Technical Liftoff Question

[aramis]

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.

TNE does, too. But it allows 10 metric tons (close enough to long tons for most purposes) per Td.

 

[kilemall]

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.

Okay, fair enough.

I'm inclined to go the hard way just because that makes the TU more 'real' if there is differentiation.

So 1-G Beowulf can't lift and take off in full 1-G given that it's a streamlined box and so is stuck on station to station runs for full size planets, something like Type R can because it has wings, Type S just has the juice to get it done scout style, there is a reason and need for those 3-6G small craft, tech up enough and the conformal wing tech allows for lift for the 1-G hoi polloi, and a posh liner will whisper up to the clouds with nary a bump or harsh plasma scoring the paint job.

Makes for a reason to have GCarriers as the poor man's shuttle, since my verse has 5/10-ton containers standard probably more like a 14-ton grav truck (mounts a container standard, so only costs 4 ton extra and has a back seat in the cab for passengers/crew).

 

[kilemall]

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).

Well, LTA craft like blimps of course, but something in my head just doesn't relate those to our starships.

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.

I'm assuming antigrav is a 'push' tech, but is not worth much once you get out of a high-gee field because there is nothing to push against. Of course there is less 'pull' too so I suppose one could argue that you could have push vector even with .001 G in play, but not fast enough to merit the volume/install cost as opposed to cargo, reduced cost or better deep space maneuver drives with more rapid travel under a greater variety of conditions.

I do have an Amber Zone for Earth and Centauri Prime being protected biome zones that requires grav-only vehicles, so I am not adverse to their use at all, just not without a pressing economic, scientific or military reason.

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 some sense my question is pedantic anyway, I just wanted an answer I can live with, or at least a reality check that CT's mechanics for leaving 1-G ground in a 1-G ship had a bit of a design flaw.

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.

Ya I understood the fuel thing as a third grader back in the Space Race, and I have quite a bit of G mechanics built into IMTU, because Space is Hard.

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.

Think I'm going with what I posted above, will just really shock them into feeling the hard.

 

[martletsquire]

So you have counter-grav for small vehicles but not for starships? Urrrrrr, uh, okay.

So, how much to retrofit counter-grav on my freetrader?

 

[kilemall]

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.

My TU is with the first 120 years of human space civilization and something like only 40 years of jump, so for now most people on colonies welcome the roar of the engines as a natural part of the area signifying commerce (like in Wichita Kansas, NO one complains at all about any plane flying over day or night, because that's the city's life blood).

For those more gentle souls already on Centuari Prime, everything is whispery grav shuttles and liners and so their reveries are not disturbed by the sound of interstellar travel. I wasn't letting players go downport directly either place anyway, but the frontier is another matter.

I'm going CT and not going to make this a hard physics exercise, I'm looking for the feel. Cargo/mass dumping does make for high drama, but I expect I will impose that sort of thing only if say their main M-drive is out but they are trying to escape a situation on thrusters only.

I have a whole treatment in my IMTU article re: grav and whatnot, but to summarize very quickly-

* Traveller Gravtech is push/repulsor, no pull/tractor

* Normal deck gravity therefore is a 'push' repulsor plate in the ceiling

* For adjusting against accel there are push plates in the aft bulkhead of any compartment, and possibly 1-G in the floor for gas giant refueling runs or landing on a 2-G world (exploration ships might have a 2-G floor plate for 3-G enviornments)

* Gravtech is limited by TL, TL9 is 1-G compensation and TL10 is 2-G, my TL limit

* TL 7-8 craft used constant 1-G and vertical deck design to create 'artificial gravity' under accel (at least enough push for objects to drop 'normally' even though its more like the floor rushing up to the object and to provide a push resistance for muscles)

* Some TL 9-10 craft still use vertical decks to allow for constant 2-G/3-G accel with no special equipment or impact

* Accelerating 1-G over compensation will just involve strapping in, not moving around much, and a long time to affect people

* Accelerating in a 6-G fighter 2-G over compensation means 4-G constant, which is no big deal if it is like an Apollo launch, but IS a big deal for people to do it for several hours or days, and it HURTS

* Cargo is normally in containers and bolted onto each other and the floor/ceiling like modern containers are on ships or well/spine railcars, the rest will be secured one way or another, likely by netting for light items

* Example of game color this allows, most surplus Type S are TL9 craft with fission plants nearing End Of Life and 1-G compensation, so they are uncomfortable and not entirely safe. So there is quite the competition to get a Detached Duty scout that is TL10 and has the new fusion reactors, a fact that is not lost on the Scout Service and can be used to show favor or punish miscreants

* Vertical ships have a retro charm or annoyance depending on people's aesthetics or whether you are the poor cargo master dealing with lift and gantrys/grav loads off smaller cargo holds off the ground, and an association with the faster thrust only liners (although high social standing passengers likely would not be caught dead in anything but a grav liner)

 

[kilemall]

So you have counter-grav for small vehicles but not for starships? Urrrrrr, uh, okay.

So, how much to retrofit counter-grav on my freetrader?

I talked about it above already, it's not a problem for ships to have it, it just COSTS. Space that could be revenue producing, and extra build costs that shows up in a bigger loan with less ability to earn debt service.

So some ships are clunky garbage scows not worth a second glance but which carries the wealth of systems, and others are svelte grav equipped liners and gleaming reflec yachts that never see unsightly reentry plasma marring their perfectly alluring hulls.

Again, HG ships only, costs the next level up in maneuver drive, doesn't cost more in powerplant unless you want to run both (or have the extra capacitors built in to do it in an emergency). I'm going as simple/stupid as possible for the effect I want.

Type A traders are using standard components and so it would have to be a whole drive redux, several million credits, so probably not worth it except as a vanity move.

Better off selling the old bird to fund your new moneymaker.

 

[aramis]

easyest way...
MDrives don't work above .1G... a bit above 3.16 radii from barycenter
canonical gravitics have traction from 1 (surface) to 20 radii. That would give a good overlap,. describe it as too steep a slope to grab...

 

[kilemall]

easyest way...
MDrives don't work above .1G... a bit above 3.16 radii from barycenter
canonical gravitics have traction from 1 (surface) to 20 radii. That would give a good overlap,. describe it as too steep a slope to grab...

Perhaps so, and perfectly reasonable for most people's purpose I'm sure.

I'm going with M-Drive as a combo of eM-drive and MHD thrusting above 1G, eM-drive doesn't work in atmo so there is fuel burn via MHD, otherwise economy thrusting outside of atmo at 1-G which given two out of the three main reactor types do not burn L-Hyd, means the cheap ships can go a long time in normal space without fuel per se.

 

[kilemall]

Thanks to all who responded!

 

[kilemall]

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.

I'll have to think about this one, if you can double fire your lasers you should be able to risk overload to get aloft. Hard on the engineering, especially if you do it routinely, but shouldn't be more then a 5 minute thing most of the time.

 

[McPerth]

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.

Remember that in the case of an aircraft carrier, the plane dos not begin at zero velocity, but (assuming the carrier sails against the wind, as they use to when making air operations), at the carrier speed plus the wind speed, as the plane must calculate its speed relative to the wind, not to the wáter.

So, if a carrier is sailing at 20 kn against a 20 kn wind, the initial (before engaging the engine) speed for the plane relative to air is about 40 kn...

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.

(bold is mine)

Never thought about that, but could deflectors be used to lower the air drag when you intend to use this same air for liftoff?

 

[kilemall]

Never thought about that, but could deflectors be used to lower the air drag when you intend to use this same air for liftoff?

<Shrug> why not, that's what jets are designed for, to provide lift as desired for it's function while most efficiently cutting through the air to avoid drag.

And since I am postulating conformal capabilities, that means the deflector fields can act like the swing wing concept behind the F-14, F-111 and MiG-27, configured one way for maximum lift and then reconfiguring mid-flight for maximum low drag.

 

[WistfulD]

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.

Yeah, and Han and Chewie's ship didn't have a maneuver drive rated in Gs. In fact, it probably had a max speed instead of an acceleration. I was just highlighting the mix of hard science and space opera.

 

[kilemall]

Well, settled on a double the G rating burn for a 1000 second turn at 1% of the fuel supply, with successive turns not only burning up fuel but also potentially damaging the M-Drive. Effectively the starship equivalent of dumping fuel into afterburners.

They'll hate every 1000Cr of fuel they see burning out the tailpipe, and next time likely stick to highport if possible, but they won't be stuck and I can keep my IMTU separation of low-end and high-end landings.

Also not worried about the landing bits, figure the attitude thrusters can put on .5 G pushing one way and the main thrusters can swivel down in most cases for landing and takeoff.

Again, thanks to all!

 

[esampson]

If you really want to get a good idea what certain orbital mechanics are like I would highly recommend downloading Orbiter. It is designed as a very 'hard science' simulator so it doesn't naturally handle all the things that you may experience in Traveller but it does still do a really good job with teaching (and it is possible to build ships that function more like Traveller ships, but it takes a bit of programming).

In the environment that you've laid out this is what I expect would happen:

First, the 'air shaping' technology that you were talking about earlier to reduce drag would probably also be used to generate lift. Just like you use the deflector field to make a faux nose cone to channel air around the ship you can use the field to make a faux wing to lift the ship. I can't give you any idea what the stall velocity of such a ship would be since I have no idea how big a 'wing' you're able to create but the stall speed will also vary depending on lots of other external parameters. On planets with lower gravity the stall speed is lower. As atmospheres get thinner the stall speed gets higher.

At 1G of thrust a ship should probably be able to take off in standard situations. Your average airplane engine produces far less than 1G of thrust (which is why they can't climb straight up) but has little difficulty keeping the plane in the air.

Because of the way engines work in Traveller if an engine is able to produce slightly more thrust than is required to keep the ship in the air then the ship can break orbit. The reason for this is fairly simple. Both the formula for calculating lift and the formula for calculating drag are based on a bunch of semi-constant numbers (coefficient of drag, coefficient of lift, cross section of shape, surface area of wind, etc.) multiplied by the velocity squared. This means that if you travel twice as fast you have both four times the resistance and four times the lift. Both formulas also are multiplied by the atmospheric density without any form of geometric progression. That means that if the atmospheric density is one half then both the drag and lift are one half of what they were.

This means that if the ship is able to fly at a given speed with 1 atmosphere of pressure it can also fly at a height where the pressure is 1/4. It just needs to fly twice as fast and it can manage that because the drag for flying twice as fast in 1/4 pressure is the same as flying at the base speed at 1 atm. Since the engine could handle that, it can handle the higher altitude.

Since the engine has at least a little more power than is required to keep the ship flying it is able to gain altitude, which lowers the atmospheric pressure, which lets the ship go faster, which generates more lift, which lets the ship go higher. It is an endless cycle until the ship reaches a velocity that is so high that it leaves orbit. This isn't, by the way, escape velocity, but significantly higher. EV is the speed you need if you are moving directly away from the planet.

But if all of this is true, why can't airplanes just keep gaining altitude and speed until they launch into space? The answer to that is simple; the engines of airplanes don't provide a consistent level of power at all altitudes like a ship's engine. As a general rule the higher an airplane goes the less power its engine produces (there are exceptions to this curve that occur in some planes as they near ground level because their engines are tuned to be more efficient at higher elevations where they spend the majority of their operations). In our earlier example a plane that is operating at 1/4 atm has to fly twice as fast as it does at 1 atm. This requires the same amount of force from the engine to overcome drag but if the engine is now only producing 75% of the energy that it produces at 1 atm it may not be able to provide that much thrust. Not enough thrust to overcome the drag, the plane slows down, the plane slows down and it loses lift. It loses lift and it drops down. Eventually it will reach a point where the engine produces just enough energy to offset the drag when it is moving fast enough to generate just generate enough lift to keep it from descending.

So why don't we fire our rockets that way? Why not build them like planes and fly them into space? Because launching into space this way means you spend a great deal of time inside the atmosphere. The longer you are in the atmosphere the more time you are dealing with drag. To counteract drag you have to burn fuel. Also, all those airfoils are additional weight which means even more fuel has to be spent. It is simply much more efficient to build larger motors with enough thrust to push the rocket straight away from the planet so that you are in the atmosphere for the absolute shortest time possible. But that's with our engines in the real world. In the world of Traveller where engines either don't use any fuel or else they use very, very little fuel that's a whole different story.

So in summation, the majority of the time ships in your universe probably take off more or less like airplanes. Unlike an airplane, however, instead of 'leveling out' after gaining some altitude they probably just keep going up at more or less the same angle. Ships with powerful enough engines (greater than the gravitational pull of the planet) will probably angle up to a more or less directly away from the planet slope as soon as they are able. They might need to wait until they are over water or uninhabited areas or at a high enough altitude to remove any potential danger to the people on the ground before they do that, but once that threshold is crossed there's not a lot of reason for them to be fighting the atmosphere. Ships with weaker engines will continue on a slope appropriate to their engine and the environmental factors (gravity and atmospheric density). This may result in a path that sort of spirals out from the planet but I doubt it would take very long at all even for them to break orbit from the planet (probably around .5 to 1 trip around your average Earth like planet).

 

[aramis]

For the more lighthearted, but obnoxiously realistic at all the painful points, Kerbal Space Program is a fun lesson in orbital mechanics.

 

[jawillroy]

For the more lighthearted, but obnoxiously realistic at all the painful points, Kerbal Space Program is a fun lesson in orbital mechanics.

For fun read brutal. <barbie> Matching orbits is HARD.</barbie>

 

[aramis]

For fun read brutal. <barbie> Matching orbits is HARD.</barbie>

Upgrade your mission control to get better planning assistance.

 

[atpollard]

Back to the OP topic:

If gravity is that strong, then the landing will probably be a bit hard as well.
So why not kill two birds with one stone and install a really heavy spring and latch under the landing pad.

When you are ready to leave, trip the latch. The spring releases the energy stored at landing and gives you that extra boost to reach orbit.

 

[kilemall]

Back to the OP topic:

If gravity is that strong, then the landing will probably be a bit hard as well.
So why not kill two birds with one stone and install a really heavy spring and latch under the landing pad.

When you are ready to leave, trip the latch. The spring releases the energy stored at landing and gives you that extra boost to reach orbit.