Planetary landers

[chornedsnorkack]

What do you think is a minimum performance for a spaceship to land on Earth and then take off to reach even low Earth orbit - entire and unrefuelled?

This is probably impossible by known chemical rockets. And nothing has ever taken off even from Mars.

[PeZook]

Depends on what you want the ship to do, and thus how much mass it has to carry back and forth

Now, for Earth it borders on the impossible because of the atmosphere. According to the Tsiolkovsky Rocket Equation, you could accelerate a 104 ton payload to 7.9 km/s in one go if you started at 1000 tonnes, or about half the launch mass of the Shuttle (assuming engines as efficient as the Space Shuttle Main Engine) ; Problem is, the atmosphere and gravity well makes it a bigger problem than the equation would suggest. Drag and gravity means that the Shuttle has to weigh 2000 tons to loft a 112 ton orbiter into low earth orbit.

So with chemical rockets that's the ballpark ; Starting in space, you need to be able to land a 2000 ton spacecraft full of fuel like a glider, and then have it (violently!) take off again. Mind you, the Shuttle isn't really an SSTO ship like you want, it sheds mass during launch, and of course soft-landing something the size of a WW2 light cruiser is a problem in and of itself. For a 2000 ton ship to glide at all it would need absurdly large wings, which mean mass...you get the idea.

Yeah, doing it SSTO with chemical rockets is probably quite impossible. Mind you, improving your fuel's specific impulse by a factor of 2 reduces the engineering problems considerably, but to get anything remotely feasible you'd need some form of a highly advanced nuclear rocket (10x more specific impulse than chemical rockets, generally speaking), which reduces the total launch mass to something a bit larger than a heavy airliner - mind you, provided it lands by gliding. Any requirements to maneuver on the way down increase the amount of fuel you will need and thus the total mass of the system.

On the other hand, a Mars surface return mission is vastly simpler, and perfectly doable with chemical rockets alone. In fact, NASA currently has a proposal on the table to use the SLS to run a robotic Mars sample return mission.

[chornedsnorkack]

Another matter is - land where?

Lots of SF mention huge spaceships. Well, assuming HANDWAVE propulsion systems - how would such a ship endure the forces of resting on surface of planet in 1 g?

The existing ships, of up to 100 000 tons, do endure 1 g - distributed along its length and then on specially shaped blocks at the bottom of the dry dock. The big ships would, I expect, break apart if beached.

Of course, large spaceships might be built in space and only ever exist in space. But then you could hear of, e. g. fleets of tenders ferrying soldiers from ground to giant motherships unable to take off from ground...

Anyway... assume that you do have spaceships specifically designed to land and take off, not at designed spaceports but in unprepared landscape. Either because you are exploring an uninhabited planet where the spaceports are yet to be built, or you are attacking an enemy planet where the spaceports that exist are defended by enemy.

What kind of landing gear would your spaceships have and where would you prefer to land? On land, or into bodies of water?

[Simon_Jester]

So with chemical rockets that's the ballpark ; Starting in space, you need to be able to land a 2000 ton spacecraft full of fuel like a glider, and then have it (violently!) take off again. Mind you, the Shuttle isn't really an SSTO ship like you want, it sheds mass during launch, and of course soft-landing something the size of a WW2 light cruiser is a problem in and of itself. For a 2000 ton ship to glide at all it would need absurdly large wings, which mean mass...you get the idea.

Your best bet would probably be to make it a flying boat, but the glide problem is an issue, and wings do NOT do good things for a vehicle's buoyancy, because they have lots of surface area (and heat-shielded weight) but very little volume because they're so thin...

On the other hand, a Mars surface return mission is vastly simpler, and perfectly doable with chemical rockets alone. In fact, NASA currently has a proposal on the table to use the SLS to run a robotic Mars sample return mission.

It REALLY helps if you can manage on-site propellant production. Landing a fuel factory on the planet first, and having it produce the takeoff fuel, saves you a lot of trouble because it means you don't need to treat the takeoff fuel as part of the landing payload. Total spacecraft mass decreases by a factor of lots, which means you don't need such stupidly advanced engines.

This is very much a possibility on Mars but poses some challenges (see Zubrin and Mars Direct for reference).

On Earth it actually wouldn't be hard at all to build a robotic lander that could, say, liquefy oxygen straight out of the atmosphere, and create kerosene or alcohol or whatever out of local biomass. You could then use this to fuel your rocketship's ascent stage, at which point the whole SSTO issue becomes simpler.

It also helps if you have an airbreathing engine capable of getting your MAGNIFICENT SPACELANE up to hypersonic speeds within the atmosphere, no?

Another matter is - land where?

Lots of SF mention huge spaceships. Well, assuming HANDWAVE propulsion systems - how would such a ship endure the forces of resting on surface of planet in 1 g?

The existing ships, of up to 100 000 tons, do endure 1 g - distributed along its length and then on specially shaped blocks at the bottom of the dry dock. The big ships would, I expect, break apart if beached.

On the other hand, they are also very big for their mass, since these huge cargo ships are actually designed to maximize cargo volume. If you have technomagic propulsion systems you might well build the spacecraft a lot sturdier, and physically able to survive "crunchdown" landings where the ship settles into a soft surface.

[chornedsnorkack]

Largest production device capable of vertical takeoff and landing is Halo. Maximum takeoff weight 56 t, of which the empty weight is 28,2 t. The payload is quoted as 20 t.

How well do the big helicopters handle landing on ground that is rough and uneven, or soft, etc?

Given a HANDWAVE reactionless drive box, instead of the big rotor/s, what would be the practical size of a spaceship fit to land on unprepared grounds? You do not want the pressure hull to crack or be pierced as soon as you switch off your lift engines and have to support its weight in 1 g.

[Dass.Kapital]

One thing that has not been mentioned so far.

1) Shape of landing/returning vehicle: Aerodyne ( http://www.astronautix.com/craft/x24a.htm , http://www.jrbassett.com/X15/X24A_2a.JPG )

This allows for lift, controlled flight and a shape which is good for putting heat-shields on.

[chornedsnorkack]

Yes, but how will X-24 hover?

Typical endurances for current state of the art helicopters are in the region of 3 hours.

Note that 3 hours of 1 g would sum up to delta-V of 106 km/s! Yet a helicopter will run out of performance at a horizontal speed of just 80...90 m/s... and at climb of mere 5 m/s!

How long hover endurance in atmosphere would be necessary for a planetary lander to choose a landing spot - or carry out a battle mission in atmosphere and return to orbit without landing?

[lPeregrine]

How long hover endurance in atmosphere would be necessary for a planetary lander to choose a landing spot - or carry out a battle mission in atmosphere and return to orbit without landing?

Why would you spend hours hovering looking for a landing spot instead of finding one from orbit? It seems like you're stuck on a really soft-scifi idea of what you want your ship to do, but you want hard-scifi numbers for it. It's probably a doomed effort.

[chornedsnorkack]

Checking it, Apollo initial missions budgeted about 60 seconds hover time for landing. The later missions managed to save fuel on the approach and extend the hover time to two minutes.

So yes, landing with one minute hover time is feasible. Of course, providing that one minute in 1 g is harder than in 0,16 g...

[Lord Revan]

you also have to remember that Moon as practically no atmosphere to spreak of (there might be stray gasses hanging near the moon but that nothing that would have any real effect) so it's not only gravity you have to work against if landing onto Earth style planet.

[Simon_Jester]

Largest production device capable of vertical takeoff and landing is Halo. Maximum takeoff weight 56 t, of which the empty weight is 28,2 t. The payload is quoted as 20 t.

How well do the big helicopters handle landing on ground that is rough and uneven, or soft, etc?

Given a HANDWAVE reactionless drive box, instead of the big rotor/s, what would be the practical size of a spaceship fit to land on unprepared grounds? You do not want the pressure hull to crack or be pierced as soon as you switch off your lift engines and have to support its weight in 1 g.

Heh.

You make an excellent point. At the hard-SF end of the scale such worries reign supreme. At the other end, with sufficient freedom to imagine, you get things like Doc Smith's Dauntless "landing" on a planet by settling belly-down on a concrete field and crushing a hollow into the concrete with its own weight. Or, wackier yet, landing point-down on its sharp, conical tail by crushing down into random rock on a mountain.

With the right materials things like that would be possible- but they entail an almost inconceivable amount of bulk, advanced technology, and magic engines.

Checking it, Apollo initial missions budgeted about 60 seconds hover time for landing. The later missions managed to save fuel on the approach and extend the hover time to two minutes.

So yes, landing with one minute hover time is feasible. Of course, providing that one minute in 1 g is harder than in 0,16 g...

One minute turned out to be pushing it; Apollo 11 damn near didn't come back because of all the fuel they had to burn up hunting for a new landing site when the first one turned out to be a boulder field.

If I were Buzz Aldrin, I'd probably have occasional nightmares about Armstrong taking "just thirty more seconds, I am NOT giving up on this!" after being unable to find a landing spot at first... and eating into the takeoff fuel.

[chornedsnorkack]

So unless your lander has the capacity to hover for two minutes in 1 g, it is not practical to use.

And you still need to take off.

If you can sustain, say, 1,4 g diagonally upwards - 1 g to hover/rise, 1 g to accelerate horizontally - you´d be in orbit in 800 s. But your delta-V would sum up to 11,2 km/s - both ways. So 22,4 km/s for landing and takeoff, plus the hover time - 2 minutes in 1 g amounts to 1,2 km/s.

Brief hover to choose a landing spot is fine to visit one spot on planet, chosen from orbit. If you want to carry out exploration, or search and rescue, the ability of prolonged hover would be nice.

And how about confronting an enemy who may not have spaceship landing HANDWAVE capability (at all, or available nearby), but who does have atmosphere-bound vehicles?

Messerschmitt 163 was impossibly fast and high against the propeller adversaries. But the short endurance of the rockets - just 7 minutes of engine run - meant that the propeller planes could outlast the Me-163, and they knew they could.

If you have a planetary lander, which does have the capability to keep accelerating to any speed and altitude all the way to orbit, where it is invulnerable, confronted by jet fighters and helicopters, both of which can outlast it (endurance in 1-2 hours for fighters, 3-4 hours for helicopters) and of which the fighters can use the aerodynamic wings to outmaneuver it... how much advantage is controlling the orbit?

[Simon_Jester]

So unless your lander has the capacity to hover for two minutes in 1 g, it is not practical to use.

Well, that depends on conditions. If, for example, you can send down a probe-bot to assess the conditions of the landing site directly, you don't have a problem. And such a probe might be a good deal lighter than the fuel your craft would need to hover!

Likewise if you are landing on a known solid surface, or are confident that your craft's rough-field landing capability can handle the known parameters of the surface. This would be typical for a craft that acts as the equivalent of a military helicopter- such helicopters usually aren't landed on completely unknown surfaces even though they could be called upon to operate anywhere in the world.

If you can sustain, say, 1,4 g diagonally upwards - 1 g to hover/rise, 1 g to accelerate horizontally - you´d be in orbit in 800 s. But your delta-V would sum up to 11,2 km/s - both ways. So 22,4 km/s for landing and takeoff, plus the hover time - 2 minutes in 1 g amounts to 1,2 km/s.

...Uh, what? If you can sustain 1.4g diagonally upwards, you're better off pointing it straight up to get above the atmosphere as fast as possible. Or almost straight up.

There's a reason rockets don't "tip over" and start accelerating sideways until they've climbed many kilometers. That said, your calculation of the equivalent delta-v expended to hover in place for two minutes seems to be correct.

Brief hover to choose a landing spot is fine to visit one spot on planet, chosen from orbit. If you want to carry out exploration, or search and rescue, the ability of prolonged hover would be nice.

True. It depends on exactly what the lander is for. You might have limited-hover landers and "drone" craft which contain a much higher mass fraction of payload as engines/fuel and can hover rather longer, for example.

If you have a planetary lander, which does have the capability to keep accelerating to any speed and altitude all the way to orbit, where it is invulnerable, confronted by jet fighters and helicopters, both of which can outlast it (endurance in 1-2 hours for fighters, 3-4 hours for helicopters) and of which the fighters can use the aerodynamic wings to outmaneuver it... how much advantage is controlling the orbit?

Considerable advantage. Your orbit-capable fighter can be launched from orbit, bomb a target, and return to orbit, without ever flying low or slow enough to be counterattacked. How much of that sort of thing will it take, before the enemy's airstrips and air defenses are too battered to pose a threat?

[chornedsnorkack]

If you have a planetary lander, which does have the capability to keep accelerating to any speed and altitude all the way to orbit, where it is invulnerable, confronted by jet fighters and helicopters, both of which can outlast it (endurance in 1-2 hours for fighters, 3-4 hours for helicopters) and of which the fighters can use the aerodynamic wings to outmaneuver it... how much advantage is controlling the orbit?

Considerable advantage. Your orbit-capable fighter can be launched from orbit, bomb a target, and return to orbit, without ever flying low or slow enough to be counterattacked. How much of that sort of thing will it take, before the enemy's airstrips and air defenses are too battered to pose a threat?

Quite some. This goes into the matter of payload needed for effective orbital bombardment.

Blitzkrieg against Britain, in something like 9 months, August 1940 to May 1941, is estimated to have delivered something like 60 000 tons of bombs. I hear numbers like 90 000 individual bombs.

Britain managed to keep airstrips, air defences and war production running and expanding, and to gain air superiority. The only example of gaining air superiority, seems to be.

V-1 launched 8000 bombs of about 15 000 tons to Britain in some three months, June-September 1944. Did not manage to slow down the advance in France. V-2 delivered about 3000 bombs in 6 months, and this was mere nuisance terrorism.

RAF and USAF combined claim to have delivered something like 2,7 million tons of bombs to areas under German control in something like 3 years, early 1942 to 1945. They did batter air defences and divert resources from war production, but the Germans managed to keep war economy and production of air defences into the early months of 1945.

So yes, these planetary landers can bomb with impunity - either launch their bombs from orbit at 100...200 km, or take the fuel/engine costs (whatever they are) to decelerate from 8 km/s to 1...2 km/s at 20...30 km, and back - and either way, they may find that most of their bombs explode harmlessly hundreds of metres from any worthwhile target. And if their total payload is limited by their being at the end of their logistic line across space, while the defenders are exploiting the efficiencies of aerodynamics and oxidizing air...

With V-2, the British could, and did, deliver millions of tons of materiel and millions of men to chase the launching sites over surface of water and land.

What if they couldn´t?

How would Britain, or USA, endure against an enemy who can in total safety and impunity deliver a few hundred tons of bombs per year (but no more!), year after year - and is waiting to see if the airspace ever gets safe to land in?

[Dass.Kapital]

Yes, but how will X-24 hover?

Typical endurances for current state of the art helicopters are in the region of 3 hours.

Note that 3 hours of 1 g would sum up to delta-V of 106 km/s! Yet a helicopter will run out of performance at a horizontal speed of just 80...90 m/s... and at climb of mere 5 m/s!

How long hover endurance in atmosphere would be necessary for a planetary lander to choose a landing spot - or carry out a battle mission in atmosphere and return to orbit without landing?

Um....why hover?

As pointed out...one can safely peruse a landing site from orbit. Second, are we talking about earth? If so...then what's the problem of doing a water landing? The shape of an aerodyne works well for a 'flying-boat'. Unless you're coming iinto shallow water, there's very little risk of any 'boulder fields'...

As for 'cruising'. Then a mix of air-breathing engines once the craft has de-orbited. One only needs the rockets to get back up to orbit. (See the previous point about dropping a refueling system first)

As for the bombing thing.... Again, why do this in atmosphere? Simply drop rods ( ala G.I. Joe ) and have things ready to stop craft climbing out of the gravity well.

[Simon_Jester]

Quite some. This goes into the matter of payload needed for effective orbital bombardment.

Blitzkrieg against Britain, in something like 9 months, August 1940 to May 1941, is estimated to have delivered something like 60 000 tons of bombs. I hear numbers like 90 000 individual bombs.

Britain managed to keep airstrips, air defences and war production running and expanding, and to gain air superiority. The only example of gaining air superiority, seems to be.

V-1 launched 8000 bombs of about 15 000 tons to Britain in some three months, June-September 1944. Did not manage to slow down the advance in France. V-2 delivered about 3000 bombs in 6 months, and this was mere nuisance terrorism.

RAF and USAF combined claim to have delivered something like 2,7 million tons of bombs to areas under German control in something like 3 years, early 1942 to 1945. They did batter air defences and divert resources from war production, but the Germans managed to keep war economy and production of air defences into the early months of 1945.

So yes, these planetary landers can bomb with impunity - either launch their bombs from orbit at 100...200 km, or take the fuel/engine costs (whatever they are) to decelerate from 8 km/s to 1...2 km/s at 20...30 km, and back - and either way, they may find that most of their bombs explode harmlessly hundreds of metres from any worthwhile target. And if their total payload is limited by their being at the end of their logistic line across space, while the defenders are exploiting the efficiencies of aerodynamics and oxidizing air...

With V-2, the British could, and did, deliver millions of tons of materiel and millions of men to chase the launching sites over surface of water and land.

What if they couldn´t?

How would Britain, or USA, endure against an enemy who can in total safety and impunity deliver a few hundred tons of bombs per year (but no more!), year after year - and is waiting to see if the airspace ever gets safe to land in?

...Bomb technology has changed a lot since the London Blitz, which is an extremely bad analogy for the scenario we're talking about.

Ever heard of the atomic bomb?

Orbital bombers armed with them would be terrifying and powerful, and an enemy with orbital bombers delivering a few hundred tons of them could bring a large, rich nation like the US to its knees.

[Batman]

Not to mention that if you have the orbitals, you no longer really need explosives-just dropping something heavy and resilient enough from orbit can do terrible damage. Using conventional explosives (and to a degree even nukes) when you have the ultimate high ground would be dubious policy at best since kinetic strikes can do all the damage you wish for (or as little, within limits, as you wish for) by simply dropping kinetic penetrators, and any civilization that can reliably stop those is bound to have blown you out of the sky hours ago.

[Starglider]

One minute turned out to be pushing it; Apollo 11 damn near didn't come back because of all the fuel they had to burn up hunting for a new landing site when the first one turned out to be a boulder field.
If I were Buzz Aldrin, I'd probably have occasional nightmares about Armstrong taking "just thirty more seconds, I am NOT giving up on this!" after being unable to find a landing spot at first... and eating into the takeoff fuel.

What? The LEM ascent engine was completely separate from the descent engine, with dedicated fuel tankage not connected to the lower stage. Completely running out of descent fuel would simply have forced an abort (immediate state separation and ignition of the ascent engine).

[Simon_Jester]

Not to mention that if you have the orbitals, you no longer really need explosives-just dropping something heavy and resilient enough from orbit can do terrible damage. Using conventional explosives (and to a degree even nukes) when you have the ultimate high ground would be dubious policy at best since kinetic strikes can do all the damage you wish for (or as little, within limits, as you wish for) by simply dropping kinetic penetrators, and any civilization that can reliably stop those is bound to have blown you out of the sky hours ago.

The kinetic bombardment angle isn't quite as simple as it's sometimes made out to be, but to some extent yes.

Nuclear bombs still win on area effect, though, unless you scale up to outright asteroid-dropping. And asteroid dropping is such a major undertaking that producing nuclear weapons would probably be less work.

Rocks are not free, citizen.

What? The LEM ascent engine was completely separate from the descent engine, with dedicated fuel tankage not connected to the lower stage. Completely running out of descent fuel would simply have forced an abort (immediate state separation and ignition of the ascent engine).

[blink]

Fuck, you're right, I'm an idiot. Sorry. I don't know what happened to my brain.

[Batman]

Actually, at least for the time being, yes, rocks are, and even if they weren't, I seriously doubt the people about to drop them onto your planet are in a mood to discuss the legalities.

[lPeregrine]

Bomb technology has changed a lot since the London Blitz, which is an extremely bad analogy for the scenario we're talking about.

And navigation technology. Instead of primitive WWII-style "maybe we'll get near the right city" navigation at night the bombers are going to know exactly where they are, and the bombs are going to be precision guided weapons (with or without nukes depending on the target).

Actually, at least for the time being, yes, rocks are, and even if they weren't, I seriously doubt the people about to drop them onto your planet are in a mood to discuss the legalities.

Rocks themselves are free, but the fuel to move them around isn't. Either you have to grab them from the asteroid belt or get them off the planet and into orbit, and neither of these is a trivial problem.

[Dass.Kapital]

About the perpetrators.

The nearest source of 'Rock' (Again if we are talking about Earth and not just an earth-like planet) is Luna.

Low gravity and lots of mass to scoop up and compact it into something to drop.

Which gets away from the lander issue of the original post.

Very much cheers to all.

[chornedsnorkack]

Remember that the defenders on any inhabitable planet have a shield against kinetic missiles. Atmosphere.

Look at Chelyabinsk. 300 kiloton explosion - and Chelyabinsk, in Urals, is one of the prime targets for an attack, being a centre of Sovier/Russian military industry.

300 kilotons - and nobody died. Just 1491 injured. Many of them because they misused the over two minutes after flash, to look out of window that was going to be blown to shards. They must have forgotten their civil protection training.

At something like 23 km height, 300 kt was almost a harmless firework. And it was a 10 000 t or so projectile!

A 20 t payload of a spaceship, with 600 t explosion WOULD have been a harmless firework at 20+ km.

Now, you could drop rocks at even smaller speed - and hope they do not explode harmlessly high in atmosphere. But then, for one, they will be even less powerful. Yes, they´d create small craters, and kill people nearby. And some blast effect. But the thing is, your rock is still going to be tumbling randomly for the 100+ km path through atmosphere... your crater is going to be quite some distance from its intended target.

[energiewende]

Even with current technology and only conventional bombs, strategic bombing would be economically devastating although it would cause far fewer direct casualties than WWII. Rather than trying to destroy everyone's house in the hope they won't show up to work tomorrow, factories will simply be repeatedly bombed directly with precision munitions. The explosives expended per vital target destroyed efficiency will have increased more than 100x. Expecting a future strategic bombardment to look like WWII is not reasonable. More likely it would look like a nuclear exchange (and even they were somewhat precision targetted, as technology allowed) except with only one side to the exchange.

If such a ship were able to orbit and operate with impunity, the planet would be helpless and this would be a surrender-inducing moment, if the surrender hadn't occurred already. It's like sailing a fleet of battleships into Tokyo Bay. Landers would deliver police forces/COIN troops.

[Starglider]

The nearest source of 'Rock' (Again if we are talking about Earth and not just an earth-like planet) is Luna. Low gravity and lots of mass to scoop up and compact it into something to drop.

In absolute distance yes but not in energetic terms. If you don't mind waiting, asteroids can be brought to earth orbit using (relatively) cheap solar-ion engines. The energetically 'nearest' objects will be the ones at the earth-sun trojan points and in earth-resonance solar orbits. If you need bombardment ASAP then yes the moon is the obvious source, although for larger bombardments it quickly becomes cheaper to emplace / build a launcher on the surface of the moon vs hauling individual rocks up with spacecraft.

But the thing is, your rock is still going to be tumbling randomly for the 100+ km path through atmosphere... your crater is going to be quite some distance from its intended target.

Tactical orbital bombardment pretty much assumes enclosing the mass in an aeroshell (if soft or rubble) or carving the projectiles into a stable aerodynamic shape (if nickel/iron). This is required for any sort of accuracy, although simply spraying the rock with ablative material will greatly improve chances of making it to the surface. Strategic orbital bombardment with dinosaur-killer class asteroids, not so much.