How to catch a comet ?

[MartG]

Sorry I haven't posted anything for a while, as I've been busy doing other things. Anyway, this scenario has been bubbling at the back of my mind for a while now.......what could have been done if a comet or asteroid had been spotted heading towards the earth back in the 60s/70s ?

When astronomers working at the MKO observatory in Hawaii first spotted the new comet in June 1976, it caused little excitement as when it's orbit was calculated it was shown that it would cross the Earth's orbit on the far side of the Sun, so probably wouldn't even give a decent light show due to it's distance. However when it's orbit was plotted ahead it was realised that while there was no danger in it's current pass around the Sun, the next pass in 17 years had a very high probability of impacting the Earth.

Many initially believed that 17 years would be ample time to come up with a solution, but it was soon accepted that with current technology attempting to divert the comet's orbit when it was next approaching the Sun would be impossible, as the velocity change needed to alter it's course away from the earth would be too great, especially when it's huge mass was considered. It was therefore decided to launch a mission to alter its orbit as soon as possible, possibly using nuclear explosives to impart thrust to the comet nucleus, though as a backup planning for a later mission would also be carried out.

Due to the distance the comet reached from the Sun, any mission to intercept it would have to be launched within 18 months  to catch it outbound from the Sun ( the earlier the launch, the less delta V the spacecraft would need to intercept the comet ), which ruled out developing completely new launchers and spacecraft for the mission. At the time the Soviets lacked the massive boosters needed to put the required tonnage into orbit, so most of the spacecraft would have to be launched by the American Saturn V.

Again, the short deadline for the launch ruled out building all new Saturns, but fortunately there was just enough hardware left over from the Apollo and Skylab programmes to get the job done. The comet chasing craft was designed depending on reused Apollo & Saturn technology, some of which had to be hurriedly retrieved from various museums.

The habitable section of the spacecraft was largely made from the space station already built as a backup for the Skylab project, using the S-IVB-515 stage, though the Apollo Telescope Mount was removed leaving just the core workshop, the airlock module, and docking adaptor.  This section was launched by the SA-514 Saturn V originally assigned for the cancelled Apollo 18, with the only modification being the same as carried out on the Skylab 1 booster where the S-II stage carried out the orbital insertion. The launch was manned ( removal of the ATM meant a standard Apollo payload shroud could be used ), and carried the Apollo CSM-119 which had been converted to a 5 man configuration when it was designated as the rescue craft for the Skylab 4 and ASTP missions. Responsibility for this portion of the spacecraft was given to NASA.

The Apollo CSM would also be used to approach the comet and place the nuclear weapons, as the main spacecraft was thought too large and unwieldy for this task. Modifications to the CSM for this task involved increased fuel capacity for the RCS control jet system which would be used for most manoeuvres near the comet ( the comets negligible gravity meant the primary SPS motor would be too large for the delicate work needed approaching the comet ), and landing legs fitted to the Service Module as well as external manipulators to carry the nukes. When the main spacecraft was under power the CSM was docked with the forward docking port, with additional framework supporting it against the acceleration forces involved. Additional spacecraft could be similarly supported at the side docking ports.

The main propulsion section of the spacecraft was based on a Saturn V S-II second stage ( the S-II-F test stage was used for this as the two remaining flight-ready S-II stages were required for launch duties ), with the five J-2 engines replaced by a single NERVA-2 nuclear rocket motor fuelled by liquid hydrogen. While this single engine gave similar thrust to one of the original J-2s ( hence the spacecraft had only 1/5th the S-II's original total thrust ), the specific impulse of the engine was double that of the J-2 hence needed only half the fuel for the same overall velocity change. Additionally, as the NERVA engine only required liquid hydrogen and didn't need liquid oxygen, the S-II's oxidiser tank could be amalgamated with it's hydrogen tank, allowing a greater load of fuel to be carried ( this simply required the removal of the common bulkhead between the tanks ). Replacement of the LOX with LH2 reduced the also stage's fuelled mass from 491 tonnes to 133 tonnes, though the addition of oxygen, nitrogen, and water tanks within the forward skirt area for life support for the extended mission added around 70 tonnes back onto the weight.

To launch this part of the spacecraft a two stage Saturn V was again used, though due to the increased payload mass required the launcher needed additional thrust to get off the ground ( the standard Saturn V could have lofted the stage, but only if it was almost empty of fuel ). After various proposals were considered, it was decided to attach four of the solid boosters being developed for the space shuttle to the first stage, and uprate the engines of both the first stage ( to F-1A standard ) and second stage ( to the J-2S version ).

Even with the enhanced Saturn V launching it, the propulsion stage couldn't be launched fully configured, so the remaining Saturn 1B launchers delivered various additional components and the crew for the spacecraft.

The idea was to use a couple of old Airfix Saturn V kits I had in the stash as a basis, plus some Shuttle SRBs. Unfortunately the build was delayed, as when I was doing some research into the Saturn rockets I got sidetracked – with the result that I'm not only building the whatif 'Comet Chaser', I'm also building two 1/144 Airfix Saturn Vs ( one standard, the other as Skylab using RealSpace Models conversion kit ), one Revell 1/96 Saturn V ( plus RealSpace CSM and F-1 engines, and NewWare's detail kit ), and  a RealSpace 1/144 Saturn 1B too !

Anyway, finally got around to actually beginning construction of the Airfix Saturns last week, and looking at them I got to thinking that they could do with the 1st stage engines being replaced with ones with the insulation fitted as on the flight items, as the kit F-1 engines omit this. While there are reasonable replacement engines available in 1/96, the only 1/144 ones didn't look particularly accurate to me ( and the engine bells apparently aren't hollow ), so I've made my first foray into resin casting.

My efforts used a modified Airfix F-1 engine as the master, from which I made a two part silicone rubber mould - two part so I could make the engine bell hollow as per the original kit engines. I also made a new S-1C aft heatshield for the engines to attach to, as the kit item has ribs which aren't there on the real thing.

The kit of parts
http://thumbsnap.com/s/4wZYxthA.jpg[/img]|http://thumbsnap.com/v/4wZYxthA.jpg]|http://thumbsnap.com/v/4wZYxthA.jpg

As test fitted, after painting
http://thumbsnap.com/s/bGzJfHyU.jpg[/img]|http://thumbsnap.com/v/bGzJfHyU.jpg]|http://thumbsnap.com/v/bGzJfHyU.jpg

Revised 2nd stage thrust structure - the kit one has the engines too far apart, is completely wrong around the centre engine mount, and doesn't have the fuel pipes or electronics boxes

http://thumbsnap.com/s/lY6gTR1r.jpg[/img]|http://thumbsnap.com/v/lY6gTR1r.jpg]|http://thumbsnap.com/v/lY6gTR1r.jpg
Above left, the raw casting, above right assembled with kit engines and plasticard heatshield

Also, when Airfix fixed the SM to the correct diameter, they didn't do anything with the CM, which means it doesn't fit the Boost cover anymore ( and looks very undersized ). They also persisted with the original SPS engine nozzle, which is the wrong shape. So, a couple of castings to fix those problems - the new CM fits the revised BPC and the nozzle is now the right shape.

http://thumbsnap.com/s/rhkb0Dmb.jpg[/img]|http://thumbsnap.com/v/rhkb0Dmb.jpg]|http://thumbsnap.com/v/rhkb0Dmb.jpg

( BTW If anyone wants any of the above bits, just drop me a line )

[chrisonord]

Hya Mart,
Your first attempt at casting resin is looking really good mate
I was half expecting a whiffed MR2 build I could be interested in some of those bits myself in the near future, as I want to build a moon based interceptor and a transatmospheric craft for my S.H.A.D.O. builds.
Cheers,
Chris.