Space Travel

Part 1 – Getting to Orbit

For those of us interested in space travel however we live in interesting times; as there is probably more development going on in the world today than at any time since NASA was spending an astounding 4.5% of US federal budget trying to get to the moon.

Taking the first step for space travel for humans is not easy. It pushes the known boundaries of Chemistry, Engineering and Materials science.  Indeed if the Earth’s radius was only slightly larger than it is we probably couldn’t get into space at all using any current technology. As it is most launch systems available today are only capable of delivering about 3% of their launch mass into orbit, let alone getting it to another planet.

You can’t go anywhere else in space until you first get to orbit but that first step is hard; really hard.

Our brains tend to think about getting into space as a problem in “going up high enough” but really it is a problem in going fast enough horizontally. Rockets take off straight up to shed the treacly grasp of air resistance as soon as possible, but around 90% of their energy is used in going horizontally to attain orbital velocity. This night picture of a recent night-time rocket launch gives a good illustration of that.

Ross, Going Postal

The Kármán Line is used as a fairly arbitrary definition for where space starts – it is an imaginary line at an altitude of 100 Km; the theory being that at that height there is no prospect of aerodynamic lift holding anything up, so it can’t be reached by an aeroplane.

The V-2 built in the 1940s had the capability of technically “getting into space” i.e. above the Kármán Line, if you fired it straight up. But such a trajectory would have made it no more than a firework as it would have come straight down again (with the launch team at least able to console themselves with a fantastic Darwin award). If launched on a more rocket like trajectory the V2 got to a maximum velocity of 5760 Km/h (about 1.6 Km/s), not bad I hear you say.

To graduate from being a missile to being a space vehicle however one needs, at a minimum, to achieve low earth orbit (LEO) – to do that needs a velocity of just over 28,000 Km/h  (or about 7.8 Km/s – and to get at least a couple of hundred Km up – orbits decay super quickly – days or hours – below 150 Km-ish)

delta V (DV) is the most basic measurement of Orbital navigation – it is a measure of the change in velocity / impulse required to get from A to B.

To get from Earth to LEO requires a delta V of roughly 9.5 Km/s (the 7.8 we saw above plus some extra to overcome initial air resistance and lift the body from surface to 200 Km).

As an aside the Virgin galactic thing for taking tourist trips – only manages a delta V of about 1.6 Km/s so whilst interesting in terms of vehicle design – is nothing more than a fairground ride in practical space travel terms.

To get some perspective on the difficulty of getting into orbit its worth comparing the impulse required to get other places. It takes a delta V of about 9.5 to get to LEO but then it only takes a further delta V of just over 4 Km/s to get from LEO to a low orbit around the moon.

To get from LEO to a low orbit around Mars only takes a deltaV of between 5.5 and 6 Km/s (or only a bit more than half as much as getting from the surface into orbit).

So getting to LEO is “harder” than getting from LEO to Mars by some margin.

(aside: Measurements of the impulse needed to travel between bodies in our solar system are not actually constants – they depend upon a variety of factors such as the relative orbits of the bodies, and when the engines are fired [see Oberth effect], but generally figures are quoted assuming you are taking the most favourable conditions.)

The “road map” for navigation in our solar system is mainly given by deltaV diagrams – such as this one.

Mars/Moon/Earth Delta-Vs

Ross, Going Postal

So the first step for us humans, getting into orbit, is really hard because we need to go really really fast. In most other walks of life if you need something to go faster – most of us think ‘we’ll lets just put a bigger engine in it’, this answer will only get you so far in rocketry as the engine and all its fuel has to be carried off the launchpad. So what matters is (a) the efficiency of the engine in turning its fuel mass into thrust (b) that the engines on the craft can actually provide enough thrust to overcome gravity and get it off the launchpad.

The propellant on a rocket is used for two things; firstly, reacting energetically to get a nice high velocity of exhaust  and secondly to be the mass that we chuck out the back. A rocket moves courtesy of Newton’s third law – chuck stuff out the back and you move forward – but because in space there is no other “stuff around” you have to carry all that mass with you when you start (compare with an aeroplane powered by a jet engine which doesn’t have to carry all the air that it uses for reaction mass with it,).

Without turning this into “Monday maffs”, there is one equation that captures a lot of the difficulties in getting into orbit – The rocket equation:-

Ross, Going Postal

This equation is remarkably simple – it says that if you want a delta V of some number then it can be calculated from the exhaust velocity (Ve) of the rocket engine you are using and the mass ratio i.e. the ratio of the initial mass (with propellant) and the final mass (after propellant has been used).

The need for a really high exhaust speed, and also enough thrust to get off the launchpad says we need really powerful engines, but the mass ratio says we need really light engines and fuel tanks. In practice today, for most launch vehicles, this means using Liquid Oxygen (LOx) as the oxidiser with one of Liquid Hydrogen (LH), Liquid Methane or plain old Kerosene as the fuel. With the exception of Kerosene, none of these propellants are easy to store or pump at high speed – and have all sorts of other nasty characteristics.

Here is a really odd US armed forces film on the dangers of dealing with Liquid Oxygen (nasty pictures of LOx burns at the end)

Although the Ve part of the rocket equation is best served with a LH/LOx mix – this being significantly higher energy than the other two fuels – LH carries a lot of downsides – for example it has to be kept so cold it embrittles the metal it touches and it has low density so needs a great big fuel tank – remember the big red centre fuel tank on the Space Shuttle – that was full of LH/LOx.

Containing LH is really tricky – now do it in a vehicle that is huge, needs to be accelerated rapidly from 0 to thousands of metres per second, and to meet the rocket equation must be super light and you start to get some idea of the challenges.

As a comparison a typical modern soft drink can, that one can still marvel at for being so light and thin is 94% drink and 6% can when full – a good mass ratio you’d think. Well the shuttle external tank, that had to contain two of the hardest liquids to store known to mankind – was 96% fuel and 4% container!

The barriers even with fuels that have been in use for a long time are still being pushed – SpaceX has begun using “deep cryo LOx” which is cooled to -207C well below its boiling point, as this is 8% greater in density but has all sort of disadvantages – one that you will see when the first manned Falcon 9 mission flys next year is that the fuel is loaded after the crew – as if the deep cryo LOx heats up it expands dangerously so the whole launch has to be scrubbed while the tanks are emptied.

All of these challenges mean that even today after 60 odd years of orbital flight, a significant fraction of launches fail – 6 out of 96 total in 2017. Even manned launches over the last 20 years still fail at close to 1%. Personally I’d still be prepared to take the risk!

Next in the Series – A Look At Rocket Engines.
 

© Ross 2018
 

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