Re: [pygame] Rocket + Bike = Physics

["Nirav Patel" <olpc@xxxxxxxxxxxxxx>]

I'm picturing a bicycle duct taped to the side of a Saturn V here...

This also reminds me of the Top Gear where they convert a Reliant into
a Space Shuttle. http://www.youtube.com/watch?v=_b4WzWFKQ20

Nirav

On Wed, Jan 7, 2009 at 10:58 PM, René Dudfield <renesd@xxxxxxxxx> wrote:
> Would you need rockets pointing left and right too?  Then your rocket
> bike can go around corners at rocket speed too.
>
> Fuck tron... rocket bikes for the win!
>
>
>
> On Thu, Jan 8, 2009 at 2:53 PM, Ian Mallett <geometrian@xxxxxxxxx> wrote:
>> Hello,
>>
>> A little bit ago a idea of attaching a rocket to bike came up (Thank you JP)
>> The pygame community is a fun-loving group, so in the interest of fun, let's
>> talk about the implications of it in a TOTALLY off-topic thread.  :D  I
>> encourage responses, as I think they will be to the amusement of all
>> concerned.
>>
>> I'll start.
>> Yanom points out that the size of these things is pretty big and hence a
>> rocket bike would really be "more like attaching your bike to the rocket."
>> Because of the bike part, I'm assuming all propulsion is lateral.  Let's
>> likewise assume that a) the rocket can support itself horizontally this way,
>> and b) the bike can hold the whole thing up as well on it's small two
>> wheels.  We're not working against gravity here, so acceleration is going to
>> be considerably higher.  The Atlas V, the rocket mentioned, masses
>> 546,700kg.  A bike masses < about 20kg.  The total mass of the apparatus =
>> 546,720kg.  The first stage of the rocket exerts 4,152,000N of force over a
>> 253 second burn time.  Let's assume friction is negligible--(and duh, we're
>> going to ignite it).
>>
>> F = MA says that we'll accelerate at approximately 0.77 G (darn was hoping
>> for something more) But:
>> After 253 seconds, we'll be traveling at about 1.9 kilometers/second = 5.6
>> mach, or fast enough to cross the continental US in about 35 minutes.
>>
>> BUT...the fuel burned lightens the payload (more specifically, "thrown" out
>> the back, thereby pushing the rocket forward (Newton's Third Law)) so we
>> actually go faster.
>> This kicks us up to over 2.7 km/second = 8 mach, or fast enough to cross the
>> continental US is about 24 minutes.
>> At the highest acceleration, we get a more severe, but certainly tolerable
>> 1.6 Gs.
>> See attachment for the program I used to derive this.  I didn't bother to do
>> calculus--a second by second approximation is good enough.
>>
>> I actually thought of putting four rockets on this guy so it's balanced--a
>> bike with a rocket on one side would be lopsided, duh.  Again, for all you
>> practical people, shut up :-)  Turns out though that the acceleration
>> remains nearly the same, which I should have known if I'd thought about it.
>> Oh well.
>>
>> Now, how fast are the bike wheels spinning?  At 2.7 km/second, and a wheel
>> diameter of 559mm (this is a mountain bike) each wheel will spin at 165,540
>> RPM, which would probably cause a violation of the wheel's structural
>> integrity and a catastrophic failure, i.e., the wheel would explode.
>>
>> All this is interesting, but boring, if you know what I mean--so let's
>> calculate what happens when our contraption (let's use the four rocket
>> variety) crashes headlong into an invincible wall traveling at 2.7
>> km/second.  The total mass just before impact is approximately 1,053,841kg,
>> and it's traveling at a good 2.708km/sec.  This means that at impact, we
>> have a kinetic energy of 3,864,047,133,512j or about 3.8 terajoules.  Now
>> we're getting somewhere fun!  This collision is comparable in scale to a
>> small atomic bomb.  Now, as far as acceleration goes, a bike is pretty
>> short, say 2.5m.  When you sit on it, you're probably not longer than 1m
>> yourself, meaning you'll come to a full stop in about 0.369 milliseconds,
>> equaling a brain-splattering 748,852Gs, which would very probably kill you.
>> Moral: don't ride bikes with big rockets into immovable objects.
>>
>> Which is not to say it isn't done--at least attaching rockets to bikes.
>> Actually, the smaller rockets probably work better for pure acceleration.
>> This video uses several small rockets and accelerates to 163 m/s in about 5
>> seconds.  French people on rocket bikes is always fun.  Then there's
>> completely random stuff getting shoved by rockets, like buses (rocket only
>> helps here), skateboards, small cars and real cars.  This German rocket car
>> is a fizzle.  An unmanned rocket car is launched peculiarly here.  Water
>> rocket cars still go fast, and for our grand finale, in '76 someone tried to
>> jump the St. Lawrence River with a rocket powered Lincoln Continental (skip
>> to 2:55), which pretty much disintegrated in midair.
>>
>> Ian
>>
>