When does the perspective from the cockpit of a spaceship change?

[musicgold]

Hi,

Maybe this is a foolish question but I am not able to wrap my mind around it.

I imagining a spaceship approaching the Earth as shown below. The ship is planning to land at the red cross in the first picture, somewhere in Europe. I think, from this distance the pilot must feel that he is headed to giant ball, right angles, and if the ship could pass through it, he would emerge on the other side of the ball. (perspective #1)

Now having been to in the cockpits of many planes while they were landing, I know how it looks and feels (perspective #2).

I wish to know when anw how would the perspective of the spaceship pilot change from #1 to #2?

Thanks

Perspective #1

Perspective #2

 

[hutchphd]

Perspective #1 is wrong for landing on a planet with an atmosphere.

 

[Vanadium 50]

Perspective #1 is wrong for landing on a planet with an atmosphere.

The USS Lawn Dart!

 

[Bandersnatch]

At some point during the descent you orient the spaceship so that the landing gear points towards where the land is - which is all that #2 is. Maybe imagine you're aiming for the limb of the planet instead of straight down. E.g. towards Japan. As you approach, the limb of the planet flares out and flattens until it's just the horizon.

Plenty of video games featuring landing a spaceship, of varying degrees of magical physics involved. Try searching youtube for e.g. "elite dangerous landing on a planet" to get a cockpit view during the whole process. There's the free Orbiter simulator if you want to play with it yourself.

 

[Halc]

It is best to approach anything butt-first. You want to slow down. The picture you depict is Hollywood, ship with engines thrusting the ship towards its destination which is exactly the worst thing to do.

WRONG:

In actuality, the ship (like the returning Apollo spacecraft ) can orient itself any direction as long as its coasting. The ship's best trajectory is to go off to one side, letting gravity pull you in and hitting the atmosphere at nearly horizontal trajectory on the far side of the planet. This hits Earth at maximum speed and allows the atmosphere to do the braking for you. The ship is oriented butt-down the whole way because it's shaped that way. It has no engines.

Your pictured futuristic ship doesn't seem designed for re-entry, in which case it has fuel to spare and can do the slow down itself. So assuming it only has big engines in the back like that, it needs to point away from Earth and come to a near halt and lower itself into the atmosphere. Once sufficiently slow, it can roll and fly like the space shuttle landing, but your ship doesn't seem to have wings, so it will be more like the spaceX boosters coming down where it either burns fuel all the way down (like they did when landing on the moon), or it comes down sideways for maximum friction and only orients its engines downward at the last moment to use them to come to a halt, spaceX style.

 

[musicgold]

It is best to approach anything butt-first. You want to slow down. The picture you depict is Hollywood, ship with engines thrusting the ship towards its destination which is exactly the worst thing to do.

WRONG:
View attachment 275988

In actuality, the ship (like the returning Apollo spacecraft ) can orient itself any direction as long as its coasting. The ship's best trajectory is to go off to one side, letting gravity pull you in and hitting the atmosphere at nearly horizontal trajectory on the far side of the planet. This hits Earth at maximum speed and allows the atmosphere to do the braking for you. The ship is oriented butt-down the whole way because it's shaped that way. It has no engines.

Your pictured futuristic ship doesn't seem designed for re-entry, in which case it has fuel to spare and can do the slow down itself. So assuming it only has big engines in the back like that, it needs to point away from Earth and come to a near halt and lower itself into the atmosphere. Once sufficiently slow, it can roll and fly like the space shuttle landing, but your ship doesn't seem to have wings, so it will be more like the spaceX boosters coming down where it either burns fuel all the way down (like they did when landing on the moon), or it comes down sideways for maximum friction and only orients its engines downward at the last moment to use them to come to a halt, spaceX style.

Maybe my choice of pictures was not right.

What I am trying to understand is when does the pilot's perspective change from "going sideways into a spinning ball" to "hovering overhead a steady flat surface". The second perspective is akin to that of person on the landing strip - i.e. the person feels they are on a flat Earth; he doesn't feel like he is jutting out of the surface of Earth, at right angles to its axis of rotation.

Not sure if I am making sense.
Thanks

 

[Bandersnatch]

It changes whenever they pull up for level flight. If the pilot on pic #2 were to dive straight for the ground, they'd have the same perspective as on pic #1.

 

[russ_watters]

Not sure if I am making sense.

It's not clear to me if your intent was to ask about how real spacecraft work, or if you are looking to develop a science fiction scenario. If you are looking for science fiction, the answer can be whatever you want.

People have been describing real life spacecraft . Keep in mind that there's only been around a dozen models of manned spacecraft and of them, one landed on a runway and one left Earth orbit (not the same ship).

 

[Halc]

What I am trying to understand is when does the pilot's perspective change from "going sideways into a spinning ball" to "hovering overhead a steady flat surface".

This sounds like a craft that lands the same way it takes off: engines down. SpaceX does it this way. It rotates at the latest possible moment and only then ignites the landing engines. The lunar module comes down with engines pointed in its direction of motion mostly in order to reduce horizontal velocity. It can't use air friction like the spaceX ships do. So only once that horizontal motion is gone, it should be close to the ground and it can rotate to pointing straight up to make the landing. The view from the pilot window depends on which way the window faces, which is kind of towards the engine in the lunar module case, and the complete opposite end in the spaceX case.

The space Shuttle on the other hand has an airplane shape with windows facing 'forward' and 'up'. The shuttle tends to orbit with 'up' facing the Earth, so upside down and backwards. It can briefly fire the engines to put in in an elliptical orbit that intersects the top of the atmosphere 40 minutes later, giving it time to rotate its belly into its direction of motion. Once most of its speed is lost (down to mach 2 say), it rotates forward a bit and begins to fly like the aircraft it is, landing without benefit of engines on an airstrip as you depict in your 2nd picture. Only a flying wheeled craft lands on a airstrip like that.

The view of the shuttle pilot as he approaches Earth from a great distance (Mars?) is that of whence he came. He's slowing down, so he'd necessarily pointed his nose away from his destination, the opposite direction as the one depicted in your first photo.

 

[musicgold]

If the pilot on pic #2 were to dive straight for the ground, they'd have the same perspective as on pic #1.

While physically he would doing the same thing, he would be "feeling" as if he was going vertically DOWN to the ground and not like "running into a horizontal wall".

When a spaceship is away from Earth thousands of miles, it sees that it is a approaching a ball. Say the landing strip is on the equator and is oriented East-West. So for the most of its journey, it is going right angles to the surface of Earth, like an arrow about to pierce a balloon (let's ignore Earth's rotation and revolution for the sake of simplicity). However when he is about to land on the surface, the pilot feels like he is descending down vertically, and not like he is sticking into the the side of the ball.

 

[Bandersnatch]

That's not physics, that's psychology. I can make myself 'feel' about the perspective however I please, and so can you. Watch those gameplay videos I suggested earlier and decide for yourself when the switch happens, and whether it's an arbitrary distinction or not.

 

[Janus]

While physically he would doing the same thing, he would be "feeling" as if he was going vertically DOWN to the ground and not like "running into a horizontal wall".

When a spaceship is away from Earth thousands of miles, it sees that it is a approaching a ball. Say the landing strip is on the equator and is oriented East-West. So for the most of its journey, it is going right angles to the surface of Earth, like an arrow about to pierce a balloon (let's ignore Earth's rotation and revolution for the sake of simplicity). However when he is about to land on the surface, the pilot feels like he is descending down vertically, and not like he is sticking into the the side of the ball.

An real approach and landing would look more like this:

Coming in on a tangent to the surface, slowing down, and using gravity to curve you around the planet into a landing pattern. You wouldn't come in aimed at the center of the planet.

 

[Vanadium 50]

You wouldn't come in aimed at the center of the planet.

Because that's where you'd end up!

 

[musicgold]

An real approach and landing would look more like this:
View attachment 276017
Coming in on a tangent to the surface, slowing down, and using gravity to curve you around the planet into a landing pattern. You wouldn't come in aimed at the center of the planet.

Now it makes sense! Thanks.
I think the key is to make use of the planet's gravity to orbit around it.

The following video also shows the same thing at the 3.15 mark.

 

[sophiecentaur]

This hits Earth at maximum speed and allows the atmosphere to do the braking for you.

Without an atmosphere, you would need a similar amount of energy to that which is needed to get up there in the first place. Fine for the Moon; significantly worse for Mars (very thin atmosphere).

I remember, a long time ago, discussing the process of atmospheric braking in a manned craft. The ablating tiles may appear to be a needless risk. Why not just take a different path and brake slowly, taking several orbits to 'fly down'? Well, the amount of energy that needs to be dissipated against the hull is of the same order, whatever the path but if it's dissipated slowly, the internal temperature would fry the crew and content. If the outside of the hull is allowed to get 'ridiculously' hot then the process is over much quicker and the hull surface can be cooled later as it flies for a few minutes through the stratosphere and troposphere. There is adequate cooling before the inside gets too hot. (Interesting fact is that the skin of Concord got uncomfortably hot at maximum speed, due to friction.)

However efficient future engines may be and however much fuel can be carried, I would imagine that basic technique would always be chosen. More fuel would much more profitably used to carry higher payloads and the system seems to work pretty well.

 

[DrStupid]

Why not just take a different path and brake slowly, taking several orbits to 'fly down'?

You can do that as long as you exceed the minimum energy for the lowest possible orbit. It's called aerobreaking. But as soon as you drop below that limit you will 'fly down' in less than an orbit.

 

[sophiecentaur]

You can do that as long as you exceed the minimum energy for the lowest possible orbit. It's called aerobreaking. But as soon as you drop below that limit you will 'fly down' in less than an orbit.

But has it been (can it ever be) done from Earth orbit in a manned craft? If not why not? (I thought I had this sorted out in my head.)

 

[DrStupid]

But has it been (can it ever be) done from Earth orbit in a manned craft? If not why not? (I thought I had this sorted out in my head.)

Manned space crafts in lower Earth orbit already have the minimum energy. So far the Apollo command modules have been the only manned space crafts coming in with higher energy. But they didn't used aerobreaking either. I guess it didn't make sense to extend the mission by hours just to slow down from 11 km/s to 8 km/s.

 

[sophiecentaur]

Manned space crafts in lower Earth orbit already have the minimum energy. So far the Apollo command modules have been the only manned space carfts coming in with higher energy. But they didn't used aerobreaking either. I guess it didn't make sense to extend the mission by hours just to slow down from 11 km/s to 8 km/s.

I'm not surprised they didn't add more complexity in such early flights. But I am talking about actual re-entry. Apart from the heating problem, aerobraking would be possible all or some the way down from LEO. That could limit the temperature of the skin of the hull by reducing the dissipated power. It must be to do with the convectional cooling versus the frictional heating. Once in the troposphere, the heat tiles get cooled and the frictional heating has reduced (low enough speed by then).
I guess my problem is reconciling the cooling and braking effects. Presumably aerobraking in low density atmosphere doesn't increase the surface temperature much so the hull will keep cool enough over the necessary time taken (is time the key issue in this?). In your example, the aerobraking would have dealt with about half the KE of the approaching craft. (11² is about twice 8²) but the navigation would need to be just right - they always used to talk in terms of "skipping off" the Earth's atmosphere but that could be dealt with now, I'm sure.
I accept that you have to get rid of a lot of the KE but, if you can take long enough, then why can't cooling (convective) take care of it? Up till now I have gone along with the bald statement that it 'doesn't work'. Perhaps it's just a matter of the weight of a suitable hull, compared with the weight of fuel involved in the initial slowing down. I imagine it's a complicated process.

 

[DrStupid]

Apart from the heating problem, aerobraking would be possible all or some the way down from LEO.

You mean just waiting until the ship is slowed down by the resistance of the residual atmosphere? That would take months (if not even years) and is very unpredictable. Thus it is no option for manned ships. The transition from the orbit to a suborbital trajectory is made using retro-rockets (if possible even for large unmanned objects).

 

[sophiecentaur]

That would take months

We're clearly not discussing the same scenario. I'm not suggesting 'waiting until'. I'm suggesting a suitably chosen glide path, under controll at all times. If it took a few hours instead of a few minutes then the power dissipation could be less than one percent of what's involved in a 'direct' entry.
Same total KE to be dissipated, of course. It seems that the crew would need an unreasonable amount of insulation for the heat to be dissipated outside before the cabin temperature gets embarrassing.

 

[DrStupid]

I'm suggesting a suitably chosen glide path, under controll at all times. If it took a few hours instead of a few minutes then the power dissipation could be less than one percent of what's involved in a 'direct' entry.

But how does that solve the heat problem? Extending the decent to a few hours is only possible by increasing the time between deorbit burn and reentry. I do not see the benefit because there is almost no energy dissipation before reentry. The thermal problems start with reentry but than you can't reduce the sink rate below a certain limit without using a rocket engine.

 

[sophiecentaur]

the time between deorbit burn and reentry

What do you mean by "re-entry"? Once the craft has started losing energy then it is on a re-entry process. Are you referring to the period when the craft is falling passively? That is the time I am suggesting could be extended in a glide of a few hours (using aerobraking). But even the uncontrolled plummet is using aerobraking - it just happens fast.
Edit: I can't think of another way to describe it that will help you understand what I'm actually proposing.

 

[DrStupid]

What do you mean by "re-entry"?

With reentry I mean the begin of the critical final phase of the decent, when the spaceship crosses the entry interface (according to NASA in around 400,000 feet) and kinetic energy starts to be dissipated to heat in significant amounts.

That is the time I am suggesting could be extended in a glide of a few hours (using aerobraking).

You can't glide a few hours using aerobraking. The usual controlled reentries already are the slowest possible decent. There is just not enough lift at hyporsonic speed in the upper atmosphere. You would need rocket engines to slow down the decent even further.

 

[sophiecentaur]

There is just not enough lift at hyporsonic speed in the upper atmosphere.

Cheers for that. It sounds like a very relevant consideration. Do you have a good link for that information. I saw in a NASA article that there is a lot of heat generated in hypersonic flight. That may or may not be relevant, i.e. the energy from that must come from somewhere so it would presumably involve some loss of KE - not a good fact for transport but not necessarily bad for re-entry.

I always appreciate new info and I'm not hanging onto this idea when I'm given valid reasons. We can usually assume that almost nothing gets neglected when NASA selects its methods.

 

[DrStupid]

Do you have a good link for that information.

According to this paper gliders like the Space Shuttle or Buran have a maximum lift to drag ratio of around 2 in the hypersonic regime. That is quite poor compared to normal airplanes (e.g. around 16 for an A320). Reentry capsules like the Apollo command module or Soyuz are even worse with around 0.35.

 

[sophiecentaur]

I read that too. But the comparison doesn't have to be between planes and a re-entering vehicle, its to be compared with a 'falling brick'. Imo, the relevant thing would be glide angle. A reasonable amount of drag would be what's wanted.
From skipping through a number of articles (including this one) I gather that the shuttle (the only example to study) seems to be doing its best. However, its design didn't have the benefit of high power computing for control and the problem of landing on a pocket handkerchief, the other side of the world demands a lot from any control system. Perhaps the basic constraints that applied to the shuttle would be relaxed with better control. But it's basically got to be little more than a glider with only a single go at landing where you want.
Basically perhaps 'those tiles' are actually a good solution, even in the future but they could surely be helped by better control.

 

[DrStupid]

But the comparison doesn't have to be between planes and a re-entering vehicle, its to be compared with a 'falling brick'.

If you want to extend the reentry phase to several hours than you need to compare it with planes. Bricks make ballistic reentries only.

Imo, the relevant thing would be glide angle. A reasonable amount of drag would be what's wanted.

Before you ask for the glide angle you must ensure that you can glide at all. Gliding with constant sink rate requires 1 g lift. With a lift to drag ratio of 2 this means 0.5 g drag and therefore around 27 minutes to brake from 8 km/s down to zero or just 8 minutes to turn half the kinetic energy into heat. That fits well to the black-out phase of 11-12 minutes for the Space Shuttle. You would need a much better lift to drag ratio in order to extend that to 'a few hours'.

 

[sophiecentaur]

Gliding with constant sink rate requires 1 g lift.

Is that strictly true? At terminal velocity, the lift force is gM but a freewill skydiver can still obtain lateral motion. I was equating a falling brick to a re-entry capsule but that may have been too much poetic licence. A glide angle is a glide angle whoever there is lateral motion and the initial entry angle provides that until the re-entering object is falling vertically (i.e. it's slowed up significantly). Or are we just discussing a definition? Can't I assume that a (any) re-entering craft will have some aerodynamic properties?

 

[DrStupid]

At terminal velocity, the lift force is gM but a freewill skydiver can still obtain lateral motion.

That's what I'm talking about. With a lift tro drag ratio of 2 at terminal velocity the lateral speed is twice as high as the vertical speed. But with a lift of 1 g the drag is still 0.5 g and brakes the ship down to zero in minutes and not hours.

Can't I assume that a (any) re-entering craft will have some aerodynamic properties?

Of course you can. The Space Shuttle was even able to land as a glider with its aerodynamic properties. But it still couldn't fly for hours.

 

[sophiecentaur]

But it still couldn't fly for hours.

Right. It seems that something like the shuttle design will have too little lift to keep it up for long but, once it slows up a bit it will fly as a dumpy glider. What you said about the braking effect on the ship is making sense to me. It can only 'fly' a bit better than a capsule during the hypersonic phase.
This has been very interesting, thanks.