Calling our technical experts - Any chance this thing works?

C M Edwards · 2006-06-13 09:14:55

Well, I'm mildly embarrassed. :oops:

It turns out that our new wiki didn't yet have sufficient information to answer Al's question #1. I've half-way fixed that by adding articles on escape velocity and kinetic energy, using Al's question as an example. However, it's still woefully deficient on any topic of gas dynamics, much less detached shocks and RTT.

That's a shame, because the questions asked pertain directly to the theoretical background of the Archimedes space probe being developed by the Germany Mars Society. It's bad when you can't even describe what your affiliates are doing...

aldiffer · 2006-06-13 17:18:45

Let's treat it as an opportunity then. 8)
If this works out, there will be a number of new pages and research quality material in them too.

publiusr · 2006-06-16 14:06:46

We were considered unresponsive.
Tustin really wanted to tear the places down and produce something to add to their tax base. Cities have to think about budgets like that, so it is understandable.

Meanwhile people fall all over themselves to build domed stadiums and sports arenas.

Sad.

aldiffer · 2006-06-19 00:50:48

8)

I'm sure the group supporting the interests of the Vets will make the City's case long and hard to finish. You have to have money to fight those kinds of fights and we aren't in that league yet.

Besides, we probably don't have to fight. There are large unpopulated areas in this country to this day. Some of them have local government that might fight FOR us is we can bring in a few jobs. That's the American Way just as much as those sports domes and arenas are.

aldiffer · 2006-06-19 01:03:34

You don't need to deal with shocks until you get to #5. Up to that you stick with the assumption that the sphere absorbs all the energy upon entering the atmosphere. We calculate that to get an upper bound for the heat load.

If you can answer #1 the second follows along pretty quick. It's all freshman physics.

The next two involve some experience with thermodynamics. If you can get through #4 you will see the bulk of the argument in favor of ATO for the descent phase of a flight.

#5 helps to make the bounds from #4 more realistic.

#6 helps point out the connection to the ascent phase of a flight. In your mind, you run the problem in reverse. 8)

cjchandler · 2006-06-26 17:29:37

I have doubts that this thing is going to have enough lift to climb rapidly enough. Unless it has a Cd of 0.001 and can lift it's gross tonage at 1 m/s there is simply not enough energy avalible to punch though all that drag. Maybe those winds disscused earlier would help, but that dosen't seem like it's going to help too too much at 7800 m/s. The massive frontal area of the proposed design is going to goble up all the possible savings from the thin air. I would like to see some numbers about the Cd, or by some one who knows more about these issues than I.

neilzero · 2006-06-28 21:18:21

At least one poster insists that the minimum approach speed for any body (assuming no atmosphere) is the escape velocity which for Mars is about 5 kilometers per second at the surface. If the sphere has some thrust, it can orbit in the upper atmosphere for the several days required to radiate away the kenetic energy at 400 degrees k. That is assuming an aluminum color surface that is 50% illuminated by sunlight 60% of the time, and a night time air temperature of 100 degrees k.
I''ll guess only 10% is lost by convection and conduction, in the upper atmosphere of Mars. I'll guess one milibarr internal pressure is sufficent to keep the sphere shape close enough to avoid hot spots over 410 degrees k. The sphere will rotate due to the air pressure being higher on the side closest to Mars. The rotation will help the sphere keep its shape, and reduce hot spots. I don't think the rotation will be fast enough to stress the sphere significantly, unless the radius of the sphere exceeds one kilometer. The heat energy stored in the N2 = nitrogen is less than the heat energy stored in the faberic = skin of the sphere which is close to negligible compared to the energy that needs to be radiated. This is also true if the sphere has H2 = hydrogen at one millibarr. Considerable thrust will be needed to avoid an excessive decent rate as the "orbital" speed falls below 0.1 kilometer per second. The sphere will loose it's shape (unless the skin is very elastic) as the Mars atmosphere reaches 5 millibarrs if landing is at a low elevation of Mars surface. The loss of shape may not cause any serious mischief. The rotation will slow considerably in the 4 or 5 millibarr atmosphere, but the rotation needs to be slowed to approximately zero for a safe landing.
Question 6: The inside pressure needs to be about 6 millibars to retain approximately sphere shape at landing in the 5 milibarr surface pressure. The internal pressure will drop as the sphere cools to perhaps 210 degrees k after landing. 6 millibars may stress the faberic dangerously while the sphere is radiating kenetic energy at 400 degrees k with an outside pressure of perhaps 0.1 milibarr.
High wind speed will deform the sphere considerably after landing. I can't prove any of this with math, so I may be dead wrong. Neil

aldiffer · 2006-06-29 01:00:02

The escape velocity for Mars is about 5.02 km/s and that is a decent first approximation for the arrival speed in #1. Unfortunately, it is a little low.

1. The sphere is not descending from infinity in the Martian gravity well. It is descending from the edge of the Martian sphere of influence (SOI) with respect to the sun. I put that distance around 38 Martian radii since that is where the gravity force vectors for Mars and the Sun have about the same magnitude. If you fall from there you develop a speed of 4.96 km/s.

I'm not nit-picking here even though this speed is essentially the same as the first approximation. I mention this so I can point out the SOI because...

2. The sphere arrives at the SOI with a non-zero residual velocity. If you just use averages, Mars is moving at about 24.13 km/s around the Sun. The sphere is sent on a Hohman transfer orbit from Earth and arrives at the Martian orbit doing about 21.48 km/s around the Sun. If it arrives in front of Mars, a Martian observer would think the sphere was falling at them with a speed of about 2.65 km/s while the sphere was still at the SOI. Once you add on the descent velocity gained by falling through the Martian gravity well, the speed could be in the neighborhood of 7.61 km/s.

Dealing with these numbers properly involves a little celestial mechanics. A physics student should be able ot do it, though someone with more practice could do a better job with patch-work oribit fits.

By the way, that 2.65 km/s residual velocity shows you why you need retro-thrusters on current spacecraft that intend to stick around instead of doing a fly-by. No matter which way you cut it, that residual velocity gives you a net positive orbital energy with respect to Mars which determines that you will fly by on a shallow hyperbolic path. The arriving spacecraft has to shed a little over 90% of its kinetic energy to enter a closed orbit.

SpaceNut · 2015-08-23 23:13:26

Posted as a place holder just fixed the 1st page for the shifting issue

Antius · 2015-08-24 04:39:12

Getting into orbit with a balloon is a bad idea, as the amount of static lift provided at the height at which ion engines would provide enough thrust to counteract drag, would be insufficient. A huge low density balloon would be needed which would in turn result in huge drag as it accelerated.

That said, the idea of launching a rocket from a low stratospheric balloon may be workable. At those heights, air pressure has declined to levels that allow rocket engines to work at a much better expansion ratio than would be possible at sea level. The balloon contributes virtually nothing in reducing delta V to orbit, but improves mass ratio of an SSTO by allowing the engines to fire in near vacuum conditions at take-off. The same principle applies to other air-launch concepts, i.e. Pegasus.

Tom Kalbfus · 2015-08-24 09:42:14

One might consider using an airship that is so big that it rests on the ground while the upper part is above most of the atmosphere.

Or one might consider launching a second stage from the airship.
Imagine an airship 225.78125 km long, there is a track on top a rocket ship accelerates on the track at 4 gs using an electromagnetic linear accelerator to reach 4.25 km/second before igniting its rockets to complete the journey to orbit.

Antius · 2015-08-24 11:18:05

Tom Kalbfus wrote:

One might consider using an airship that is so big that it rests on the ground while the upper part is above most of the atmosphere.
Or one might consider launching a second stage from the airship.
Imagine an airship 225.78125 km long, there is a track on top a rocket ship accelerates on the track at 4 gs using an electromagnetic linear accelerator to reach 4.25 km/second before igniting its rockets to complete the journey to orbit.

I like your sense of humour

SpaceNut · 2015-08-24 18:39:52

Antius wrote:

Tom Kalbfus wrote:
One might consider using an airship that is so big that it rests on the ground while the upper part is above most of the atmosphere.
Or one might consider launching a second stage from the airship.
Imagine an airship 225.78125 km long, there is a track on top a rocket ship accelerates on the track at 4 gs using an electromagnetic linear accelerator to reach 4.25 km/second before igniting its rockets to complete the journey to orbit.
I like your sense of humour

Nah just the inflatable tower......

Mars_B4_Moon · 2022-07-01 18:18:59

Gigantic nuclear-powered 'flying hotel' capable of carrying 5,000 passengers and staying airborne for months at a time is unveiled in outlandish concept design

https://www.dailymail.co.uk/news/articl … esign.html

kbd512 · 2022-07-01 20:54:10

I don't think that thing will look quite the same way the artiste imagined it. For starters, such a design would generate so much drag that simply getting airborne would be a problem, and at some point that beast will have to takeoff and land. If it was designed to land on water, that'd be more practical. If it does land on a runway, then it likely won't have fixed landing gear. To be frank, a flying boat design that can use the ocean as a runway is a lot more practical than vertical takeoff or any of the other nonsense they came up with in the 1960s and 1970s. If you put enough thrust behind anything, then it will fly.

A better question might be what the draw would be to spending a bunch of money to vacation aboard a nuclear-powered "flying hotel". Cruise ships take you places to do tourist-type activities. Where would this thing take you if it's not going to land? The logistics of the entire concept are also questionable at best.

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#101 2006-06-13 09:14:55

Re: Calling our technical experts - Any chance this thing works?

#102 2006-06-13 17:18:45

Re: Calling our technical experts - Any chance this thing works?

#103 2006-06-16 14:06:46

Re: Calling our technical experts - Any chance this thing works?

#104 2006-06-19 00:50:48

Re: Calling our technical experts - Any chance this thing works?

#105 2006-06-19 01:03:34

Re: Calling our technical experts - Any chance this thing works?

#106 2006-06-26 17:29:37

Re: Calling our technical experts - Any chance this thing works?

#107 2006-06-28 21:18:21

Re: Calling our technical experts - Any chance this thing works?

#108 2006-06-29 01:00:02

Re: Calling our technical experts - Any chance this thing works?

#109 2015-08-23 23:13:26

Re: Calling our technical experts - Any chance this thing works?

#110 2015-08-24 04:39:12

Re: Calling our technical experts - Any chance this thing works?

#111 2015-08-24 09:42:14

Re: Calling our technical experts - Any chance this thing works?

#112 2015-08-24 11:18:05

Re: Calling our technical experts - Any chance this thing works?

#113 2015-08-24 18:39:52

Re: Calling our technical experts - Any chance this thing works?

#114 2022-07-01 18:18:59

Re: Calling our technical experts - Any chance this thing works?

#115 2022-07-01 20:54:10

Re: Calling our technical experts - Any chance this thing works?

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