CR4 - The Engineer's Place for News and Discussion ®


Previous in Forum: Close Encounters of the Classified Kind  
Close
Close
Close
24 comments
Guru
Engineering Fields - Engineering Physics - New Member

Join Date: Oct 2006
Location: MSP, MN
Posts: 569
Good Answers: 6

Reentry Options

07/13/2017 12:39 PM

Was watching a program about the Space Shuttle, and the reentry business of it and the Apollo capsules, with the tiles and heat shields needed to withstand slamming into the atmosphere at 17,000 MPH. If we used retrorockets to slow the spacecraft before reentry, would this not reduce the heating problem? I realize retros would have a weight penalty, but solid rockets, like the SS boosters, would be simple and cheap to implement. Not having to deal with tiles or other heat shields would save weight and reduce risk. Thoughts?

Reply
Pathfinder Tags: Apollo heat shield reentry space
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 17968
Good Answers: 1037
#1

Re: reentry options

07/13/2017 1:47 PM

It seems that slowing down you would lose altitude and still have to deal with atmospheric friction, you would just come down faster it seems to me....

https://space.stackexchange.com/questions/8026/do-you-need-a-heat-shield-to-enter-the-atmosphere-from-non-orbital-speeds

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Reply
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 11229
Good Answers: 133
#2

Re: Reentry Options

07/13/2017 1:53 PM

That idea will "never get off the ground", as it were.

They are improving the heat resistance features of leading surfaces at high Mach number all the time, and some of the new composites coming down the pipe now will make the old stuff seem even more primitive.

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply
Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 11946
Good Answers: 757
#3

Re: Reentry Options

07/13/2017 4:31 PM

Skip or Boost Glide reentry techniques have been studied and may be used in the future. In theory, this technique can extend reentry time for heat load reduction. I think they found the irregularities of the high level atmosphere caused this approach to be abandoned. I think the abandoned Dyna-Soar project was intended to use this approach.

__________________
"Don't disturb my circles." translation of Archimedes last words
Reply
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 11229
Good Answers: 133
#4
In reply to #3

Re: Reentry Options

07/13/2017 4:58 PM

Yes that project now is a real Dyna-Soar. LOL

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply Score 1 for Off Topic
Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 11946
Good Answers: 757
#5
In reply to #4

Re: Reentry Options

07/13/2017 10:15 PM

Now you'll tell me you wish to buy a Chevrolet. But only to see the U.S.A. in your new Chevrolet.

__________________
"Don't disturb my circles." translation of Archimedes last words
Reply Score 1 for Off Topic
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 11229
Good Answers: 133
#9
In reply to #5

Re: Reentry Options

07/14/2017 9:19 AM

Nah! My next truck, I will go back to Dodge, much smoother ride, all the creature comforts, and a drive train that I cannot tear up.

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply Score 1 for Off Topic
Guru

Join Date: Feb 2013
Posts: 567
Good Answers: 22
#6

Re: Reentry Options

07/13/2017 11:03 PM

I used to think that too. Then I saw a calculation of what that would involve. There is no possible retro rocket that could put a dent in the 17,000 mph speed of a spacecraft. If you built one, it would be enormous like a booster rocket, and impossible to put into orbit. In order to put anything in orbit, most of the thrust of the rocket is used to get it going horizontally really really fast, as it must to be in orbit at all. Not much of the energy is needed to raise it up into space. So to slow it down again, you need a similar amount of thrust.

Reply Score 1 for Good Answer
Guru
Engineering Fields - Engineering Physics - New Member

Join Date: Oct 2006
Location: MSP, MN
Posts: 569
Good Answers: 6
#7
In reply to #6

Re: Reentry Options

07/13/2017 11:21 PM

Interesting. I guess if we assumed a spacecraft mass, we could calculate how much force would be needed to slow it to some speed. If we stopped it to zero, it would fall straight down. If we assume its at about 200 miles up, then it would free fall for, oh, maybe 190 miles before drag began to seriously slow it. Rough calcs have it taking 4 minutes and reaching 5500 mph at 10 miles altitude. Sound right? Still pretty fast. Wonder if drag chutes could be used...

Reply
Guru
Engineering Fields - Engineering Physics - New Member

Join Date: Oct 2006
Location: MSP, MN
Posts: 569
Good Answers: 6
#12
In reply to #6

Re: Reentry Options

07/14/2017 10:13 AM

Thinking of Apollo and Saturn 5, most of the energy was used to lift the launch vehicle, which was left behind! We ended up with a relatively small payload in orbit. Let's see; kinetic energy = 1/2 mv2. So knowing the mass of the spacecraft and it's speed, we can figure how much energy must be lost to slow to a given speed. Sound right so far? Knowing that, some rocket scientist could calculate how big a retro would be needed.

Now, I suspect that NASA is way ahead of me on this. But I still want the numbers and will press on.

Reply
Guru
Canada - Member - Specialized in power electronics

Join Date: Feb 2008
Location: Montreal, Canada.
Posts: 1323
Good Answers: 78
#8

Re: Reentry Options

07/14/2017 7:43 AM

Using the conservation of energy principle, the energy needed for the retro-rockets should be about equal to the energy needed to bring it up to space, less the friction losses in each directions.

This means that your retro rocket could be about 50 to 75% of the initial rocket (just a guess).

The present strategy of using the atmosphere to dissipate this energy seems to be the most economical at the moment.

What would be nice is if we could some how recuperate part of this energy and transfer it to the next rocket we want to bring up. We simply need a big pulley and a long cable attached to a sky hook to operate like an elevator with its counterweight.

__________________
Experienced is earned, common sense is taught, both are rare essentials of life.
Reply
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 11229
Good Answers: 133
#11
In reply to #8

Re: Reentry Options

07/14/2017 9:25 AM

How about we skip that, and just design a craft that is highly supersonic up to 60,000 or 70,000 ft altitude, with a "calf" ship that is rocket boosted, of smaller mass.

Also design a re-entry vehicle that could channel some or all the hot gases (this would be an engineering breakthrough that does not seem remotely possible at present), into a thrust turbine engine (thus it is air-breathing, even at high speed, high altitude)?

The generated thrust would brake speed to survivable levels before the "icing" melts.

Just throwing a really wild idea out there. The exhaust of this proposed engine would have to have a velocity of at least 3-10 times the velocity of the craft falling into the atmosphere, and have a comparable amount of mass flow to the gases flowing around the craft as in penetrates the atmosphere.

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply
Associate

Join Date: Mar 2015
Posts: 33
Good Answers: 1
#10

Re: Reentry Options

07/14/2017 9:20 AM

Yea wow, just how many of these large explosive devices would be needed for the average mission?

Reply
Power-User

Join Date: Nov 2009
Posts: 229
Good Answers: 15
#13

Re: Reentry Options

07/14/2017 10:14 AM

I have no idea whether this is true or not but I once read that the space shuttle has to turn itself 90 degrees to the atmosphere and use its wings to "knife" it's way into the atmosphere.

The reason stated was: if it tries to re-enter in normal flat flight mode it will just bounce off the atmosphere.

Does anyone know if it's actually true?

Can't find anything at all about it on a quick search.

Reply
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 11229
Good Answers: 133
#14
In reply to #13

Re: Reentry Options

07/14/2017 11:14 AM

In any re-entry, attitude of the craft must remain within a tightly controlled envelope, or she will truly "skip" out and miss the LZ. If attitude control is bad enough, it simply acquires too much frictional heat and ablates everything, including the crew.

Therefore, attitudes must be maintained with an eye toward the safety envelope at all times during this mission critical step. Any fidget of the controls, or misfire is a bad day for all concerned.

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply
Power-User

Join Date: Nov 2009
Posts: 229
Good Answers: 15
#18
In reply to #14

Re: Reentry Options

07/15/2017 9:59 AM

Definitely a need for cool heads in that situation.

Reply
2
Guru
Engineering Fields - Engineering Physics - New Member

Join Date: Oct 2006
Location: MSP, MN
Posts: 569
Good Answers: 6
#15
In reply to #13

Re: Reentry Options

07/14/2017 11:39 AM

From Wikipedia: The vehicle began re-entry by firing the orbital maneuvering system engines, while flying upside down, backside first, in the opposite direction to orbital motion for approximately three minutes, which reduced the Shuttle's velocity by about 200 mph. The resultant slowing of the Shuttle lowered its orbital perigee down into the upper atmosphere. The Shuttle then flipped over, by pushing its nose down (which was actually "up" relative to the Earth, because it was flying upside down). The vehicle started encountering more significant air density in the lower thermosphere at about 400,000 ft, at around Mach 25, (18,000 mph). The vehicle was controlled to fly at a 40-degree nose-up attitude, producing high drag, not only to slow it down to landing speed, but also to reduce reentry heating. As the vehicle encountered progressively denser air, it began a gradual transition from spacecraft to aircraft. In a straight line, its 40-degree nose-up attitude would cause the descent angle to flatten-out, or even rise. The vehicle therefore performed four steep S-shaped banking turns, each lasting several minutes, at up to 70 degrees of bank, while still maintaining the 40-degree angle of attack. In this way it dissipated speed sideways rather than upwards. This occurred during the 'hottest' phase of re-entry, when the heat-shield glowed red and the G-forces were at their highest. By the end of the last turn, the transition to aircraft was almost complete. The vehicle leveled its wings, lowered its nose into a shallow dive and began its approach to the landing site.

The steep banking turns sound like what you read. Hell of a ride, eh?

Reply Good Answer (Score 2)
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 11229
Good Answers: 133
#16
In reply to #15

Re: Reentry Options

07/14/2017 12:31 PM

with the amount of lift produced even in upper atmosphere at those speeds with 40 ° nose up attitude, yes, she would be "skipping" back upward. By using high-bank S turns, while maintaining nose up attitude 40, the energy is lost as would be with any aircraft making steep bank turns, rather than increasing altitude, thus maintaining the braking action of the atmosphere, limiting heat up (the extra energy going into producing the turn), and keeping the shuttle on her targeted flight path to the LZ.

__________________
If it ain't broke, don't fix it. Just build a better one.
Reply
2
Guru

Join Date: Mar 2007
Location: at the beach in Florida
Posts: 17968
Good Answers: 1037
#20
In reply to #16

Re: Reentry Options

07/15/2017 6:30 PM

Here's an animation of the reentry and landing....

First stage of the Saturn V rocket....

First stage is 33' across and 138' long......5 mil lbs loaded....

__________________
Life is like riding a bicycle. To keep your balance you must keep moving. A.E.
Reply Good Answer (Score 2)
Power-User

Join Date: Nov 2009
Posts: 229
Good Answers: 15
#22
In reply to #20

Re: Reentry Options

07/16/2017 9:27 AM

Thanks SE, that was awesome.

Reply
Power-User

Join Date: Nov 2009
Posts: 229
Good Answers: 15
#19
In reply to #15

Re: Reentry Options

07/15/2017 10:00 AM

Thanks SSCpal, that is a lot more complex than the simple sentence I quoted but it does convey the difficulties extremely well.

A hell of a ride, no question about that.

Reply
Guru
Engineering Fields - Engineering Physics - New Member

Join Date: Oct 2006
Location: MSP, MN
Posts: 569
Good Answers: 6
#17

Re: Reentry Options

07/15/2017 9:39 AM

Big picture, we've been using aerobraking since the dawn of spaceflight and, to my knowledge, never had a failure. The media says things like "THE ASTRONAUTS ARE ENGULFED IN A BALL OF FIRE!" Makes it sound really risky. But none of our failures have been associated with aerobraking. Seems like the easiest way to shed energy on reentry. Until we get the space elevator, I think we'll keep using it.

Reply
Guru

Join Date: Apr 2010
Posts: 5279
Good Answers: 513
#21
In reply to #17

Re: Reentry Options

07/15/2017 9:45 PM

Actually, the space shuttle Columbia did indeed fail on aerodynamical braking.

Still, retro rocket braking is totally infeasible, and aerodynamic braking is almost free, if you can stand the heat!

Reply
Guru
Engineering Fields - Engineering Physics - New Member

Join Date: Oct 2006
Location: MSP, MN
Posts: 569
Good Answers: 6
#23
In reply to #21

Re: Reentry Options

07/16/2017 11:19 AM

Columbia was compromised by a hole in a wing leading edge, which had been caused by a chunk of insulation foam that hit it during takeoff.

Reply
Guru
Engineering Fields - Electrical Engineering - Been there, done that, still doing it. Engineering Fields - Control Engineering - New Member

Join Date: Dec 2008
Location: Long Island NY
Posts: 11946
Good Answers: 757
#24
In reply to #23

Re: Reentry Options

07/16/2017 6:01 PM

Yes, what you say is correct. However, this compromise did not become a fatal flaw until the "controlled" aerodynamic braking of reentry.

__________________
"Don't disturb my circles." translation of Archimedes last words
Reply
Reply to Forum Thread 24 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be Good Answers:

These comments received enough positive ratings to make them "good answers".

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

Canary (1); James Stewart (6); jhotmer (1); marcot (1); redfred (3); Rixter (1); SolarEagle (2); SSCpal (5); wrenchtwirler (4)

Previous in Forum: Close Encounters of the Classified Kind  

Advertisement