Space Exploration Asked by Kevin bueno on December 28, 2021

Is the current design paradigm of space engineers still too anti-naturalistic?

Why not design Space Cruisers first large & slow but steady into outer orbit, over months rather than in a day? I think it would be an exciting spectacle to see spaceship flybys a couple times a year like: "there she goes, up up & away!"

I think the question is a little ambiguous. I'm not sure what is meant by "get to space overnight". The development process? Anyway, here's a few possible answers...

It's hard to get into space. The faster a rocket goes as it takes off from Earth, the better use it makes of its fuel. The limiting case of a "slow" rocket is one that hovers in place, using fuel but not going anywhere (that was an example of argument from extremes). So refer to tfb's answer. Humans put a limit on the upper end, and human-carrying rockets accelerate at about 3g. I think the early astronauts tolerated 6g, but that's really a lot.

If you're going to another planet, it's going to be a slow process. It could be years. If you have powered flight at all, the most efficient engines, like ion engines, have very low thrust.

If you're sending humans, you want the trip to run as quickly as possible because it's a psychological and life-support challenge to maintain humans for months or years, and because of the radiation load they will accumulate over time. You want it to go fast because of the human factor, but it's not going to be fast, sorry. That's the basic reason that we haven't sent humans to Mars yet, it's like six months one-way. That's too slow, and we can't speed it up!

Answered by Greg on December 28, 2021

I would like to add on to the other answers.

You can't get to space slowly because that would be horrendously ineffective.

But once you get to space and reach an orbit, what about getting to *further* places?

Spacex is planning to do something that somewhat comes close to your suggestion:

- Throw many spaceships to space (quickly every time, but one at a time)
- Take your time preparing for the journey
- This means fueling the important spaceships by depleting the fuel in the not-so-important spaceships

- Once you're ready send the important spaceships to a destination far away, like Mars

This isn't directly what you meant but certainly is not an overnight launch.

Note that missions to Mars already take quite a long time, but never stop to prepare or take their time like Spacex will for in-orbit refueling.

Answered by Speedphoenix on December 28, 2021

There are two parts to an answer to this question. I am going to assume that you want to get into space with some kind of flying machine, so in particular I'm not talking about a space elevator: space elevators are a hugely cool idea but we're not very close to being able to build one.

The first thing, as other people have said, is that the main bit of getting into orbit is not getting *high*, but getting *fast*. As an example, at a height of $400,mathrm{km}$ (about the height the ISS orbits at), orbital speed for a circular orbit is about $7670,mathrm{m/s}$. This is about $22$ times the speed of sound in air.

Well, if you look at the design of aircraft which are designed to go much faster than the speed of sound you'll find several things:

- they are made of special materials as they get so hot due to forcing their way through the atmosphere;
- they burn a
*lot*of fuel; - they don't go anywhere
*near*$22$ times the speed of sound – the SR-71 Blackbird could do somewhere shy of Mach 3, an it was an extremely exotic bit of machinery.

And things get a lot worse as you go faster than that. And to get into orbit you need to go about *eight times* as fast as that.

What this means is that, if you want to get into orbit, you have to get out of the atmosphere as fast as you can, because you really do not want to be designing a vehicle which can travel at anything like orbital velocity in the atmosphere unless it's doing that in order to *slow down* from orbital velocity. Indeed it's almost certainly not possible to construct such a thing with materials we have, and if it was it would burn a stupidly large amount of fuel.

This is why rockets go straight up initially, for instance: they want to get out of the atmosphere as quickly as they can so they can do most of the acceleration they need to do out of the atmosphere.

So the first conclusion is that **getting to orbit means getting out of the atmosphere really quickly**: once you've started accelerating to orbital velocity you need to be out of the atmosphere as soon as possible.

Well, the result of this is that, for most of the passage into orbit, you are doing it in something which is more-or-less a vacuum. In particular you can't rely on the air for lift or for reaction mass, because there is (almost) no air.

But why does this mean you have to get into orbit quickly? Why couldn't you, for instance, just drift gently up to orbital height and then zoom off sideways? Or not bother with the whole orbit thing at all and just go vertically upwards all the way to Mars or something?

Well, the answer to that is that is a thing called $Delta v$, which is, in simple terms, the amount of speed change you need to do something. Let's consider a really simple rocket: it will go vertically upwards until it gets to where it wants to go, then it will stop. We can give a slightly hairy expression for the $Delta v$ needed (the maths here gets simpler in a bit):

$$Delta v = int_{t_0}^{t_1}left|frac{GM}{r^2(t)} + ddot r(t)right|,dt$$

Here $r(t)$ is the distance of the rocket above the centre of the Earth, as a function of time, and $t_0,t_1$ are the times it start & stops. $ddot r(t)$ is the acceleration of the rocket, and the $GM/r^2(t)$ term is the part due to gravity.

Well, let's assume that $r(t) = ut, u > 0$: the rocket just goes upwards at a steady speed. Then we can actually do this integral:

$$ begin{align} Delta v &= int_{t_0}^{t_1}left|frac{GM}{r^2(t)} + ddot r(t)right|,dt\ &= int_{t_0}^{t_1}frac{GM}{u^2 t^2},dt\ &= left.-frac{GM}{u^2 t}right|_{t_0}^{t_1}\ &= frac{GM}{u^2}left[frac{1}{t_0} - frac{1}{t_1}right] end{align} $$

And let's say we want it to go from $h_0$ to $h_1$ above the surface, corresponding to

$$ begin{align} t_0 &= frac{R + h_0}{u}\ t_1 &= frac{R + h_1}{u} end{align} $$

This gives us the following expression for $Delta v$:

$$Delta v = frac{GM}{u}left[frac{1}{R + h_0} - frac{1}{R + h_1}right]$$

Well, the detail of this is not very important, but the important thing is that there is a factor of $1/u$ in this expression: the lower $u$ is the higher the $Delta v$ is. Let's write

$$Delta v = frac{Gamma}{u}quadtext{where }Gamma = GMleft[frac{1}{R + h_0} - frac{1}{R + h_1}right]$$

And $Delta v$ translates into fuel in a rather horrible way. The important quantity to think about is the ratio of the initial mass, $m_o$ to the final mass, $m_f$ of the rocket: this is the ratio of how much mass you start with to how much mass you end up with. If it's a big number then the rocket you start with has to be that much bigger. This ratio can be given in terms of $Delta v$ using the Tsiolkovsky rocket equation

$$ begin{align} frac{m_o}{m_f} &= e^{frac{Delta v}{v_e}} &&text{$v_e$ is exhaust speed}\ &= e^{frac{Gamma}{v_e u}}\ text{or}\ m_o &= m_f e^{frac{Gamma}{v_e u}} end{align} $$

OK, so what does this mean? What it means is that the initial mass of the rocket goes *exponentially* as $1/u$, which is the ascent speed. Well, speed is distance over time, so the initial mass of the rocket goes *exponentially* as the time taken to ascend.

This is why rockets want to get into orbit quickly: **not getting to orbit quickly absolutely catastrophic in terms of fuel requirement**!

Answered by user21103 on December 28, 2021

Imagine you have a very heavy book and a bookcase, and your goal is to put the book on the top shelf of the bookcase. How much time would you spend doing that? Maybe five seconds, maybe fifteen. Would going much slower help you? No, it would not, because simply carrying the book is exhausting to you. You would never be able to hold the book up for an entire day while lifting it to the top shelf very slowly.

Now you might argue that the bookcase has other shelves, and a good strategy would be to move the book up shelf by shelf while catching your breath after every shelf. This would be a good strategy in the bookcase case, but unfortunately, in real life between the ground and low earth orbit there are no other shelves. You need to carry the rocket all the way up, then accelerate it to orbital velocity, without being able to refuel or rest. If you stop the rocket's thrusters too early, the rocket just falls back down to Earth and your effort is wasted.

This is a large simplification, but that's the essence of the matter: each second not in orbit is another second spent fighting against gravity. Fighting against gravity costs fuel, lots of it. And you already need a huge amount of fuel just getting to orbital velocity so you can stay in orbit. So much fuel in fact, the vast majority of the rocket is just fuel, and that rockets have to work in several stages: once a large fuel tank is emptied, it is dropped and you're basically left with another, smaller rocket that can start from higher altitude and velocity. Repeat one or two more times, and the actual bit of the rocket that reaches orbit is only maybe 2-5% of the rocket you started with. So you really don't want to require any more fuel than necessary. You want to reach orbit as fast as you can without damaging the rocket, the payload or the passengers.

Once you're in orbit, though, you do not need to spend any more fuel, because essentially you're just falling around the Earth forever without actually hitting it. Now you can take all the time you need to plan your next course of action. Of course, if you're on a manned mission, more crew supplies cost more fuel to bring into orbit, so since we want to save on fuel we plan on making the mission as short as possible while achieving all the mission objectives.

Now you're probably wondering why we would need so much fuel to get into orbit? Airplanes can travel all over the planet and they don't need near as much fuel. Well, the main issues here are Newton's laws of motion: if you want to accelerate forward, you need to push something backward. Airplanes have an easy job, they can just take the air that's around them and push it back using jet propellers. It's not as easy for rockets, because a) they need to go much faster than planes and b) there is much less air where they fly. So basically they need to carry all the stuff they ever want to push behind them with them, and this is the fuel. It's like if you had a rowboat on the sea but you weren't actually allowed to row, you had to accelerate by throwing pebbles behind you.

There's more difficulty though! Consider that the momentum you gain by throwing a thing behind you is equal to the thing's mass times the thing's velocity. So you gain the same amount of speed by throwing 10 kgs of pebbles at 10km/h as you would if you threw just 1 kg of pebbles at 100km/h. Unfortunately, since the kinetic energy of an object is equal to its mass times its velocity *squared*, the latter approach costs much more energy. For an aircraft, air is always in large supply, and it can just chuck a bunch of air behind it at medium velocities to continue accelerating forward. But a rocket? The only thing it can throw is the fuel it has, so it needs to throw the fuel behind it much faster to compensate. That takes bonkers amounts of energy, all of which needs to come from the fuel itself.

But wait, it gets worse! Suppose you designed a cool rocket engine, with which if you have 1 ton of fuel, you can accelerate 1 ton of payload to 1 km/s. (This rocket engine isn't actually as powerful as modern rocket engines, but it illustrates the problem nicely.) If you have 2 tons of fuel, you can accelerate 2 tons of payload to 1 km/s, and if you have 100 tons of fuel, you can accelerate 100 tons of payload to 1 km/s. Now how much fuel to you need to get 1 ton of payload to 2 km/s? You guessed correctly, it's 3 tons of fuel. Why's that? Well, think about it. What fraction of the rocket do you need to get to 1 km/s? Of course you need the payload, but you also need 1 ton of fuel that allows your payload to accelerate one more km/s. So that's two tons of stuff you need to accelerate to 1 km/s, which means you need two more tons of fuel.

This repeats. For every single km/s you want to go faster, your rocket would have to be twice as large. So to reach the 7.8 km/s at which you can stay in low Earth orbit, your rocket would need to weigh hundreds of tons on the ground just so that final one ton of payload can reach orbit, even disregarding the additional fuel you need just to escape the gravity well and atmosphere.

That brings me to the final point: orbital velocity. Yes, you need to go 7.8 km/s just to stay in low earth orbit. Go any slower, and you fall down *onto* earth instead of just falling *past* it. Just reaching orbital *altitude* is relatively easy compared to that, it takes about 2-3 km/s of upward velocity from ground level (ignoring atmospheric drag), but after that you'll just fall back down again. That's why rockets to orbit spend most of their burn time accelerating horizontally, after a vertical start to get out of the dense and draggy parts of the atmosphere.

Low earth orbit is called low earth orbit because it's the lowest altitude at which you can orbit earth without getting significantly slowed down by the edge of the atmosphere. It's also possible to orbit at higher altitudes than low earth orbit, and in those orbits you don't need to be that fast, because gravity weakens the higher up you go, and gravity weakens faster than the radius of your orbit increases. For example, in geostationary orbit (which is high above Earth), you only need to go about 3.1 km/s, and the moon orbits at only 1km/s. Unfortunately, there's a lot more room between Earth and geostationary orbit, or the moon, and moving all the way over there means you'll need to fight a lot more against gravity, which costs energy. In total, the additional potential energy you have to gain is greater than the kinetic energy you save by orbiting slower, so reaching higher orbits costs *even more fuel* than reaching low earth orbit.

Hope this illustrates the main challenges of going to space without falling back down, and why rockets go fast instead of slow.

Answered by Magma on December 28, 2021

There are some concepts for slow access to space being developed. The most fameous one is probably the space elevator. However there is a concept more like the one you are asking for called airship to orbit (more details in this question Is the "airship to orbit" mission profile feasible?) this is currently beyond our engenering skills to construct but there are people working on it. It would involve slowly ascending in a baloon, transfering to an even lighter balone to fragile to be at ground level at altitude and the take tath to orbit by slowly building speed with solar powered ion theusters.

Answered by lijat on December 28, 2021

The general concept of doing things slowly generally allowing better efficiency in a design is certainly correct. The difficulty with applying this to rocket launches is the concept of gravity loses that in general mean getting to orbit as quickly as possible is better.

Slow/efficient space access via space elevator and related designs is the logical extension of the concept but well beyond current technology.

Relevant is this XKCD along with questions tagged air launch.

Answered by GremlinWranger on December 28, 2021

You would not save fuel by going "Large & Slow but Steady into Outer Orbit over months rather than in a Day", you would need a lot more.

The longer it takes to reach orbit, the longer you have to fight gravity with a lot of fuel. Keeping the same height below orbit needs fuel.

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