Deep Space Questions - Episode 8 - Heavy Payloads, Cool Rockets & The Most Difficult Question Yet

Continuing to answer questions from supporters over at my Patreon

NASA's open Source Software

Weakless Universe

Fundamental Physical and Resource Requirements for a Martian Magnetic Shield

American Rocketman - by Robert C Truax

Most of the questions are still taken from the first post:

[eyemastervideo]

The problem with a trebuchet on the moon is that they used to try and find the biggest tree around to build the trebuchet's main arm. There are no trees on the moon.

[LaunchPadAstronomy]

I asked John Grunsfeld about returning Hubble many years ago. If I remember everything he said correctly, NASA looked into this, but by the end of SM4, HST had more than doubled its original mass. So a return mission would have necessitated four EVAs to remove instruments, the solar arrays, and batteries in order to fit within the shuttle’s landing weight, leaving behind a lot of space junk. Considering this would have mandated an end to HST within 2 years of SM4, I’m glad they didn’t do this mission 😀

[wdavidwoods]

Top video, as ever. On the Saturn V hydrogen tanks, the vehicle had two hydrogen tanks. The one in the third stage had insulation on the inside, as you said. The second stage's hydrogen tank had insulation on the outside. Initially, the insulation was in the form of panels glued to the tank. This was problematic due to air pockets liquidising with the cold. From Apollo 13 onward, spray-on foam was used, a technology that was carried on to the Shuttle External Tank.

[mewintle]

Wow, those were amazing questions. Thanks, everyone!

[hamslicemcdooogle]

I don’t think I’ve ever seen Scott as happy as when he got stumped by a seemingly simple yet not at all obvious question.

[tekkx_avery]

To answer the first question is easy! The way home is always faster (not necessarily shorter).
I remember a one-day trip with a horse-drawn carriage. The horses have been two mares. As they recognized we're on our way back home they get faster and faster as they want to go back to their foals to feed them. I remember how strong we had to pull the reins to prevent them from galloping. ;)

[captainjack-ash]

Maybe the outbound voyage has to be slower due to there not being an atmosphere to slow down but you can come back faster and dump excess energy in earths atmosphere?

[JC-IV]

Let’s be honest, this channel is the best master class. Hopefully in person in the future we could have a lecture hall packed for Scott Manley Lecture series

[ebenwaterman]

Going to the moon you are starting near earths escape velocity and slowing down to the moons.
Coming back you accelerate to the moons escape velocity and don't brake at all on the way back.
They were going really fast when they hit the atmosphere. A lot faster than from LEO.
Translation: They were cookin' on the way back. :)

[dondeas]

No research on this, but for Q1 I would say that the return trip was faster because it could be. On the way out, every pound of fuel you burn going out will be matched by a pound of fuel to capture orbit (not 1:1 obviously but you get the point). On the way back the only speed limitation is the capacity of the heat shield, since braking is 'free' from a fuel standpoint, so you can come back faster.

[brianjaber]

Thank you for that Scott I completely enjoyed it although I completely enjoy everything you do, so I guess it’s just thank you so much and keep up the great job.

[JulianDanzerHAL]

6:00
in a 2 dimensional universe you'd expect literally everything to be a black hole

[michaelgoetz]

With regards to Landing with a heavy payload, anything the shuttle could bring to orbit, it could also land with. It had to be able to do this: any launch abort mode other than abort-to-orbit would require it to land with its entire payload still onboard. Even if it was capable of theoretically launching more than it could land with, they would never do this since an abort would result in the loss of crew and vehicle.

[jeromethiel]

Hot damn, those were some good questions!

One thing i find that always separates people who actually know things from people who pretend to know things is how often they say "hmmm... I hadn't thought of that, let me go and find out."

So i would class you, Scott, as someone who knows things. Because the more you know, the more how little you know. And that inspires you to go and learn more.

People who are absolutely sue about things are liars, prophets, or politicians. Because in the real world, there is no certainty. There is only questions that need to be answered. And the answers change as we learn more.

[piranha]

13:10 : Minotaur I is a rocket that uses a solid first stage, yet still has thermal insulation blankets, which it sheds at liftoff. I'm not sure why it needs those. (Perhaps to prevent the propellant's rubber binder from cracking if it's too cold? Or softening and deforming if it gets too hot?)

Also, the rocket is white, and the blankets bright yellow, and they just "peel" back at liftoff. Which has earned it the nickname of "banana rocket".

[kennethng]

Moon to earth faster: how about. you don't have to accelerate the LEM on the way back?

[QwazyWabbit]

Re: Apollo trip times. It’s all about conservation of energy and initial mass at TLI. The kinetic energy (KE) at TLI was dependent on the mass of the vehicle which included a fully fueled and assembled LM and the mass of the SIVB at the TLI burn. The velocity of the coupled CM/LM had to be calculated to give a specific velocity at the moon and a specific ΔV at LOI. The velocity was constantly decreasing on the way up, out of the gravity well of earth and slightly increasing once the force of lunar gravity matched the force of earth. KE=1/2mV^2. Separation from SIVB reduced the mass arriving at the moon. We have now converted the KE of the vehicle at TLI to gravitational potential energy at the moon with a lower mass vehicle. Coming back, we are reduced to CM/SM mass plus a couple kg of rocks. The total KE at reentry must equal the KE at TLI and we have much lower mass so it results in a higher velocity on the way back and shorter return time. All this and the fuel budget was calculated on earth long before launch. The trajectory and course corrections had to be recalculated for Apollo 13 safe return because they were bringing the full LM back with them. Evaluate all the Apollo logs and compare them with Apollo 13’s to see the difference a change in vehicle mass makes. All the KE has to be dissipated in the atmosphere on reentry.

[mikeo]

Spacehab hardware apparently flew 22 shuttle missions and Spacelab 32 missions - wow, would love a deeper dive Scott. And I’ll get on to Lego about an add to the Shuttle set

[ryanhamstra]

Answer to first question. I would guess that the reason is because every FPS you add on the way there you have to remove it to get in orbit. On the way home you can use the rest of your fuel plus the craft is lighter. In Apollo 13 they do a burn after finishing the free return trajectory. On the way there you need to follow the free return, way home you don’t have to, you just have to hit the atmosphere. You also don’t have a fully fueled LEM to tow on the way home.

[danmorrison]

Since the Earth is roughly 80 times the mass of the moon, a spacecraft climbing out of the Earth's gravity well would necessarily end up going much slower. A spacecraft falling into that gravity well would be gaining speed nearly the entire way, making the trip shorter. Or is this Biz Geek crazy?