don't shoot us we're harmless', rather then the usual player 'I'm Han Solo and I'm takin' off cause I play by my own rules' sort of thing.

I also have to assume 1-G ships are taking their lives in their hands doing a gas giant scoop and having no way to get out if something goes wrong and they don't maintain the vector for the pullaway phase.

 

[whartung]

1G vs 4G, still, isn't the point.

The point is staying in the sky long enough to reach the escape velocity, whatever it may be.

Consider this.

For a plane to take of, it must travel at a high enough speed that it's wings gain enough lift to lift the plane. Below that speed, the plane stalls, and falls out of the sky.

Now consider an aircraft carrier.

The combination of the catapult and jet engine thrust does not need to accelerate the plane to take off speed by the time that he plane leaves the deck.

Rather, the plane must reach flight speed BEFORE IT HITS THE WATER. The deck is somewhat above the water, and it's not uncommon to see a plane dip down below the deck during takeoff. The height of the deck effectively extends the amount of time the plane has to reach takeoff velocity. Not a lot, but some, and enough to matter. Don't think that the flight engineers did not consider this when specifying takeoff weights for carrier launched aircraft.

So, by the same notion, as long as the starship has SOME mechanism that will keep it aloft for the duration of the acceleration, it can make orbit.

At 1G acceleration, it takes ~20 minutes to reach escape velocity on earth. At 4G, it'll take 5 minutes. Space rockets are conveniently pointed straight up, and use a combination of thrust, inherent stability on the design to remain aloft while accelerating in order to leave orbit. Apollos peak acceleration was about 4G.

20 minutes at 1G is roughly 7000km. Apollo was at 185km before it punched out to the moon. So, that's a long trip up at 1G, but, as long as it can stay up, and accelerate, it'll work. The ship could lazily corkscrew up.

The problem with lifting bodies is simply they lose lift (due to thinning air) before they can reach escape velocity. Also, things don't like to fly at Mach 32 -- the air kind of gets in the way. So that's another issue.

 

[BlackBat242]

I find people getting hung up on "escape velocity" without considering one simple thing.

If you are not going to run out of fuel, you can start on a planetary surface, go no faster than 1 kph (or even slower) relative to the planetary surface, and still reach safe jump distance. You might start moving back towards the planet as soon as you cut off your thrusters/M-drive, but you can get very far away without ever reaching "escape velocity".


Remember, "escape velocity" is the velocity at which, if you shut off your thrusters/M-drive completely at that point, you will continue to move away from the planet indefinitely. If you are close in, EV will be high - if you are 100km out, it will be much less - and if you are 100D out, it will be miniscule - but you do not ever have to reach EV if you have a "operate for weeks without refueling" STL drive system.

 

[kilemall]

Whartung, I'm not worried about the air, again deflectors- optimal conformal configuration, probably some sort of virtual aerospike set ahead of the hull, and the reason our TL9+ streamlined ships don't really care about hull lift/hypersonic issues. IMTU of course, just makes sense as an explanation for Why Things Are This Way.

Clear on the takeoff/lift thing re: planes.

But in our case, we are talking constant accel engines, a different bird then the jet in that it has a slower start but higher potential top end and theoretically no requirement for lift at all.

I'm looking for visualization and tactical realities for players to deal with, so again, it's that first 20 km out of the downport.

Now if we are headed straight up as per rocket launches, wouldn't the gravity cancel most of our lift for the first few minutes and miles at 1-G, 9.8 mps vs. our accel of 10 mps and therefore we are accelerating at .2 mps until we get further up?

In one minute that gets us to 12 meters per second, or 26 mph, not exactly a dramatic getaway.

I know we are fudging massive amounts of physics every time we play Traveller with our 'maybe that might work someday but not exactly this way' tech, but just within the simplistic 'this maneuver drive equals this accel for this big a craft' mechanics, seems to me Free Trader Beowulf is better off sticking to size 5 planets or less.

Am I missing something within those parameters as to what takeoff would be like, thrust only?

 

[kilemall]

I find people getting hung up on "escape velocity" without considering one simple thing.

If you are not going to run out of fuel, you can start on a planetary surface, go no faster than 1 kph (or even slower) relative to the planetary surface, and still reach safe jump distance. You might start moving back towards the planet as soon as you cut off your thrusters/M-drive, but you can get very far away without ever reaching "escape velocity".


Remember, "escape velocity" is the velocity at which, if you shut off your thrusters/M-drive completely at that point, you will continue to move away from the planet indefinitely. If you are close in, EV will be high - if you are 100km out, it will be much less - and if you are 100D out, it will be miniscule - but you do not ever have to reach EV if you have a "operate for weeks without refueling" STL drive system.

Good point, and feeds into the 'Salvage One' example cited above, but doesn't make for much drama for our adventurers.

At this rate the authorities could take a beer break, set up the plasma artillery, and be popping our heroes with no time issues.

I'm thinking more a flight profile where those .5G thrusters are working overtime keeping the ship 'out of the dirt' while building accel at tree level and get out of LOS, then when we have a good speed built up and out of range THEN point at the sky, and hope they don't have planetary defenses with 5000 km range online.

 

[aramis]

Whartung, I'm not worried about the air, again deflectors- optimal conformal configuration, probably some sort of virtual aerospike set ahead of the hull, and the reason our TL9+ streamlined ships don't really care about hull lift/hypersonic issues. IMTU of course, just makes sense as an explanation for Why Things Are This Way.

Clear on the takeoff/lift thing re: planes.

But in our case, we are talking constant accel engines, a different bird then the jet in that it has a slower start but higher potential top end and theoretically no requirement for lift at all.

I'm looking for visualization and tactical realities for players to deal with, so again, it's that first 20 km out of the downport.

Now if we are headed straight up as per rocket launches, wouldn't the gravity cancel most of our lift for the first few minutes and miles at 1-G, 9.8 mps vs. our accel of 10 mps and therefore we are accelerating at .2 mps until we get further up?

In one minute that gets us to 12 meters per second, or 26 mph, not exactly a dramatic getaway.

I know we are fudging massive amounts of physics every time we play Traveller with our 'maybe that might work someday but not exactly this way' tech, but just within the simplistic 'this maneuver drive equals this accel for this big a craft' mechanics, seems to me Free Trader Beowulf is better off sticking to size 5 planets or less.

Am I missing something within those parameters as to what takeoff would be like, thrust only?

Which all depends upon lift past bouancy. The amount of upthrust needed to counter gravity is subtracted from the upward thrust generated to get the available thrust for time to orbit calculations.

If you have more than 0, and unlimited endurance, you can make solar orbit without ever having hit orbital velocity. In point of fact, it's possible to break orbit on 0.001G - if you can sustain it long enough. Ion drives currently have that kind of endurance.

A 1G takeoff is doing 9.8*60= 588 m/s after 1 minute, and has crossed 9.8*0.5*(60^2)=9.8*1800=17460 m.

The first minute at 1G over buoyant is, again, 17.46km.

At 4G above buoyant, it's 2332 m/s, and 70.56km away. And the next minute, you're 4704 m/s and 282.24 km away. 3 min, and doing 7056 m/s and 635 km away - you've flown past the ISS, which is doing 7.8 km/s 90° away. Matching up will be about 4 minutes maneuvering.

Hell, even at 3G... orbital range is only 5-6 minutes total, especially if using an arced course. That's what the shuttle did after the initial burn at 2G... (due to fuel mass, it's not consistent thrust.)

Now, if one wanted to be truly pedantic, one could run the big numbers and figure out the integrated math to orbital altitude and velocity...

But even at 1G: 5 min is 2940 m/s and 441 km away from start. If fully vertical, you're at a residual velocities that are going to carry you well past the ISS... 2 min is 1176 m/s, and 70km away from start...

All the above done using 9.8m/s/s.

ALL that really matters is air resistance and the thrust past buoyancy.

 

[kilemall]

I can do the multiples of 10 mps just fine so if that was the only major issue (and I've already got handwave set up for the air resistance) doing the calc for takeoff is no problem and I can do 'you are here in x seconds after takeoff', if it's that simple.

But I don't see where the negative vector gravity represents factors into any of your numbers, you seem to be assuming successful accel will be the same under full gravity as not.

Is the term 'buoyancy' where you are saying we are assuming we have 1-G positive thrust over our mass and therefore we don't have to worry about gravity?

I understood 1-G in Traveller terms to mean 10 mps accel constant in a 0G state. But as we get closer to any gravitional field we have the (admittedly simplistic) G value where we impose the vector towards the body generating the gravity with it's mass. The ship's drive may be providing that constant thrust, but our effective speed is going to be affected by the celestial body.

Match forward velocity with the gravitional field and we have orbit. Slow forward velocity with firing mains in the retro attitude and we slow down and enter atmo (where we slow down a LOT more and generate a good deal of heat in the process).

So if we are mapping our vectors on takeoff, we're drawing a line near straight up, but there is still a near equal vector drawing straight down representing the 1-G gravitational field, and Im just not seeing that reflected in your numbers (some of which I am sure I am not getting).

You're probably going to have to explain buoyancy to me in this arena, I only encountered the term regarding 'wet' ships previous to this.

Are you also saying that a 1-G ship is going to be able to get off the surface of Jupiter if the ship does not break down and is able to maintain constant accel?

 

[aramis]

Buoyancy is the means (whichever is used) of matching local gravity in order to float. Usually, it's used to refer to displaced water buoyancy, but it's also used in aviation for air displaced lift, and sometimes in flight theory texts for other lift=gravity calculations.

Traveller has used several methods canonically

• CT ignored the issue - kind of - by giving a table that ignores the effect of local gravity upon getting to orbit.
• MT allowed overthrust for up to 30 min, and that overthrust at up to 400% of rated.
• TNE used Contragrav - which negates 98% of local gravity. So, in TNE, you need at least 2% of local to be bouyant. Note that it's theoretically possible to build a ship with CG and take off by air-buoyancy alone...
• T4 used Contragrav, as well.

And with gravitic thrusters, no matter how high local g is, if you have lift sufficient to be bouyant, you can spike it by some means to get upward motion instead.

Very few people are going to fly NOE to get to a better vertical launch. They're FAR more likely to strap on SRB's and go zooming straight up to minimize exposure times.

Also note; Safe jump from a size 8 world is about 4:24:00 at 1G...

Note that, as long as the up vector is longer than the down, you're not going to fall, but climb. Since (outside of TNE) gravitic thrusters are de rigeur, if it can go up at all, it goes up until it decides not to.

Note also: The CT travel times to orbit are NOT adjusted for local gravity. The presumption is that full maneuver g's will be used. Doesn't explain how/why, just does.

 

[whartung]

Buoyancy is neutralization of the gravity well. A helium ballon is buoyant, as it is lighter than the surrounding atmosphere. Once the atmosphere gets too thin, the balloon reaches equilibrium and stops rising (normally, they simply explode from lack of pressure).

Apollo uses sheer raw thrust to overcome gravity. Typical Traveller ships seem to use gravitics to do that. In some TUs, Anti gravity negates the local gravity, and can also provide thrust. Others it simply negates gravity and thrust must be achieved through other means.

If you want to just use raw thrust, then a 1G drive isn't enough. A 1.1G drive is, but not a 1G drive. The simple point being that your drive has to have a net positive thrust to overcome the gravity. A .1G net thrust is going to take you some time to get to orbit, but you will get there. And to be pedantic, the net thrust must overcome both gravity and atmospheric drag (which increases with velocity).

In the crunchier systems, your drive has thrust, and the G rating is dependent on the current mass of the ship. Obviously a properly spec'd drive will be one that is powerful enough to move the loaded ship.

For example, many modern rockets, particularly large ones, are too heavy to launch initially. The motors do not initially generate enough thrust. However, as fuel is consumed, the weight comes down to cross the threshold and soon the vehicle begins to accelerate.

This has other value, as it doesn't subject the vehicle to an instantaneous high G event, rather the Gs advance smoothly and slowly. That's why something like Apollo slowly lifts off the pad vs a SAM that goes streaking in to the sky.

But in typical Traveller, you don't lose weight, so the drive simply has to be more powerful than the load.

Finally, the simple truth is that in your TU, if a 1G drive won't work, then ships won't have 1G drives. Your TU isn't "canon", so "canonical" ships won't necessarily work.

 

[martletsquire]

Do you also take in total mass of expendables/cargo in consideration? If not, that could make for some exciting times as the crew dumps cargo and fuel to lighten mass.

Do you have internal grav plates/inertial dampers? How do you handle acceleration effects upon your crew, cargo and ship components? How do you handle the effects of a powerful thruster/acceleration system upon the outside environment?

As to the buoyancy question, once you break the grav level of the planet, you got it made (if your fuel lasts long enough). IMTU, slow and steady liftoffs are considered normal, with transit times being from 10 minutes up to an hour. Lumbering merchants is standard. Kicking in your M drive full strength near habitation, or in atmospherics is considered a not-nice thing to do.

This is all originally stemming from the days before M drives were reaction-less. Kicking in a giant nuclear torch in someone's backyard was a not-neighborly thing to do and generally peeved off the locals. Once M drive was reactionless and we switched over to it, we considered some of the field reactions could be disturbing to sophonts and equipment at high powers, therefore same reaction by locals who just had their house shook down or their guts vibrated.

 

[BlackBat242]

Do you also take in total mass of expendables/cargo in consideration?

Only in the versions of the game that consider mass rather than volume (CT and MT use volume and completely ignore mass for spaceships/starships maneuver calculations).

 

[WistfulD]

Only in the versions of the game that consider mass rather than volume (CT and MT use volume and completely ignore mass for spaceships/starships maneuver calculations).

It's not that I never noticed before, but has anyone ever thought about what an unusual design decision it is to use Gs of thrust as your basic movement metric, but not have loaded mass as the primary ship qualifier?

 

[Brian Smaller]

It's not that I never noticed before, but has anyone ever thought about what an unusual design decision it is to use Gs of thrust as your basic movement metric, but not have loaded mass as the primary ship qualifier?

GT addresses this. Ships have an empty and loaded mass separate from their displacement weight. Cargo is calculated as an average of (and I am trying to remember here without looking it up) about 5 stons per 1 dton. Thrust always ends up as something other than a whole number. It is quite cool because you can build lumbering freighters that slog along loaded at some fraction of a g.

 

[whartung]

It's not that I never noticed before, but has anyone ever thought about what an unusual design decision it is to use Gs of thrust as your basic movement metric, but not have loaded mass as the primary ship qualifier?

Because it's easy. Because Traveller, especially originally, is an RPG, not a space sim.

Roll a character, get a free ship, hop on board and treat it as a horse or mom's hand-me-down Vanagon. You know, stick the key in and go. I don't recall Han and Chewie discussing pounds of fuel on board or cargo load balancing issues when they blasted out of Mos Eisley.

Obviously, over time, Traveller got crunchier.