Author Topic: Basic Rocket Science Q & A  (Read 272041 times)

Offline Malderi

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Re: Basic Rocket Science Q & A
« Reply #540 on: 10/05/2010 07:48 PM »
I read that plane change maneuvers use less fuel in higher orbits compared to lower ones. This is why, for example, GEO birds launched from Canaveral will do their GTO burn in a 28.5 degree orbit, and make it equatorial only when they're all the way up there.

Why is this the case? I don't have any understanding of the equations (or the reasoning behind them).

Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #541 on: 10/05/2010 07:53 PM »
Orbital velocity is much lower in high energy orbits, since much of the kinetic energy has been converted into potential energy. You can also combine the plane change with the circularisation burn which is inevitable and relatively large.
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Offline DavisSTS

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Re: Basic Rocket Science Q & A
« Reply #542 on: 10/06/2010 12:20 PM »
Having an argument with a friend. "200g acceleration on a human body wouldn't necessarily kill you, the time over which this acceleration occurs has to be taken into account."

He's wrong, yes?

Online ugordan

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Re: Basic Rocket Science Q & A
« Reply #543 on: 10/06/2010 12:33 PM »
My 2c is that your friend's probably right. What's the deceleration for example if you bang your head against a wall or jump from a table and the force your feet experience the instant you hit the floor? Human body is a good shock absorber so can withstand some punishment if it's short enough.
« Last Edit: 10/06/2010 12:34 PM by ugordan »

Offline kevin-rf

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Re: Basic Rocket Science Q & A
« Reply #544 on: 10/06/2010 12:41 PM »
Colonel John Stapp, did the rocket sled research that determined the limits. His record as listed by wiki is 45g  ( http://en.wikipedia.org/wiki/John_Stapp ).
« Last Edit: 10/06/2010 12:42 PM by kevin-rf »
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Offline DavisSTS

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Re: Basic Rocket Science Q & A
« Reply #545 on: 10/06/2010 12:46 PM »
The argument started when I said there was no way the crew of Challenger could have drowned as they would have died in the 200G impact with the water if they were still alive.

Offline kevin-rf

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Re: Basic Rocket Science Q & A
« Reply #546 on: 10/06/2010 03:45 PM »
Well, the Columbia report did point a flaw in the seat design that resulted in the head not being properly restrained (broken neck) during tumbling of the crew compartment. (Don't know if that would apply to Challenger, since they where dressed differently).
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Offline Proponent

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Re: Basic Rocket Science Q & A
« Reply #547 on: 10/07/2010 03:24 AM »
Well, the Columbia report did point a flaw in the seat design that resulted in the head not being properly restrained (broken neck) during tumbling of the crew compartment. (Don't know if that would apply to Challenger, since they where dressed differently).

I suspect the friend is right, in that a 200-g deceleration can be tolerated for a very short time.  That time, however, is probably shorter than the time it takes for a human body or crew cabin falling from great height to decelerate to zero speed on impact with the water, putting survivability of Challenger's crew in grave doubt.

If I remember correctly, by the way, there were signs that emergency oxygen supplies aboard Challenger had been manually activated following disintegration of the orbiter.

Offline Jorge

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Re: Basic Rocket Science Q & A
« Reply #548 on: 10/07/2010 03:28 AM »
Well, the Columbia report did point a flaw in the seat design that resulted in the head not being properly restrained (broken neck) during tumbling of the crew compartment. (Don't know if that would apply to Challenger, since they where dressed differently).

I suspect the friend is right, in that a 200-g deceleration can be tolerated for a very short time.  That time, however, is probably shorter than the time it takes for a human body or crew cabin falling from great height to decelerate to zero speed on impact with the water, putting survivability of Challenger's crew in grave doubt.

If I remember correctly, by the way, there were signs that emergency oxygen supplies aboard Challenger had been manually activated following disintegration of the orbiter.

Emergency *air* supplies, not oxygen. The distinction is non-trivial, since air would not have kept the Challenger crew conscious in the event of a cabin depressurization.
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Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #549 on: 10/07/2010 05:34 AM »
CxP 70024 requires less than 500 G/s of jerk.
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Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #550 on: 11/05/2010 01:17 AM »
Having an argument with a friend. "200g acceleration on a human body wouldn't necessarily kill you, the time over which this acceleration occurs has to be taken into account."

He's wrong, yes?

Attached is a figure from NASA STD 3000 on shock Gs allowed.  It shows 200 Gs as an upper limit to short G loads.  20 Gs is a good rule of thumb for ascent and entry Gs that last for several seconds.

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Offline tnphysics

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Re: Basic Rocket Science Q & A
« Reply #551 on: 11/24/2010 03:29 PM »
Here is an idea for a high (SSME-esque, at least in vac) performance GG engine.

The engine runs on LH2/LOX. The GG exhaust, after expansion in the turbine, is  used to cool nozzle parts. This heats it hot enough that it has an exhaust velocity equal to that of the main chamber gasses. The extremely low molecular weight should make this possible despite the low temp.

Is this possible? If so, is it practical?

Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #552 on: 11/24/2010 05:26 PM »
If you've got 1500F H2 and H2O coming out of the turbines, it's probably not enough of a heat sink vs -425F H2.

Also, you'd need a lot of pressure left after the turbine to get it through cooling passages.  That's not as efficient if you're not going to burn it the rest of the way.

I could imagine, though, basically a shell over the inner nozzle with many wide channels instead of hundreds of tubes.  Structural design would be a lot different than we're accustomed to, but it's doable.

Still, though, I think the much weaker heat sink makes it a non-starter.  Interesting academic exercise, though.
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Offline drbobguy

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Re: Basic Rocket Science Q & A
« Reply #553 on: 11/24/2010 10:18 PM »
Here is an idea for a high (SSME-esque, at least in vac) performance GG engine.

The engine runs on LH2/LOX. The GG exhaust, after expansion in the turbine, is  used to cool nozzle parts. This heats it hot enough that it has an exhaust velocity equal to that of the main chamber gasses. The extremely low molecular weight should make this possible despite the low temp.

Is this possible? If so, is it practical?

I'm no expert so can'd do the math, but it seems very unlikely and you might as well do the easy thing and just use it for film-cooling with an annular injector midway down the nozzle, as in the J-2.

Offline Malderi

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Re: Basic Rocket Science Q & A
« Reply #554 on: 12/10/2010 05:10 PM »
What is the relationship between delta-V (in m/s, for example) and degrees of plane change capability, depending on altitude? I'm just curious for a rough rule of thumb, especially for LEO. So, when a satellite has 500 m/s of delta-V, how many degrees of plane change could you do in an ISS-like orbit?

Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #555 on: 12/10/2010 05:53 PM »
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.

Offline Malderi

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Re: Basic Rocket Science Q & A
« Reply #556 on: 12/10/2010 06:23 PM »
Ah, I should remember to look through Wikipedia more thoroughly. Thanks.

Offline scienceguy

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Re: Basic Rocket Science Q & A
« Reply #557 on: 12/31/2010 12:37 AM »
I just wanted to know, what is a good power to weight ratio for power systems for spacecraft? What is it for say a small nuclear reactor? What is it for a fusion reactor?
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Offline Robotbeat

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Re: Basic Rocket Science Q & A
« Reply #558 on: 12/31/2010 04:49 PM »
1) I just wanted to know, what is a good power to weight ratio for power systems for spacecraft?
2) What is it for say a small nuclear reactor?
3) What is it for a fusion reactor?
1) Modern conventional solar arrays typically do about 50-80W/kg, though the UltraFlex arrays (being built for Orion, was baselined for ST-8 before ST-8 was canceled, and is baselined for many new missions in the latest planetary science decadal survey and already used on the Phoenix lander) will be able to do at least 150-175W/kg, at the solar array-level (not the cell-level, which could be much higher), and could likely be extended to 300 or 500W/kg with new ultra-high efficiency (~35%) IMM cells which are reportedly one fifteenth the thickness of typical multijunction cells (and thus much lighter, allowing the structure to also be lighter). So, I would say anything above 75W/kg is "good," but in a decade, that will probably seem like very low performance.

RTGs have much lower specific power, though don't need sunlight, so work well even in the deep outer solar system, though their power decreases considerably over time as the fuel decays and the thermocouples degrade. MMRTGs (the modern RTG design used for MSL) have about 2.8W/kg, which is, to be honest, pretty bad (which is part of the reason for the push to ASRGs which have about 4W/kg, which also are more efficient, though they haven't been proven in space). Older RTGs, like used on Cassini, have a tad over 5W/kg specific power, but the last one is on its way to Pluto right now.

2) The specific power for the only nuclear reactor ever orbited by the US is a little less than 1.75W/kg unshielded (and even worse when shielded). So, pretty horrible... in fact, at 1 AU from the Sun, that's one hundredth of the performance of the latest Ultraflex arrays. And the prototype fission reactor only produced about 500W electric power. Not exactly fair, since it was pretty much just a proof-of-concept, but nuclear fission power in space has a lot left to prove before it competes with solar power (or RTGs). However, for a lot of power in the outer solar system, there are not really many other options than nuclear fission, though the amount of development left to do is pretty enormous (and billions of dollars need to be spent). In the inner solar system, solar power is awesome.

There are some notional concepts for very high specific power fission space reactors (~200W/kg), but the cost of development would be... um... out of this world. Solar power could much easier reach this specific power level in the inner solar system (and has actually been demonstrated at more than this performance on the IKAROS solar sail spacecraft with thin film solar cells).

3) Has never been demonstrated to produce electricity, even on Earth (unless you count solar power...). Unless there is some kind of breakthrough, it's not an option. And if fusion power only will work with ITER-style tokamaks for the next century, I highly doubt fusion would be better than fission power for space propulsion in the next century.

Regarding solar versus nuclear:
Power on the surface of celestial bodies is a little different story. Lunar night is a long time to store electricity in some kind of battery, so nuclear power has an edge, here (also, it would be easier to shield on the Moon, where you can put the reactor behind a hill or use regolith). A similar situation is faced on Mars, where, although night isn't nearly as long as on the Moon, there are some pretty intense and long-lasting dust storms. Also, the Martian atmosphere can be used as a heat sink instead of just having to rely radiators. Also, the Martian winds can cause problems for the very lightweight solar arrays that can be used in microgravity and vacuum. So although solar power is a pretty good solution, advanced "RTGs" (like improved, high power ASRGs) and new nuclear fission reactors are probably superior for surface power, if you have the money for them.

EDIT:For reference, solar power specific power figures given above are for 1AU distance from the sun when fully lit. Solar power available per area decreases as you get further from the Sun (and increases as you get closer to the Sun).

Power at Mars = ~50% of power at 1AU
asteroid belt   = ~10-20% of power at 1AU
Jupiter           = ~4% of power at 1AU
Saturn           = ~1%
Uranus           = ~0.25%
Neptune         = ~0.1%

Power at Venus =~200% of power at 1AU (if you can handle the high temperatures)

(there are some design issues with operating a solar array with very low sunlight, but they can be addressed)
« Last Edit: 12/31/2010 05:14 PM by Robotbeat »
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Re: Basic Rocket Science Q & A
« Reply #559 on: 12/31/2010 08:54 PM »