Spaceflight on high gravity planet.

[Sky Captain]

There are Earthlike planets several times massive than Earth discovered with stronger surface gravity and faster escape velocity. If civilization develops on such a planet how could they achieve spaceflight?
On Earth reaching orbit is pretty demanding task with velocity of 7.8 km/s in low orbit and total delta v required when accounting for looses around 9,5 km/s. Now imagine a planet with 1,5 to 2 g surface gravity and ~15 km/s low orbit velocity and total delta v needed for low orbit somewhere around 20 km/s. Chemical rockets would lack delta v capability to reach orbit although suborbital flight would be possible. Nuclear systems tend to have poor thrust/weight ratios and on high gravity world thrust requirements would be demanding.

Are there any other options for spaceflight or civilization on high gravity world would be essentially locked never to explore their solar system?

[Dass.Kapital]

One quick thing I can think of is,

https://en.wikipedia.org/wiki/Project_O ... pulsion%29

https://www.ted.com/talks/george_dyson_ ... anguage=en

While some sort of chemical booster (And lots of it) might be needed to get the ship above the planets radiation belts, once at a good height the nuke pulse should keep lifting.

The shape would probably be a lot wider and flatter?

Also, a two stage system, as in a bigger version of Richard Branson's "White Knight" one and two?

If the surface gravity is higher... won't that effect the atmosphere density? So, while air resistance would be higher... wing lift would also be greater?

[Sky Captain]

One quick thing I can think of is,

https://en.wikipedia.org/wiki/Project_O ... pulsion%29

https://www.ted.com/talks/george_dyson_ ... anguage=en

While some sort of chemical booster (And lots of it) might be needed to get the ship above the planets radiation belts, once at a good height the nuke pulse should keep lifting.

The shape would probably be a lot wider and flatter?

In theory it could work at least in a way that there is sufficient thrust and delta v available from nuclear fuel. Real dealbraker would be engineering and materials science. Very high liftoff thrust would mean nukes would have to detonate much closer to ship to provide more thrust than if taking off from Earth. So the ability of blast shield and shock absorbing system to take extra strain without becoming too heavy would be critical here.
There are people who think structural engineering even for Earth surface launched Orion craft would be extremely challenging, maybe beyond capabilities of existing materials. Real thing was only tested on small scale with chemical explosives so it is very difficult to say what kind of obstacles would be encountered if full scale surface launched ship was attempted to construct.

Also, a two stage system, as in a bigger version of Richard Branson's "White Knight" one and two?

If the surface gravity is higher... won't that effect the atmosphere density? So, while air resistance would be higher... wing lift would also be greater?

Not sure how helpful it would be. Air launch from subsonic speed at 10 - 15 km altitude only takes something like 1 km/s delta v away so not much help if ~20 km/s is required. Fueled rockets are heavy, Fueled monster rockets capable of 20 km/s delta v are many thousands of tons - far too heavy for reasonable aircraft to lift. Even fairly small Atlas v or Falcon 9 rockets when fueled are too heavy for largest aircraft to lift.

[Elheru Aran]

The physics don't really change all that much. The only thing is you need more delta-vee to achieve orbit. It might be an option to use chemical to throw the ship into sub-orbit and then use a couple of nuke pulses to shove the rocket the rest of the way. Alternatively, you might see a combination of chemical and magnetic accelerators.

But yes, it's certainly possible that a civilization on a high-gee planet might not explore their solar system much given the increased cost of putting stuff into orbit.

[Simon_Jester]

Not sure how helpful it would be. Air launch from subsonic speed at 10 - 15 km altitude only takes something like 1 km/s delta v away so not much help if ~20 km/s is required. Fueled rockets are heavy, Fueled monster rockets capable of 20 km/s delta v are many thousands of tons - far too heavy for reasonable aircraft to lift. Even fairly small Atlas v or Falcon 9 rockets when fueled are too heavy for largest aircraft to lift.

The big advantage of air launch is that it gets you above the bulk of the atmosphere, so you don't have to waste delta-v laboriously climbing out of the atmosphere.

Also, with present technology air launch is likely to be subsonic, but numerous proposals for supersonic or hypersonic launch vehicles have been proposed, at which point the aircraft can at least somewhat contribute to the delta-v budget.

More generally, though, yes, higher gravity would very much interfere with the development of space flight, just as lower gravity would be a tremendous asset.

I'm not sure our present exoplanet knowledge is a representative sample of what's out there, honestly; our surveys are strongly biased towards finding heavy planets and planets in close orbits around their parent stars, so that's what we find first.

[Elheru Aran]

Air launch does bring an interesting question to mind. Air travel can be more or less safely presumed to precede space travel... right? So how would a higher gravity planet affect the development of *that*? And how would those developments relate to the advancement of space travel?

For example, would lighter-than-air craft become more cost-effective, as it's cheaper to keep them in the air than to expend a lot of fuel on keeping powered aircraft in the air, maybe? And I'm reminded of something I saw a while ago... I honestly don't recall if this was an anti-satellite weapon or something like that, but it was basically a balloon carrying a rocket into high altitude, where the rocket would ignite and take a shortcut into orbit.

[SpottedKitty]

Going to a bit of an extreme, I'm thinking of Hal Clement's Mission of Gravity. I wonder if the Mesklinites would ever be able to develop space flight without a bit of tech assistance?

[Simon_Jester]

Air launch does bring an interesting question to mind. Air travel can be more or less safely presumed to precede space travel... right? So how would a higher gravity planet affect the development of *that*? And how would those developments relate to the advancement of space travel?

For example, would lighter-than-air craft become more cost-effective, as it's cheaper to keep them in the air than to expend a lot of fuel on keeping powered aircraft in the air, maybe? And I'm reminded of something I saw a while ago... I honestly don't recall if this was an anti-satellite weapon or something like that, but it was basically a balloon carrying a rocket into high altitude, where the rocket would ignite and take a shortcut into orbit.

Sounds like something you might try with a sounding rocket (which really has only one job and that is to physically reach an altitude of X miles, explicitly not meant to reach orbit).

Hm. I'm not sure how much of an airplane's fuel is needed to 'keep the plane in the air,' as opposed to providing forward movement. People have designed planes that can fly around the world without refilling the gas tank, as I recall; it's just that if you want to go fast you need thousands of horsepower.

[Zeropoint]

but it was basically a balloon carrying a rocket into high altitude, where the rocket would ignite and take a shortcut into orbit.

Play Kerbal Space Program long enough to get a craft into orbit, and you'll see why this is a small savings: getting into orbit is more about velocity than altitude. Starting above the thick, goopy sea-level air WILL save you some wasted delta-v, though.

Hm. I'm not sure how much of an airplane's fuel is needed to 'keep the plane in the air,' as opposed to providing forward movement

Forward movement keeps the plane in the air. The plane's various aerodynamic features push/pull air downward, creating an upward reaction force on the plane. This creates "induced drag" which is an unavoidable side effect of producing lift, not reducible by any feat of streamlining. The kinetic energy of forward motion lost to this force can be replenished with the plane's stock of gravitational potential energy, in which case you have a glider . . . or by using the chemical potential energy (or nuclear, if you're feeling ambitious) to turn engines.

[LaCroix]

Wouldn't the much denser atmosphere also help providing lift to a conventional aircraft? They should be able to fly at much higher altitudes, making an in-flight launch even more desireable.

[Simon_Jester]

The atmosphere doesn't extend out to a significant fraction of the depth of a planetary gravity well in any case.

The real obstacle to getting into orbit isn't height, it's speed- your rocket has to accelerate until it's moving at the planet's orbital velocity. It's trivially easy to reach orbit altitude; the V-2's ballistic trajectory topped out a hundred or so kilometers up as I recall. But the V-2 could never, never dream of attaining a stable orbit, because it simply didn't have enough energy in its stored fuel to accelerate its mass to such a speed.

If the surface gravity increases, so does the orbital velocity requirement, and that's potentially crippling.

Air launch helps not because it gets you higher, but because it gets you going faster. This is mainly because you don't need to waste time and rocket fuel lifting your rocket through the lower atmosphere, where it is very inefficient to move fast. Normally, rocket launches have to launch vertically and then 'tip over' because they cannot attain the needed sideways speed while traveling through dense atmosphere. Almost all the fuel expended during the vertical phase of a rocket's flight is wasted.

If you launch from a 15000 to 20000 meters up, you expend much less fuel that way, and you recover much of the lost delta-v.

If you launch from 20000 meters up in a hypersonic aircraft, you gain one or two kilometers per second of delta-v that are provided by the aircraft, which helps noticeably... the problem then becomes building a hypersonic aircraft capable of carrying a rocket more or less able to reach LEO under its own power, which is not a trivial exercise.

[Eternal_Freedom]

Great, now I have a mental image of a kitbashed XB-70 Valkyrie carrying an Ariane V or something.

[Simon_Jester]

That's... pretty much how it was actually supposed to work, although a fully fueled Ariane V would weigh about, oh, 15 or 20 times as much as any figure I can get for the B-70's payload.

This is why nobody does much of anything with air-launch in real life. To make it viable would require absolutely huge, and extremely fast, aircraft.

A friend has seen drawings for a B-70 that was set up to launch a heavily modified Titan II upper stage, which could handle a small spaceplane or Gemini capsule.

[Sea Skimmer]

Air launch also has a problem in that the rocket must be capable of holding together in a horizontal position and good enough engine ignition characteristics and control authority to pitch up, stop falling and then begin accelerating before it looses a huge amount of speed and altitude. Conventional large booster rocket designs cannot do these things, and modifications would push up the empty weight considerably. The Pegasus air launch rocket manages this set of problems by having its own full wings! That is not an idea that will scale well. Nor will supersonic planes in general and some turbine engine problems would come into play here as well, not unsolveable ones but ones that will only further push up aircraft size. Because our engines are temperature limited overall pressure ratios will have to stay lower meaning more fuel burn at lower altitudes.

Hypersonic launch brings a whole new world of problems concerning how absurdly violent that flight regime is and problems like thermal expansion getting worse with overall aircraft size. Also the payload would have to be internal which is the real deal breaker. Maximum speeds with a hydrogen scramjet in any case are about mach 12-14 depending on whom you ask, and while the altitude this is obtained at would rise in this context the air speed won't change because it's a thermal-drag issue. Jet fuel powered will max out at around mach 7 due to actual thermal breakdown of the fuel.

Oh also if sea level pressure is higher normal rocket nozzles won't work as well, because the shapes are already compromises. That could be mitigated in large part by making aerospike engines. Its really kind of surprising that none of them have ever gotten into production, as existing engine designs can generally be modded to become one. I suspect its in large part because the actual demand for very heavy space launches IRL is low. Building space bases on low mountains or plateaus might also become a useful thing for any launch option, though the real problem with this aside from simply finding a suitable spot is bad weather will simply be more prolific.

Given that earth has actually managed to launch a 1000lb probe at 16.25km/s actual velocity, implying nearly 18km/s delta vee though, I don't think chemical rockets can be ruled out unless someone has a specific cite to that effect. That used an Atlas V which is hardly a bleeding edge design either, if superior to the earliest space boosters. A chemical launch should be feasible, it will just require something the size of a Saturn V to put a single person in orbit. I suspect the compounding effect will be to put back manned space travel by several decades.

But hey even if that doesn't work, and NEXUS can't be built with nuclear upper stages and a rocket piston catapult based launch pad, then Nuclear Salt Water Rockets are a thing. An ISP of 6-10,000 for the lower end forms of that concept would solve everything as far as reaching orbital speed goes...at least as long as a nozzle can be built. As far as the environment impact goes it'd probably matter a great deal less then coal power actually does to human health.

Also keep in mind all that some chemical fuels may be possible in the far future which simply are not right now, such as N8 nitrogen and even metastable hydrogen. These would give motors in the ~800 ISP range or greater which should already be enough to solve this with normal sized space boosters. We simply don't know if they'll actually be possible to manufacture.

[Adam Reynolds]

Great, now I have a mental image of a kitbashed XB-70 Valkyrie carrying an Ariane V or something.

There apparently was a Russian proposal to do this using the Tu-160(given that it actually went through full production, unlike the B-70). Another such suggestion involved just using the extra weight that would be possible with either a C-5 or An-124, thus allowing a sufficiently larger payload. The large aircraft would also allow the rocket to be dropped out the back of any standard aircraft rather than relying on the specialized designs as used by Orbital Sciences with Pegasus. Interestingly enough, a Minuteman was dropped out the back of a C-5 in 1974 in exactly this fashion. PDF here.

The USAF has also apparently been conducting tests with this idea as well, under ALASA. It was intended to give low cost on demand microsatellite launch from an F-15(as well as somewhat stealthy launches, but that wasn't the official reason). It was terminated recently after the monopropellant had a bad habit of exploding. So one again sees the problems that can occur.

[Sea Skimmer]

Lol yah they were surprised the hyperglobic propellent exploded on the ground. That's DARPA for you.

A MiG-31 air launch project actually got hardware cut and tried to get civilian contracts in the 1990s. The main Tu-160 air launch concept required a dedicated variant of the bomber because normally it has no means of carrying an external payload and the weapons bays are too short for a decent booster compared to the size of the plane. A small one would still work but that isn't worth operating such a large aircraft to do. So it was nearly as far away from being operational as unbuilt planes.

The problem in any case is you aren't getting more then about forty tons on any of these planes no matter what you do. The Minuteman launch kinda illustrates that, impressive but its only a 30,000lb missile and its still so long it needs something like a C-17 or C-5 to carry it because of bulk. And that's with solid rocket motors, liquids have much higher ISP but also much higher bulk.

NASA drew a bunch of mach 3.5 launch planes. I shall dredge up the choicer ones when I'm not heading out the door.


Like this but a whole PDF of them is around.

[LaCroix]

OTher question - given the restriction for aircraft - is there a possibility to use a launch catapult for missile launch, to add the speed necessary?

I am playing with the mental image of a railgun firing a sabot-encased rocket. Basically replacing the first stage by a railgun. I'd figure you'd need a sabot that eats the wear of the railgun firing, and a much sturdier rocket design to withstand the acelleration (and a shitton of power generation, of course). Give the rocket a kick in the ass to Mach X, and get it high enough to make the nozzle work properly, and let it do the rest.

Too far into sci-fy land or just VERY expensive?

[Sky Captain]

Given that earth has actually managed to launch a 1000lb probe at 16.25km/s actual velocity, implying nearly 18km/s delta vee though, I don't think chemical rockets can be ruled out unless someone has a specific cite to that effect. That used an Atlas V which is hardly a bleeding edge design either, if superior to the earliest space boosters. A chemical launch should be feasible, it will just require something the size of a Saturn V to put a single person in orbit. I suspect the compounding effect will be to put back manned space travel by several decades.

One problem would be extra thrust required to lift off the ground. Saturn V if put on 2 gee planet would not be capable of takeoff because thrust/weight ratio would be less than one. More engines and/or powerful boosters would be required to achieve liftoff. Fuel tank structure also would have to be stronger and heavier to not collapse from own weight. All that would make higher dry mass of entire rocket reducing total delta v.

But hey even if that doesn't work, and NEXUS can't be built with nuclear upper stages and a rocket piston catapult based launch pad, then Nuclear Salt Water Rockets are a thing. An ISP of 6-10,000 for the lower end forms of that concept would solve everything as far as reaching orbital speed goes...at least as long as a nozzle can be built. As far as the environment impact goes it'd probably matter a great deal less then coal power actually does to human health.

Nuclear systems certainly have the performance, question is if such systems can be built to have necessary thrust/weight ratio and not melt down. Huge NEXUS like first chemical stage that puts nuclear stage above atmosphere and gives it some upward and sideways velocity may help to reduce thrust/weight demands from nuclear stage.

In any case spaceflight on Super Earth type planet would be insanely expensive. Imagine Saturn v or Nova class rockets just to launch few ton communications satellite or Sojuz capsule.

OTher question - given the restriction for aircraft - is there a possibility to use a launch catapult for missile launch, to add the speed necessary?

I am playing with the mental image of a railgun firing a sabot-encased rocket. Basically replacing the first stage by a railgun. I'd figure you'd need a sabot that eats the wear of the railgun firing, and a much sturdier rocket design to withstand the acelleration (and a shitton of power generation, of course). Give the rocket a kick in the ass to Mach X, and get it high enough to make the nozzle work properly, and let it do the rest.

Too far into sci-fy land or just VERY expensive?

It really depends on how much velocity railgun could give to a rocket and how much heavier rocket structure have to be to survive launch. There have been proposals for such systems, but those involved small payloads with solid rocket booster to circularize orbit after electromagnetic launch.

[biostem]

I would think that you might need to resort to more stages on these rockets, so as to drop excess weight on the way up. Would it be at all feasible to use a really long fuel line to feed the rocket from the ground, or is that just asking for trouble/would be way heavier?

[Simon_Jester]

You'd be pumping the fuel uphill against two gravities as it climbed rapidly.

Not gonna happen.

[SpottedKitty]

I would think that you might need to resort to more stages on these rockets, so as to drop excess weight on the way up.

I'm not sure that would work. Each added stage means there's more weight in fuel, engines and support structure (plus the extra fuel needed to lift all that) that isn't currently contributing to engine thrust. Eventually you'll hit a point where the whole thing can't be designed with enough thrust to take off, or can't carry enough fuel to reach the orbit you want.

Note that the Saturn V does actually sort-of fit into this category, but deliberately; its launch weight was slightly more than its first stage thrust, so it couldn't take off until the engines had burned off fuel at full throttle for a few seconds. That's why the Saturn V launched so slowly compared to e.g. the Shuttle.

[Zeropoint]

From playing KSP, I can assure you that MOAR BOOSTERS definitely will get an arbitrary weight off the ground. You'll end up with pyramid-shaped rockets, though.

[Simon_Jester]

I am pretty sure that in real life, pyramid-shaped rockets aren't structurally viable.

The math works out in theory for ideal structural bracing of nigh-unlimited strength, but that doesn't mean it'll hold together in flight under realistic conditions. There are contraptions you can make fly in Kerbal Space Program that would never in a thousand years get out of the troposphere without blowing apart in real life.

[Borgholio]

I'm thinking rather than more stages, just more detachable boosters similar to the Delta rockets. Rather than using large (and heavy) stages, just a bunch of smaller ones. Would that work, or is it more efficient to have a few huge stages rather than a bunch of smaller boosters?

[Sky Captain]

From playing KSP, I can assure you that MOAR BOOSTERS definitely will get an arbitrary weight off the ground. You'll end up with pyramid-shaped rockets, though.

Actually right now I'm trying to make Kerbin into Super Earth type planet and see if I can make a realistic rocket from real life rocket parts that can deliver some payload to orbit.