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62 votes
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Why did the Tesla Roadster spend time in the Van Allen belt?

Up until this flight of Falcon Heavy, officially, SpaceX could not fully deliver a satellite to [nearly circular] GEO (Geosynchronous Earth Orbit), but only to a [highly elliptical] GTO (Geosync ...
geoffc's user avatar
  • 79.9k
43 votes
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Why not keep the engine for multi-stage rockets?

There are some major challenges with this. For starters, the engines of the first stage produce far too much thrust for the last stage, which would require extra structural mass to allow the rocket to ...
Nathan Tuggy's user avatar
  • 4,567
31 votes
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Why is fuel ratio different for upper stage of a rocket?

The J-2 engine used on the second and third stages of the Saturn V has a "PU valve" (propellant utilization) on the oxidizer turbopump. Adjusting the mixture ratio with this valve primarily ...
Russell Borogove's user avatar
23 votes

Why not keep the engine for multi-stage rockets?

Nathan's answer is good and cover almost everything but let me add a last bit: An engine nozzle can be optimized for only one given altitude ambient pressure. This has a great impact on the rocket ...
Antzi's user avatar
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21 votes
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Why did the Saturn V stage fairing appear to burn after separation?

The From the annotated Apollo 8 Flight Journal, in the notes after the 2:36 mark, we see that the interstage separation happens well after the J-2 engines on the second stage are running: If, at ...
Russell Borogove's user avatar
21 votes
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Why is the flame of the Falcon 9's 2nd stage (nearly) invisible?

The Merlin engine used by the Falcon 9 burns a fuel-rich RP-1/LOX mixture, like most other rocket engines. That means the exhaust coming from the engine contains unburnt fuel, mostly in the form of ...
Floern's user avatar
  • 1,479
20 votes

What would have happened if the third stage of Saturn V had exploded during TLI? Could the CM have safely returned to earth?

If the failure of the S-IVB third stage didn't significantly damage the Command/Service Module, and the CSM was able to separate from the S-IVB, then this is a surprisingly benign failure mode and the ...
Russell Borogove's user avatar
19 votes
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What was "inertial" about the Inertial Upper Stage?

I found a nice article about the evolution of the IUS in this issue of the Crosslink newsletter. Originally, NASA intended to develop a reusable "space tug" that could deliver satellites ...
Russell Borogove's user avatar
18 votes
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Why has SpaceX changed Starship from carbon fiber composite to stainless steel?

According to tweets from Elon, at least part of the decision is due to their design for dealing with the reentry heat: instead of adding ablators to cool the craft on reentry, Starship is going to ...
DarkDust's user avatar
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18 votes
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Why did Agena lose out to Centaur?

Agena is a much smaller and less powerful stage than Centaur. Astronautix gives Agena-D a loaded mass of about 6.8 tons, as compared with around 23 tons for Centaur (though this varies quite a bit ...
Russell Borogove's user avatar
15 votes
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Why does PSLV use four stages to get to LEO, and why do they alternate solid, liquid, solid, liquid?

PSLV is a bit of a weird duck because it's a transitional step from small solid-rocket designs to larger ones that rely more heavily on liquid rocket engines with higher specific impulse. It doesn't ...
Russell Borogove's user avatar
14 votes

Why two engines on stage 2 of New Glenn if it's disposable?

In 2015-2016, the New Glenn design was expected to use a single BE-4 methane-LOX engine on the second stage (about 2400kN thrust), and a single BE-3 (the same as the suborbital New Shepard's single ...
Russell Borogove's user avatar
11 votes

Is it normal for thrusters to "ice up"?

When this was captured, it was at the end of the rocket flight. Looking carefully at this, I don't see any other similar nozzles. Furthermore, I didn't see any evidence of this being used in flight. I'...
PearsonArtPhoto's user avatar
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11 votes
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Function of this big pointy thing on the Titan Transtage? Do other upper stages have something similar?

It's hard to see in your picture but there are actually two big pointy things on the Transtage. They are propellant tanks. The one in your picture is the nitrogen tetroxide tank. Reference More ...
Organic Marble's user avatar
9 votes

How can the same upper stage be optimal on a common core booster as well as on a single booster?

You haven't defined "optimal" here. The commercial launch business is more concerned with cost optimization than mass optimization. Atlas V offers two variants of Centaur, with one or two RL10 ...
Russell Borogove's user avatar
9 votes
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What is typical lifetime of GTO rocket stages before reentry?

The typical lifetime of an upper stage rocket body (in GTO) varies from few weeks to few decades. This large variation is due to the sensitivity of the orbital evolution to the initial launch ...
Harish's user avatar
  • 390
9 votes
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Are Kickstages exclusive to Rocketlabs Electron Design?

"Kick stage" or "apogee kick motor" is a loosely defined term for the last stage of a satellite launcher. Frequently it's the stage that puts the satellite into its final orbit. The term is not ...
Russell Borogove's user avatar
9 votes

How are upper stage engines chilled down?

Different vehicles use(d) different schemes. The S-IV stage on the Saturn I vehicle had overboard vents for the LH2, and fabric blowout panels for the LOX. One of the LH2 vents is indicated, the ...
Organic Marble's user avatar
8 votes

Advantages of a Solid Fueled Upper Stage

Solid fuel does make sense for a top stage, a so-called kick stage, the most notable example being the Star family. A kick stage has only smaller or similar gross mass as the payload it propels, which ...
leftaroundabout's user avatar
8 votes
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Is it normal for thrusters to "ice up"?

Is it normal for thrusters to “ice up”? The thing circled in your photo is a LOX vent, not a thruster. The puffy white object is solid oxygen. Your question is directly answered at T+1.05:25 in the ...
Bob Jacobsen's user avatar
  • 12.7k
8 votes

Is it true that hydrogen is the only practical fuel for upper stages?

No. Currently operating launchers using other fuels for their upper stages include Falcon 9 (RP-1), Soyuz (RG-1 for the 2nd stage, UDMH/N2O4 for the Fregat upper stage), Vega (solid fuels for 2nd and ...
djr's user avatar
  • 1,705
8 votes
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How rare is it in the 21st century to use five stages to get to LEO, especially for a low-mass payload?

5 stages is extremely rare -- in both the 20th and 21st centuries. Most modern launchers are two-stage-to-orbit, sometimes with a third stage for transfer to geosynchronous orbit, or added boosters ...
Russell Borogove's user avatar
8 votes
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ULA's ACES upper stage with IVF - Why are they using internal combustion engines?

What a fantastic question! I learned a lot researching this one. The use of a simple piston-in-cylinder engine on an ultra high performance in-space stage seems to be out of place in a technology ...
Anton Hengst's user avatar
  • 10.9k
7 votes

Which is overall more expensive to operate for a final rocket stage? LH2/LOX or hypergolics like UDMH/N2O4?

For an upper stage, look to the Long March rocket series as a case history. For Long March 2-4, the boosters use hypergolic fuels while the upper stage burns LOX/LH2. Given that they have decades of ...
Kengineer's user avatar
  • 1,758
6 votes
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What is this jet of material spraying out from Stage 2 during the SpaceX PAZ mission?

The thing circled in the photo is a LOX vent. The puffy white snow-like stuff is solid oxygen. It was described at T+1.05:25 in the Iridium-6/GRACE-FO webcast.
Bob Jacobsen's user avatar
  • 12.7k
6 votes
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How much fuel can Falcon 9's upper stage bring to orbit, without any other payload?

Should be pretty close to the payload to LEO: 22.8 tons. Maybe a bit more if you design a new second stage with larger tanks and a nose cone.
Hobbes's user avatar
  • 128k
6 votes

Why rotate the Falcon 9 2nd stage after deployment?

At the end of the webcast, they mention the last step is to safe the spacecraft. I suspect all they were doing was getting rid of the remaining fuel on the spacecraft. Rotating it might lead to a ...
PearsonArtPhoto's user avatar
  • 121k
6 votes
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What are the challenges in recovering Falcon 9's second stage this way?

It's likely way too heavy. Unlike the first stage, the second stage needs to accelerate all the way to LEO/GTO orbital speed. This means that all the extra stuff for reentry and landing needs to be ...
TooTea's user avatar
  • 1,765
6 votes

What would have happened if the third stage of Saturn V had exploded during TLI? Could the CM have safely returned to earth?

Almost certainly, assuming the CSM wasn't damaged. During Apollo 13, they considered (but chose not) to use "direct abort", which would have used the SM engine to return without going ...
Eugene Styer's user avatar
  • 1,852
5 votes
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SpaceX second stage altitude decrease?

This is actually pretty common in rocket launch profile design. For the Ariane rocket, we covered it in What is the reason for the Ariane 5 launcher with Intelsat 29e losing altitude?, but the same ...
PearsonArtPhoto's user avatar
  • 121k

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