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hallowatisdeze commented on a post in r/spacex
240
hallowatisdeze 6 points

About the Tesla Roadster going to 'Mars orbit'. The consensus seems to be now that we assume that the Roadster will not go to a Mars orbit, but only do a flyby of Mars and stay in a solar orbit with aphelion at Mars's orbit and perihelion at Earth's orbit. However, I didn't read any convincing reason for this assumption. As a matter of fact, this assumption does contradict the original tweet of Elon saying that the Roadster will go to Mars orbit, nothing else.

Why do we all assume that a ballistic capture to a high altitude Mars is not the case? ( https://en.wikipedia.org/wiki/Ballistic_capture ) Is this not possible with the low attitude control of the Tesla Roadster? Why not? It doesn't require any deep space insertion burn which the second stage can't execute. I can agree that it is a challenging task to do the Earth escape burn so accurate that the trajectory will lead to a ballistic Mars capture. But they can at least try it? I can't understand arguments I read that this is unrealistic because the spacecraft will not be sterilized, as a ballistic trajectory leads to a high altitude orbit with no chance of deorbiting any time soon. (An inaccurate Earth escape burn might lead to a 'contaminating' crash at Mars, but this risk is still valid when just a flyby is attempted)

Sorry for long post, I hope someone can give me some more information why a ballistic capture is realistic or why not.

theinternetftw 19 points

From this article by Phil Plait:

So I decided to contact Musk. He got back to me, and gave me the info.

[...]

No, it’s not going to Mars. It’s going near Mars. He said it’ll be placed in “a precessing Earth-Mars elliptical orbit around the sun.” What he means by this is what’s sometimes called a Hohmann transfer orbit, an orbit around the Sun that takes it as close to the Sun as Earth and as far out as Mars. This is a low-energy orbit; that is, it takes the least amount of energy to put something in this orbit from Earth. That makes sense for a first flight.

So that answers why we assume no ballistic capture.

this risk [of crashing] is still valid when just a flyby is attempted

Not if you're aiming so far away from Mars that even calling it a flyby is a stretch. It'll be interesting to see the expected closest approach distance (both this year and later years). The phrase "going near Mars" in the article above is Phil Plait's editorialization, and it may be generous.

To get past planetary protection (which Elon says they complied with), all of this had to be reckoned with.

hallowatisdeze 1 point

Thanks for that, I completely missed that article.

hallowatisdeze commented on a post in r/spacex
hallowatisdeze 36 points

This way, could they give the second stage a bit more energy, so that the second stage can cut off a bit earlier, which means that there is some fuel left for testing a controlled re-entry of that second stage?

hallowatisdeze commented on a post in r/Arianespace
hallowatisdeze 9 points

Too bad there is an abort! Some speculation of the cause:

After main (liquid fueled) engine ignition the computers calculate the generated thrust. They do this either by measuring the force on the hold-down clamps or by measuring the combustion chamber pressure (or both?).

When it is found that the engine thrust is too low, they decide to not let go of the hold-down clamps and shut down the liquid engine. This is all before the solid booster ignition, as they obviously cannot be stopped once fired.

A reason for the too low thrust can be too high propellant temperature. If this temperature is high, the density decreases. As the turbo pump works at a certain volume rate, this means that less mass of propellant is pumped to the engine for decreased propellant density. This could lead to not enough thrust. (Not relevant for Ariane, see below)

As said: this is all speculation. Maybe something completely different is the cause. SpaceX's Falcon 9 had an abort last year which looked very similar, I think that abort was caused by what I just speculated.

Jef-F 4 points

reason for the too low thrust can be too high propellant temperature.

AFAIK Ariane uses fuel and oxidizer at boiling temperatures, so they can't warm up higher than that. For F9 with sub-chilled LOX this is a problem, yes.

hallowatisdeze 1 point

Good point! I forgot about this difference between those launch vehicles. Ariane doesn't use 'superchilled' propellants like Falcon 9.

I'll strike it in my post.

hallowatisdeze commented on a post in r/spacex
NowanIlfideme 22 points

This bot always pops up, it'd probably be best for mods to contact the creator and ask to blacklist replying to u/decronym...

hallowatisdeze 8 points

That sounds like a good idea to me. However, I do appreciate this bot in typical cases. It saves me several clicks and loading times a day.

hallowatisdeze commented on a post in r/spacex
bertcox 43 points

Meme's are not allowed on /r/spacex, even the creation of meme's is banned here. Take that meme stuff to the kid pool where everything belongs. Only peer reviewed articles done by dual PHD's from Harvard's school of Philosophy are allowed here.

hallowatisdeze 16 points

Watch out, you're creating a meme.

hallowatisdeze commented on a post in r/spacex
danweber 10 points

Why is that more useful than ground-based detection?

hallowatisdeze 39 points

I don't think Putin would agree if the US tried to place a detector in Siberia.

danweber 39 points

The test-ban-treaty was a godsend for geologists, because the technology you use for detecting nuclear detonations is basically the same as for detecting earthquakes, and so there are detectors all over the globe.

You can't blow up a nuclear bomb in secret. And if you somehow got it buried deep enough that it wasn't detectable by earth-based sensors (which probably is not possible), you definitely aren't going to notice it from space.

hallowatisdeze 9 points

The detectors on the GPS sats are aiming for atmospheric nuclear detonations. I'm not sure if these are also that detectable with seismographic sensors?

Here an example of what in orbit detectors can cause: https://en.m.wikipedia.org/wiki/Vela_Incident For me, that's a very intriguing article.

hallowatisdeze commented on a post in r/spacex
436
hallowatisdeze 37 points

This is just me nitpicking:

Reading 'NROL launch' here in the comments makes me laugh. That's like saying 'LCD display', 'URL link' or 'USB bus'. :P

Kare11en 47 points

Ahem. URL.

hallowatisdeze 6 points

TIL learned what URL really stands for.

hallowatisdeze commented on a post in r/spacex
ethan829 26 points

SpaceX has always said that government missions on Falcon 9 are around $90 million. GPS III satellites are pretty light at 3,880 kg, and don't require vertical integration as far as I know.

hallowatisdeze 15 points

I'm pretty sure GPS-sats don't need vertical integration. It's basically 'just' a signal transmitter with a really accurate clock.

DoD-sats that require vertical integration are probably (as they're classified) very sensitive optical divices (lenses, mirrors...?), which must be taken more care of. Hence the vertical integration, which basically means that the sat can only take loads in one dimension/direction.

hallowatisdeze commented on a post in r/spacex
2A_is_the_best_A 23 points

They are owned by SpaceX, made mostly by the company I work for (www.orbitalsystems.com). They use several types of antennae, mostly 3m and 3.7m sized units, but they just ordered our second production 5m system.

These are located all over the place, the cape, vandy, africa, new england, bermuda. We're working on 2 additional systems for the cape to support falcon heavy right now, the'll be outside for testing at the factory next week-ish.

hallowatisdeze 2 points

Thanks, that's some interesting information!

What's the main benefit of taking the 5m ones instead of the current smaller size antennae? Is it also a useful upgrade for LEO communications, or is this another signal (haha signal, get it?) that SpaceX is really focusing on deep(er) space?

2A_is_the_best_A 2 points

A Bigger dish means more gain, so you can track things farther away or things with lower power transmitters. Or keep everything the same and use a higher bandwidth (if FCC approves) and get a higher data rate.

hallowatisdeze 1 point

Aha, thanks again for your reply. So I understand the bigger dish does not really point to specific applications. It just has better overall performance.

hallowatisdeze commented on a post in r/spacex
324
hallowatisdeze 4 points

Thanks for the analysis. I like the method just as much as the results.

My question: I understand the calculations done in MATLAB, but how did you do the tracking of the top and bottom of the stage? Do you also do this in MATLAB with some kind of image recognition tool?

hallowatisdeze commented on a post in r/spacex
gophermobile 5 points

What happens to the second stage for deep space launches? Is it still able to deorbit or does it also end up in deep space?

hallowatisdeze 7 points

My guess would be that in most cases the 2nd stage will leave the Earth's SOI. This means that it will enter an orbit around the sun which will last about forever (for any reasonable time frame).

hallowatisdeze commented on a post in r/SpaceXLounge
hallowatisdeze 8 points

I really like this kind of graphs, thanks for creating them! A few questions arise for me after seeing those. But be aware, I'm just an amateur so please correct me if I'm wrong!

  • From the first graph you can see that the CRS missions are throttling down at MaxQ to decrease stresses on the rocket at that point. This means a little loss of overall performance of the rocket. This is affordable due to the relatively low destination orbit of the CRS missions. Question: Would a significant payload mass increase be possible for the Dragon, if the rocket didn't throttle down like the other missions?

  • It looks like the last mission (Iridium) has the best performance. The acceleration and velocity are highest at any point in time. However, if I recall correctly, the payload mass of this mission was the highest. How is this possible? Did SpaceX find a way to get some more power out of this F9FT?

hallowatisdeze commented on a post in r/spacex
ticklestuff 25 points

Elon's close up of the F9 launching
https://twitter.com/elonmusk/status/820494936051097602

hallowatisdeze 8 points

That's a perfect background for my phone :)

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