Why the Indian Mars mission is more exciting than anyone seems to realize "¦ [part 2]
Mangalyaan is on its way to Mars! This is a very significant event, not just for India, but for space exploration in general. Specifically, I think that this is transformational for how we all look at the economics of space exploration. This is why.
[This is Part II and a continuation from my previous post on why media does not seem to understand the real import of Mangalyaan]
When space exploration started in the late 50's, it was a competition of prestige between the Soviets and the US. Manned missions became the most visible and tangible way to demonstrate technological prowess. Within 3 years of Sputnik, the soviets sent a human into space and the US followed suit and eventually committed to sending a man to moon. Because the payload was human, space missions committed to the least risky way of getting to space: using massive rockets to blast as quickly and directly into your target orbit. In fact, the Saturn V rocket used for launching the Apollo missions are still among the largest rockets ever flown. [Soviet missions, even if unmanned, being in competition with the US, could not fail, so they also adopted a similar approach].
To a large extent, this created a certain self-sustaining mindset about the economics of space exploration. Big rockets are expensive, so if you are going to spend that kind of money to go to space, there should be a serious purpose. This meant that the space missions had to carry important and often expensive payloads. But then, when you have important and expensive payload, you don't want to take unnecessary launch and orbital risks, so you take the least conservative route to your destination. The latest example is NASA's Maven that used a massive Atlas-Centaur rocket to go directly into a heliocentric orbit. NASA did the same with its Curiosity mission as well as the payload was the expensive rover.
An unusual set of circumstances forced Mangalyaan to take a different approach. First, ISRO's larger rocket, GSLV had suffered two recent accidents and was not ready for a launch; second, the Mars launch window occurs only every 2+ years and if this window is missed, it has to wait for two more years; third, if launched on its much smaller PSLV, it has little chance unless it does something unconventional.
This set in motion a series of interesting but risky steps.
Since the PSLV cannot launch Mangalyaan into a heliocentric orbit toward Mars, Mangalyaan had to carry extra fuel to propel itself to Mars from a low earth orbit. The 800-odd Kg of fuel it had to carry limited the amount of payload it could carry. Although this has been criticized by many in the western press – not to mention ISRO's own previous director – that Mangalyaan can serve no useful scientific purpose with its tiny payload, this tiny payload turned out to be blessing in disguise. Since the payload is not terribly expensive and important [Mangalyaan might as well carry a15Kg dumbbell], ISRO could take risks that have been unacceptable to space exploration thus far.
At this point you can ask: if the rocket could not launch it into space, how can it transport enough fuel for Mangalyaan to break out of its earth orbit. [alternatively: ∆v allowance to reach different orbits are irrespective of how you apportion the fuel; so how do you achieve a ∆v of about 14 Km/s to get to Mars]. See: delta-v on Wikipedia.
The unreported or under-reported fact is that Mangalyaan did not carry enough fuel. Period.
Instead it depended on something called the Oberth effect: when you are moving fast, the fuel you carry also has kinetic energy, so you can get more energy (and ∆v) from your fuel when you fire your engine while moving than when you are stationary.
That's why Mangalyaan's orbit raising maneuvers were performed at perigee (closest point to earth) when the craft is moving the fastest. Further, this is also why the orbit was becoming more and more eccentric (increasing difference between perigee and perigee) with each burn: the more eccentric the orbit, the faster the craft moves at perigee.
Although Oberth effect is well known, no space craft, with the possible exception of Japan's Nazomi, had banked this much on Oberth.
After PSLV put Mangalyaan on a parking orbit, Mangalyaan was moving at about 5000+ Km/s at perigee. With its first firing, not only did it get a boost from the chemical energy of the fuel, but also from the kinetic energy of the fuel that's moving at 5000+ km/hr. At each successive burn, the energy gained from a Kg of fuel is higher and higher because you are moving faster and faster [think of compound interest as an analogy]. At the last burn, the craft had an apogee of nearly 200,000 Km (2/3 of the way to the moon) and a perigee of less than 800 Km (about the distance from Bangalore to Mumbai or Chicago to New York) and moving at about 30,000+ Km/s so the extra kinetic energy it got from its fuel was considerably higher than just the chemical energy from the fuel.
Further, by going around the Earth for 3 weeks, Mangalyaan also gained angular momentum around the sun – for free – that helped to catapult it into a heliocentric orbit with greater energy than blasting directly into a heliocentric orbit. That's why Mangalyaan will arrive at Mars only 2 days later than Maven even though Maven was launched directly into orbit a heliocentric orbit on the 18th, 12 days ahead of Mangalyaan.
This strategy – until now – has been seen as too risky: you have to stop and start your engine several times; the structural stresses on the craft are extremely high at the perigee and you are adding to it by doing an engine burn; an engine burn at extremely high velocity has to be extremely accurate since a small error will lead to a huge difference "¦. There are many more risks in this strategy but you get the idea.
So what ISRO has achieved is amazing and will force other space agencies – including cash-strapped NASA – to examine their assumptions about the economics of space exploration.
Stay tuned for Part 3 ... why Mangalyaan mission could be transformational in space exploration.
PS: If Mangalyaan manages to get to Mars, India will become the first country to get to Mars on its first attempt [ESA is often quoted as the first successful agency to get to Mars, but this is incorrect: ESA's Mars Express was launched with a Russian rocket].
:dancemasti:
but it seems they are focussed on their mission!
