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Author
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Topic: Max altitude for Mercury suborbital flights
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Explorer1
Member Posts: 180
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 10-24-2019 03:44 AM
  
The Redstone booster that was used to launch Freedom 7 and Liberty Bell 7 brought both of those spacecraft to altitudes of 116 and 118 miles respectively. These altitudes were deliberately chosen but it seems clear that the Redstone could have taken either spacecraft even higher if desired. How much higher is the question. Does anyone know the maximum altitude the Redstone could have gone? It is really a question of at what point the Redstone would have run out of fuel. Secondly, what would the ramifications have been of taking Freedom 7 and/or Liberty Bell 7 to higher altitudes, say for example, up to 200 miles high? Would fuel aboard the Mercury spacecraft for a retrorocket burn have been an issue? |
ejectr
Member Posts: 1758
From: Killingly, CT
Registered: Mar 2002
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posted 10-24-2019 07:48 AM
I would think that the g forces falling from that altitude would have been an excessive duration to handle. |
Headshot
Member Posts: 891
From: Vancouver, WA, USA
Registered: Feb 2012
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posted 10-24-2019 10:08 AM
I wonder if thermal limitations of the beryllium heat shield might have also added to the altitude ceiling. |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 10-24-2019 11:48 AM
quote:
Originally posted by Explorer1:
It is really a question of at what point the Redstone would have run out of fuel.
That is mistaken. The max burn was around 143 seconds and the actual burn was set at 142 second. There wasn't more fuel to burn. The only way of changing the altitude is the angle of the trajectory. quote:
Would fuel aboard the Mercury spacecraft for a retrorocket burn have been an issue?
The retrorockets were solid rocket motors and hence fixed impulse. They were never changed for the program. The retrorockets had no real role during the suborbital flight, they were there just to test the whole spacecraft and its mission sequencing. |
Explorer1
Member Posts: 180
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 10-24-2019 12:53 PM
Taking a hypothetical situation and if the Redstone continued past 142 seconds, at what altitude might it have exhausted its fuel? |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 10-24-2019 12:59 PM
The vehicle keeps climbing after exhausting its fuel. Look at MR-2, the vehicle had higher thrust and the escape rocket pulled the capsule off. |
Explorer1
Member Posts: 180
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 10-25-2019 09:58 AM
So just to clarify, you are saying that the Redstone had only enough fuel for 143 seconds and therefore was shut down within one second of exhausting its fuel supply? And how quickly did the spacecraft separate from the Redstone after the Redstone's shutdown? |
Robert Pearlman
Editor Posts: 43576
From: Houston, TX
Registered: Nov 1999
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posted 10-25-2019 10:18 AM
Per Results of the First U.S. Manned Suborbital Space Flight: The shutdown of the launch vehicle engine occurred at T+2 minutes 22 seconds, and, at the same time, a signal was to be given to the spacecraft to separate the escape tower. Spacecraft separation occurred 10 seconds later... And from Mercury Project Summary (NASA SP-45): To meet performance requirements, use of the elongated Jupiter G tanks was necessary. These tanks give the Mercury-Redstone launch vehicle a nominal engine burning time of 143.5 seconds, 20 seconds more than the original Redstone vehicle. |
oly
Member Posts: 971
From: Perth, Western Australia
Registered: Apr 2015
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posted 10-26-2019 01:26 AM
quote:
Originally posted by Explorer1:
Does anyone know the maximum altitude the Redstone could have gone?
The maximum altitude that a Redstone could achieve would be payload and launch profile dependent. The launches that took place for Shepard and Grissom were designed, among other factors, to "control" the acceleration forces applied to the astronaut. The 1961 launch of Mercury-Redstone 2 (MR-2), placing a chimpanzee into a ballistic trajectory to an altitude of 136.2 nautical mi. and was recovered 363 nautical mi from the launch site. You could run the math using various profiles and weights, but the results would not mean much today. The Redstone launches achieved their intended goals as research flights. The Mercury-Redstone launch profile injected the Mercury capsule in a sub orbital flight at a nominal, earth-fixed velocity of 6500 feet per second. The injection angle was 41.80 degrees, cutoff altitude 200,000 feet, and Mach number 6.3. The maximum acceleration at engine cutoff was 6.3 grs. (Source) |
Lou Chinal
Member Posts: 1332
From: Staten Island, NY
Registered: Jun 2007
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posted 10-26-2019 10:16 PM
quote:
Originally posted by ejectr:
I would think that the g forces falling from that altitude would have been an excessive duration to handle.
That is right. They might have been able to go higher, but the "G" on reentry would have been too much. Both Shepard and Grissom pulled about 11 G's on reentry. The Space Task Group thought that was about enough.The orbital flights were going much faster but the angle was much less shallow. They only withstood about 8 G's. |
taneal1
Member Posts: 237
From: Orlando, FL
Registered: Feb 2004
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posted 12-01-2019 02:29 PM
quote:
Originally posted by Lou Chinal:
That is right. They might have been able to go higher, but the "G" on reentry would have been too much.
NASA required the apogee to be comfortably above 100 miles so these flights could reach "space," which wasn't precisely defined at the time. Due to the limitations of the Redstone booster NASA was forced to use a 40-degree trajectory angle which resulted in much higher peak g-loads for a shorter period than the orbital flights.To reach a higher altitude a steeper trajectory would have been flown which would have resulted in a steeper re-entry trajectory and higher g-loads than the 40-deg trajectory actually flown. Like the suborbital Redstone, Atlas also required a steeper than desired launch trajectory to achieve the required altitude. If the booster had failed near the end of the launch burn, the steep trajectory combined with the near orbital velocity would result in up to a 20g deceleration. This deceleration was the driving factor for the form-fitting fiberglass couch that was only used in the Mercury spacecraft. If not for this invention, Mercury could not have flown on an Atlas. |
Explorer1
Member Posts: 180
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 12-04-2019 07:00 PM
Could you explain that a little more about why the form fitting couch made such a difference on Atlas flights?Also a related question, does anyone know at what minimum altitude the Atlas had to reach to achieve the speed of orbital velocity? I have to imagine it was fairly low, perhaps as low as 45 miles high. Also from sea level to an altitude of about 100 miles, the speed of orbital velocity varies only by about 100 mph. Therefore, about 17,400 mph at sea level and about 17,500 mph at 100 miles altitude. Does anyone know what determines the speed of orbital velocity for Earth to be 17,400-17,500 mph? And also how is the speed of 25,000 mph determined as escape velocity from Earth? |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 12-05-2019 10:37 AM
The minimum altitude would be 80-90 miles. It does no good to achieve it lower since it would reenter quickly.The distance from the center of the planet affects the orbital velocity. The mass and diameter of Earth determine escape velocity. quote:
Originally posted by Explorer1:
Could you explain that a little more about why the form fitting couch made such a difference on Atlas flights?
It made it easier for the astronaut to handle higher g loads. |
Explorer1
Member Posts: 180
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 12-05-2019 04:35 PM
80 to 90 miles altitude sounds rather high for a rocket to get to before it reaches a speed of 17,500. I am guessing that it reaches 17,500 mph at a much lower altitude, perhaps as low as 45 miles high because atmospheric density is very low. I understand that a spacecraft won't be released from a rocket until it is at least 80 or 90 miles high because anything lower, the orbit of the released spacecraft will begin to decay immediately. Even at 80 miles high, as I understand it, an orbit might decay after two orbits. But again, what is the minimum altitude that at Atlas class rocket (or better) reaches 17,450 to 17,500 mph? |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 12-06-2019 09:58 AM
No, it isn't high. The trajectory is designed so that doesn't happen. It does no good to reach orbital velocity at a lower altitude. That is a waste of energy. Launch vehicles typically reach orbital altitude (which is easy and to get above the atmosphere) long before they reach orbital velocity. Look here; The first stage has the spacecraft and upper stage at basically orbital altitude before the upper stage fires. And the upper stage still has to burn for more than 6 minutes to get into parking orbit. Even with the stage and a half Atlas for Mercury, the trajectory would be similar, with the booster and spacecraft reaching orbital altitude and continue burning to reach orbital velocity. |
Explorer1
Member Posts: 180
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 12-07-2019 03:51 AM
For clarification, keeping to the stage-and-a-half example of the Atlas, if the main stage is going at full throttle then how does the launch vehicle "prevent" itself from achieving a speed of 17,500 mph well before reaching an altitude of 80 miles high? Do you mean the trajectory itself bleeds off the speed of the launch vehicle?And thank you for the diagram on the Mars probe launch sequence. Unfortunately, it didn't list at what point the launch vehicle reached the speed of orbital velocity. The final highest speed it listed was 18,480 feet per second (12,600 mph) at 87 nautical miles altitude (100 miles high) 5 minutes into the flight. So I am still looking for the minimum altitude at which a launch vehicle reaches orbital velocity speed preferably for manned launches into low Earth orbit. |
PeterO
Member Posts: 402
From: North Carolina
Registered: Mar 2002
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posted 12-07-2019 11:08 AM
For the SpaceX CRS-19 launch on Dec.5th, the second stage reached 198km (123mi), 10km below the orbital altitude, at a velocity of 16,300km/h (10,128mph). It then accelerated to orbital velocity of 27,154km/h (16,873mph) while only gaining another 10km in altitude. |
Larry McGlynn
Member Posts: 1267
From: Boston, MA
Registered: Jul 2003
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posted 12-07-2019 02:31 PM
The Saturn V reached beyond its orbital altitude and then flew "downhill" until it reached orbital velocity at the intended altitude. You can see that in both the launch track and on the AGC Boost cue cards. |
NavyPilot
Member Posts: 36
From:
Registered: Nov 2015
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posted 12-08-2019 10:25 AM
According to what reference system was the trajectory "downhill"? Refer for example to the AS-10 launch report: the profile illustrated in Figure 3-2. Altitude-Ascent Phase on p3-9 suggests a very flat, perhaps slightly ascending altitude trend up to S-IVB ECO. I reckon that this is stated as absolute altitude over the geodetic sphere used by the IMU, which is a normalized surface. Annoyingly, this report does not define the precise reference for altitude.I had heard that the space shuttle used a slightly "downhill" trajectory at MECO, but I thought that this allowed for an optimized separation maneuver for the external tank. Is there any evidence of a downhill trajectory for Saturn V that we can see? Also, regarding the high-then-flat profile, weren't we exploiting higher total impulse in the newer boosters to drive a trajectory optimized for a pre-circularized insertion orbit without the requirement for a backside burn rather than optimized for max payload insertion like the weaker boosters of yore? |
NavyPilot
Member Posts: 36
From:
Registered: Nov 2015
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posted 12-08-2019 10:34 AM
Larry, I think I see what you're saying: a decreasing theta? The associated altitudes aren't reducing, only the progressive pitch attitude. This gets me thinking about reference systems again (ORDEAL can be an ordeal). |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 12-08-2019 08:54 PM
quote:
Originally posted by Explorer1:
Do you mean the trajectory itself bleeds off the speed of the launch vehicle?
No, it can't reach 17,500 mph before 80 miles. There is no excess velocity to bleed off. There is no "preventing" needed, gravity takes care of that. There is this misconception that rockets have excess power. They don't, if the Atlas for Mercury had short fall in engine burn time by a few seconds, it won't make orbit.As explained before, a rocket gets to orbital altitude long before it reaches orbital velocity. As stated before, reaching orbital altitude is easier than reaching orbital velocity. See sounding rockets (they can reach 100's of miles of altitude and nowhere never orbital velocity). How about this simple explanation: The trajectory is designed so that the rocket doesn't reach orbital velocity before it reaches orbit altitude. quote:
Unfortunately, it didn't list at what point the launch vehicle reached the speed of orbital velocity.
It didn't need to. It would be obvious that it reached orbital velocity when the upper stage shut down (MECO 1). quote:
So I am still looking for the minimum altitude...
That was stated many times. It is at the point went the engines shut down. That is why they are shut down. The only time they don't shut down is if it is a special or high orbit. |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 12-08-2019 09:00 PM
quote:
Originally posted by NavyPilot:
...weren't we exploiting higher total impulse in the newer boosters to drive a trajectory optimized for a pre-circularized insertion orbit without the requirement for a backside burn rather than optimized for max payload insertion like the weaker boosters of yore?
No, that is to exploit low power (T < W), high ISP upper stages. |
NavyPilot
Member Posts: 36
From:
Registered: Nov 2015
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posted 12-09-2019 06:31 PM
Got it. Thanks Jim. |
Explorer1
Member Posts: 180
From: Los Angeles, CA, USA
Registered: Apr 2019
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posted 12-09-2019 10:38 PM
quote:
Originally posted by Jim Behling:
The distance from the center of the planet affects the orbital velocity. The mass and diameter of Earth determine escape velocity.
Okay so in returning to the question of what determines the speed of orbital velocity, it's about 4,000 miles from the center to the surface of Earth.No doubt the closer one is to the center of Earth, the speed of orbital velocity would have to be higher. So now that we are 4,000 miles distance to begin with, can anyone calculate what the speed of orbital velocity would be at the center of Earth or a mile from the center of Earth? I am looking for the multiplying number and what its meaning is. |
Jim Behling
Member Posts: 1488
From: Cape Canaveral, FL
Registered: Mar 2010
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posted 12-10-2019 08:34 AM
quote:
Originally posted by Explorer1:
I am looking for the multiplying number and what its meaning is.
It isn't a simple ratio ("multiplying number"). The velocity decreases per the inverse ratio of the radius squared. 1/r^2.You can't find the velocity at 100 miles altitude and then 120 miles using a single number. The difference is: 1/r^2 - 1/(r+100)^2 Edit: Actually, it is the inverse ratio of the square root of the radius
1/r^0.5 |
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