Space Forum / Space Flight / January 2004

Uh, unfortunately, no -- Sputnik 1's orbit was 228x947km, well above the
atmosphere (although the 228km perigee was low enough that it reentered
after only three months).

You have missed the point. What happens to you if you are dropped from 51km?
You fall. For a long time. Until you hit the surface.

Orbit is a function of speed as well as distance. You could orbit a
perfectly spherical planet (with no atmosphere, of course) at 1 meter
altitude forever, were you moving at the appropriate speed.

In the presence of an atmosphere, a sustainable orbit requires that you both
stay out of the bulk of atmospheric drag and be moving at exactly the escape
velocity for that particular body at that particular distance. Were the
earth perfectly spherical and airless, that velocity, at the aforementioned
1 meter altitude, would be about 9.8 kilometers/second. At Sputnik's orbital
distance (227 x 945km), 9 km/s would be more like it.

Getting to altitude inside the atmosphere (or even the ionosphere) is
relatively easy. Getting to 9 km/s, now _that's_ hard.

As for the lowest possible orbit - a circular orbit at 300km would decay in
30-60 days.

Jonathan Wilson

I suggest you go back and check your facts. A near-trivial google search
for "sputnik-1 perigee apogee"" found that Sputnik 1 had a perigee of
142 miles (229 km), and an apogee of 588 miles (946 km).

Zero meters --- since every time you drop something, it's in free fall,
and therefore in orbit. (Oh, you meant _stable_ orbit?  They don't exist.
You will need to first to specify how _long_ you want the orbit to last
before re-entry, and the mass to frontal area ratio of your satellite...)

43 km has been achieved, as you could have easily found by googling for
"balloon altitude record." However, in principle, the upper limit is
determine by the strongest, lightest balloon fabric or membrane available,
and it is not obvious what this limit will be.

It is very easy to get to a high altitude --- even a single-stage rocket
can exceed 100 km. Less than 5% of the total energy put into a satellite
is used to lift it to its orbital altitude; more than 95% of the energy
is spent accelerating it to a large enough horizontal velocity to _stay_
at that altitude for any significant length of time.

You are very, VERY, =VERY= much mistaken; you will only reduce the total
delta-vee requirement by a small amount.

The major benefit of launching from altitude is that your rocket won't
need an altitude-compensating nozzle; however, it will be nearly as big as
if you launched it from the ground.

For all practical purposes it is a good vacuum. The record-holding manned
balloon flights required full pressure suits, just like an astronaut would.

That depends on how big the balloon is. But you better wear nomex fire
coveralls outside it, to protect you when a stray spark causes your
hydrogen balloon to do a "Hindenburg" on you...

-- Gordon D. Pusch  

perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;'

You are in error w/re Sputnik 1.  That spacecraft's orbit was an ellpise
with the perigee at 227 km and apogee at 945 km.  It only ever flew at
31 km in the sense that it flew at every altitude between 1 and 226 km,
which is to say very briefly with a big-a.s rocket underneath to lift
the whole thing as quickly as possible to the 227+ km destination orbit.

The highest altitude for balloons is about 50 km, the lowest altitude
for anything resembling an orbit is about 150 km, and you aren't the
first to suggest balloons might be used as a first step to orbit.

But keep in mind, going up is only ~15% of the problem.  ~85% of the
difficulty of getting to orbit is the going very fast horizontally
part, and since the balloon only handles 1/3 of the "going up" part
of the problem, the rocket is going to be doing 95% of the work in
any event.

31km?!  Where did you see that?  Try something more like an elliptical
orbit from 225 to 950 km.

No, not single-stage solid.  Two to three minimum.  Getting real high
doesn't do you any good if you don't have any horizontal velocity
(unless you go REALLY high, like space elevator-type altitudes)

You'll need a full pressure suit at those altitudes.

Atmospheric density /Frontal areal density is where it's at.

The orbits for a 20m long tungsten rod, and a 1um thick mylar
balloon will be similar when the atmospheric density is different
by a factor of a hundred million.
(neglecting for the moment slight variances in orbital speed).

As a very very rough estimate, if the atmospheric mass it meets is
above the mass of the orbiting body per orbit, then it'll come down.
So, for a 40000Km long orbit, I make that around 30-40Km altitude for
a 20m long tungsten bar, and (from memory, my server with the density
-height tables on is down) around 200Km for the balloon.

I was wrong about the minimum orbit altitude, its closer to 270km than
30km. Single-stage rockets CAN reach 100km straight up, but I guess
its the other 95% horizontal thrust thats really needed.

What I was really curious about originally, was the minimum fuel cost
for say a 1kg payload in the minimum orbit for say a year. I can look
up the solid engine costs, but to make custom boosters and the amount
of kerosene needed for the liquid part, is there any source of data
anywhere? Can we safely take Sputnik's data as the MINIMUM despite the
soviet's bad reputation about fuel efficiency??? Sputnik1 was really
light.