Landing Atmo capable ships

[whartung]

A fully loaded 747 can be up to 485 tons. The main body appears to have 16 wheels, plus 2 up front.

The A380 can go up to 632 tons, it has 20 wheels plus the 2 up front.

Finally, there's nothing that says the ship has to use it's own grav. There can easily be Starport provided grav sleds that can be used to push things around the tarmac.

A 1000 ton (ton == 2000lbs), with 3 landing pads, weight evenly distributed.

1000/3 = 333 tons
333 * 2000 = 666,000lbs

Desired ground pressure = 50 lb/in^2 (a passenger car is ~30 lb/in^2)
Area = Weight/pressure
Area = 666,000lb / 50 lb/in^2
Area = 13,320 in^2, or 92.5 sq feet, which is a pad ~9 foot square.

So, a 1000 ton ship could have 3 landing pad of 9x9 feet (3x3 meters).

I read that a car tires pressure is roughly the same as its ground pressure. Using that logic, a 747 tire is 200 psi, which suggests a 200 psi impulse on landing (clearly doesn't need that much standing still). Runways are pretty thick to support 747 landings, but the overall tarmac is not.

The basic point being that 50 psi of ground pressure is not a lot, so feel free to adjust the pads as necessary.

 

[AnotherDilbert]

I still say having a ship where you need to keep the reactor going every second or you lose the landing gear is a problem.

Agreed.

I am using 10 tires (2 sets of 4 plus 1 set of 2 forward), so, 100 tons per tire, roughly.

A Cat 797 supports ~390 tonnes on six wheels and doesn't even need paved roads:

If we can do a little better at TL15, then 100 tonnes per wheel is quite feasible.


An Airbus A380 has a maximum takeoff weight of about 575 tonnes and lands on 22 wheels:

A scout would need about 40 such wheels to land on paved tarmac.

 

[kilemall]

The trick becomes getting the ground to support the weight. For example, a steel wheel on a train can hold a lot more weight, but then you need a steel rail to support it. It would cut through an asphalt or concrete road. So a super tire will either need a super-alloy runway or it will need to be wider to spread out the load.

Typically a modern steel axle works out to something like 30-tons loaded, and that's split by two wheels. The rail not only has to sustain that pounding at speed, but also stay in place and not split apart.

A more realistic option is a skid at the end of the leg, possibly with a bunch of mini-wheels good enough for moving around on surfaces but not takeoff/landing at speed.

https://en.wikipedia.org/wiki/Ground_pressure

Lets shoot for 25 PSI of ground pressure. Not good for mud, the ship will sink in, but should be good for firm ground.

916 tons assuming US tons is 1,832,000 pounds. Divide that by 25 yields 73280 square inches we need to achieve that ground pressure.

Assuming two skids of 3 feet wide each, that yields an 84 foot long skid.

Okay, 4 foot wide each, that gets us down to 63 feet.

Offload some of that onto a forward leg, and we can make those back skids shorter or skinnier.

Scouts of course may not have the luxury of firm ground, so part of the kit for exploration missions might be a fold out 'landing mat' or high tech quickcrete that the air/raft delivers to the proposed landing site ahead of main landing.

For something like the Free Trader and larger, I would expect 3 and 4 legskids to be necessary.

Arranged as so-

---X-X---
-X-----X-

I am imaging the skid portion to fold out as the strut lowers from the ship/craft.

If one insists on the conventional jetlike takeoff, it's going to take a higher PSI, smooth runways, and possibly a lot of hightech mini-tires to generate the ground pressure footprint and still be able to be retracted into the belly.

 

[Timerover51]

I agree, just leave the engine running and hover. Then there are no issues with a landing leg with a 666 tonne point load on a ground with a 10 tonne per square meter load bearing capacity.

A Sherman tank with a ground pressure of 14 pounds to the square inch of track comes out to about 10 tons per square meter, and a very large range of ground can support that. The average grassy field should support that easily. A 2000 ton mass ship would need 200 square meters of landing pads to keep the weight at that level, or a surface equal to 10 meter by 20 meters. Otherwise, the ship simply rests on its belly, which would supply more than enough bearing surface to avoid significant sinkage into the soil.

The packed crushed coral runways I landed on in the South Pacific could take a lot more weight than that.

 

[atpollard]

A Sherman tank with a ground pressure of 14 pounds to the square inch of track comes out to about 10 tons per square meter, and a very large range of ground can support that. The average grassy field should support that easily. A 2000 ton mass ship would need 200 square meters of landing pads to keep the weight at that level, or a surface equal to 10 meter by 20 meters. Otherwise, the ship simply rests on its belly, which would supply more than enough bearing surface to avoid significant sinkage into the soil.

The packed crushed coral runways I landed on in the South Pacific could take a lot more weight than that.

Yeah, I was going with ground load bearing because the surface (crushed coral in your case) needs to be thick enough to spread out the load to that level or the subgrade will fail and the strut will punch through the surface support.

This all started for me when I tried to figure out how thick a concrete landing pad was based on the landing skids shown in the illustrations. (I design roads from time to time and wanted to estimate the cost of a landing pad). The answer was a concrete slab measured in meters of thickness.

That's when I realized that Starships are more like 'flying buildings' than aircraft.

 

[atpollard]

A Cat 797 supports ~390 tonnes on six wheels and doesn't even need paved roads:

If we can do a little better at TL15, then 100 tonnes per wheel is quite feasible.

Personally, I would LOVE to see a scout capable of wilderness landings with those tires retractable into the body.

Unfortunately, the space dedicated to 'landing gear' in starships seems much more humble. It comes down to the area of the tire in contact with the ground. Note how much longer and wider the surface bearing area of the giant tire is compared to the tire on the vehicle next to it. If you put the same load on the small tire (even if it could support it), it would sink into the ground.

At work, we encounter frequent issues with those small hard tires on forklifts. They are able to support much greater loads than a conventional car tire. The problem is that if nobody said that there was going to be forklift traffic, then the roads cannot support the greater concentrated loads and the wheels sink into asphalt paving and crack (and eventually crush) the concrete slabs.

 

[AnotherDilbert]

This all started for me when I tried to figure out how thick a concrete landing pad was based on the landing skids shown in the illustrations. (I design roads from time to time and wanted to estimate the cost of a landing pad). The answer was a concrete slab measured in meters of thickness.

How does this differ from current runways? They are, as far as I understand, not much more than 1 m thick and support aircraft of up to 650 tonnes or so.


Several meters thick landing pads should be cheaper than several kilometres of 1 m thick runways?

 

[AnotherDilbert]

Personally, I would LOVE to see a scout capable of wilderness landings with those tires retractable into the body.

I agree those tyres are too big for a Scout, but we can estimate the size of the contact patch.

The tyres seem to be slightly narrower than the car, so say 1.5 m wide and perhaps 2 m long. That gives us six contact patches of 1.5 × 2 = 3 m² for a total of 6 × 3 m² = 18 m² to support ~400 tonnes.

With the same ground pressure a Scout would require a contact patch of about 45 m² to land on heavily reinforced but unpaved ground. That would be two 4.5 m square (3 squares by 3 squares) pads and a smaller front pad. Not impossible?

 

[atpollard]

How does this differ from current runways? They are, as far as I understand, not much more than 1 m thick and support aircraft of up to 650 tonnes or so.

Several meters thick landing pads should be cheaper than several kilometres of 1 m thick runways?

We are currently looking a the two smallest starship sizes (100 dTon and 200 dTon) and already forced to compare the loads to the largest dump truck and aircraft in the world ... with the 100 dTon ship dwarfing them. As you get into even the 400 dTon starship, the numbers get much worse (based on the plans and illustrations).

Is a 10 meter thick concrete pad reasonable? (personal decision)
It is NOT what I had expected.

 

[AnotherDilbert]

Is a 10 meter thick concrete pad reasonable? (personal decision)

Ok, why not? Still cheaper than several km of runway?

We can build and support much bigger structures today:

The problem is wilderness landings...

 

[atpollard]

I agree those tyres are too big for a Scout, but we can estimate the size of the contact patch.

The tyres seem to be slightly narrower than the car, so say 1.5 m wide and perhaps 2 m long. That gives us six contact patches of 1.5 × 2 = 3 m² for a total of 6 × 3 m² = 18 m² to support ~400 tonnes.

With the same ground pressure a Scout would require a contact patch of about 45 m² to land on heavily reinforced but unpaved ground. That would be two 4.5 m square (3 squares by 3 squares) pads and a smaller front pad. Not impossible?

Yeah, it is definitely doable ... especially with skids. The illustrations just tend to show skids that are closer to 1 m x 2 m and those are just too small.

Wheels can also work, but as you noted earlier, a type S needs about 40 aircraft tires, not 3 tires like a Cessna. [hyperbole] So maybe you can improve the tires and landing pad and reduce that to 20 tires.

 

[atpollard]

Ok, why not? Still cheaper than several km of runway?

Yes, it can be built.

Just checking your question out of curiosity ...
A runway (paved) for an A380 is about 2900m long x 62m wide x 1m thick for 179,800 cu.m. of concrete.

A typical starship is about 50 meters long, so let's go with a 100m x 100m x 10m thick landing pad (to accommodate longer, slender ships as well) for 100,000 cu.m. of concrete.

So a landing pad is in the ballpark of half the cost of a runway. However, one runway will land many planes per day which then drive on much thinner pavement, while each Starship will need the landing pad (or equal strength storage area) for as long as it is at the starport.

If we assume that each 100x100m pad will accommodate up to a 500 dTon ship and a 1000 dTon ship could land by taking up two adjacent pads, then a landing pad for a 1000 dTon ship costs about the same as a 2.9 km runway.

... like I said, you piqued my curiosity.

 

[Timerover51]

One thing in this whole discussion has puzzled me, and that is why are the ships being discussed so heavy, especially the merchant ships? Mass equals costs of building, and a merchant ship needs to be as cost-efficient as possible.

For a quick approximation of the mass of a merchant ship, I convert Traveller dTons of about 500 cubic feet into gross register tone of 100 cubic feet by multiplying by 5. I then divide by two to get a reasonable estimate of the mass of the ship. This is a rule of thumb I use for estimating the mass of nautical general cargo liners, that is ships carrying both cargo and a small number of passengers.

Using that, I get the mass of a 200 ton Free Trader at about 500 mass tons. Given the approximate length of the Far Trader from Supplement 7 of 50 meters by 30 meters, and assuming a belly landing, that would give slightly over 3 tons per square meter of surface weight. The standard Subsidized Merchant as depicted in Supplement 7 I would likely go with a lower figure, given the large amount of cargo space.

I am assuming that the hull of the merchant is made of what I call 40 pound high-tensile strength, HY-80, steel, similar to what is used in submarines. As I am not dealing with the very large pressure of deep diving, I can get away with lighter framing, based on standard ship building practice. I view a 1400 Traveller dTon Merchant ship are equivalent to the World War 2 Liberty ship in mass, so about 3200 tons of ship mass.

 

[atpollard]

One thing in this whole discussion has puzzled me, and that is why are the ships being discussed so heavy, especially the merchant ships?

I have no objections to your rule of thumb. The weights being discussed just happen to come from MegaTraveller and FF&S where Starship descriptions and design rules include both volume and mass. A rough rule of thumb from the MT & TNE designs is about 10 metric tonnes per dTon (916 tonnes for a loaded MT Type S).

My calculations for a Apollo Command Module/Saturn V and Space Shuttle/STS come out closer to 4 metric tonnes per dTon (fully loaded) ... the figure I use IMTU.

But for this discussion, it seemed best to stick with the 'official' mass of the starships as a starting point.

 

[atpollard]

Using that, I get the mass of a 200 ton Free Trader at about 500 mass tons.

Just a quick clarification, is that 500 mass tons empty, or fully loaded with fuel and cargo?

My quick-and-dirty 4 tonnes per dTon suggests about 800 mass tonnes for a fully loaded Free Trader.

 

[Timerover51]

Just a quick clarification, is that 500 mass tons empty, or fully loaded with fuel and cargo?

My quick-and-dirty 4 tonnes per dTon suggests about 800 mass tonnes for a fully loaded Free Trader.

That is what I would call a "light displacement" weight, i.e. the mass of the ship equipped but without cargo or fuel. A lot depends on how much mass you allow per Traveller displacement ton of volume. I tend to figure on 5 tons mass weight per Traveller displacement ton That is a reasonable figure based on my US Army logistic manuals. For dense cargo like ammunition, which averages about 40 cubic feet per ton, I have either a lot of empty volume in the cargo hold, or space for a lot of lightweight cargo. Then I might go as high as 10 tons per Traveller displacement tons for the average Free Trader. For Subsidized Merchants with a regular route, I might allow for up to 15 tons per Traveller displacement ton.

What I probably need to do is put together in one format all or at least some of my data on how much volume does a ton of a particular good occupy, including more civilian-type of goods. I do prefer a smaller container than the one used in Traveller, from the standpoint of material handling equipment. My standard container is 1.5 by 1.5 by 3 metters, of 5 X 5 X 10 feet. That is much more easily handled by equipment than the overly large Traveller container. I also assume some break-bulk palletized and crated cargo will be carried to the more frontier planets. Again, I am looking at it from the standpoint of a logistic officer and self-taught naval architect.

I probably should also put together my data on airfield construction times as well, depending on how elaborate the facilities are. A basic World War 2 fighter strip, with facilities, would take 4 aviation engineer battalion months, but a landing surface could be finished much sooner. The hard-packed coral runways in the Solomon Islands, that I would rate as equivalent to a concrete surfaced runway, could be done in 2 weeks.

 

[AnotherDilbert]

Traveller ships have armoured hulls that have significant mass, e.g. by MT a TL12 Free Trader has a minimum of 600 tonnes in just the hull.

The drives and life-support are perhaps another 700 tonnes at ~3 tonnes per m³ or 40 tonnes per Dt.

So we start at a minimum of 1200 tonnes before we even consider payload.

Max cargo is calculated at 1 tonne per m³ or 14 tonnes per Dt.

Staterooms mass next to nothing.

 

[Timerover51]

Traveller ships have armoured hulls that have significant mass, e.g. by MT a TL12 Free Trader has a minimum of 600 tonnes in just the hull.

The drives and life-support are perhaps another 700 tonnes at ~3 tonnes per m³ or 40 tonnes per Dt.

So we start at a minimum of 1200 tonnes before we even consider payload.

Max cargo is calculated at 1 tonne per m³ or 14 tonnes per Dt.

Staterooms mass next to nothing.

But I am not using MegaTraveller. I am looking at ships built under Book 2 of the LBB set. Therefore, your comment on the hull is not applicable. As for the drives, I assume that is MegaTraveller data as well? Drives at 3 metric tons per cubic meter are actually pretty dense, as is 3 tons per cubic meter for life support. As for cargo, I already indicated in a previous post my standard cargo allowance is 5 tons mass per Traveller displacement ton. That is based on the average of a wide range of military cargo, including ammunition.

Again, why the massive armor for merchant ships? Unless you totally discount what that might cost, it does not make any sense whatsoever, and does drive up the size of the maneuvering plant when it comes to acceleration. You accelerate mass, not volume.

 

[AnotherDilbert]

But I am not using MegaTraveller. I am looking at ships built under Book 2 of the LBB set.

It's the same in CT, according to several sources.

Strong Hull: The hull of a starship must not only be constructed to withstand normal space; it also must withstand the rigors of jump space.

Bulkheads: The major structural components of a ship are the bulkheads, and they represent the compartmentalization of the ship for damage control and environment maintenance as well as the outer hull of the ship. Bulkheads are very difficult to destroy. A concerted effort with an energy weapon or explosive must produce 1000 hit points of damage in order to create a hole large enough for a person to step through. Bullet firing weapons are ineffective against bulkheads.

Reasonably bulkheads are not stronger than the hull.

6. Armor: A vessel's Striker armor rating depends on its High Guard armor rating, as shown on the table below. If a vessel is hit, roll damage on the High Guard damage table, using the weapon's penetration as a negative DM and the ship's armor rating plus 6 as a positive DM.
Armor Rating
HighGuard 0 1 2 3 4 5 6 7
Striker 60 64 67 70 72 74 76 77
Add one to the Striker armor rating for each High Guard armor level over 7.

LBB2 ships have HG armour 0 = Striker armour 40 (errata) = 33 cm equivalent hard steel (about half of the max armour of a modern tank).

This is the same armour as in MT.

 

[Timerover51]

It's the same in CT, according to several sources.

Reasonably bulkheads are not stronger than the hull.

LBB2 ships have HG armour 0 = Striker armour 40 (errata) = 33 cm equivalent hard steel (about half of the max armour of a modern tank).

This is the same armour as in MT.

I would say then that you and me are operating in two different universes then. I will enjoy operating in mine and I trust that you will enjoy operating in yours.