SpaceShipTwo, VSS Unity - Fourth Flight - 12-13-2018

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The latest test flight by Sir Richard Branson's Virgin Galactic successfully rocketed to space and back.
The firm's SpaceShipTwo passenger rocket ship reached a height of 82.7km, beyond the altitude at which space is said to begin.
It marked the plane's fourth test flight and followed earlier setbacks in the firm's space programme.

https://www.bbc.co.uk/news/business-46550862
 

GLS

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Very simple anouncement...
[ame="https://twitter.com/virgingalactic/status/1073246723114381312"]https://twitter.com/virgingalactic/status/1073246723114381312[/ame]

Best response IMO (even with the typo):
[ame="https://twitter.com/Xene1042/status/1073248184191909888"]https://twitter.com/Xene1042/status/1073248184191909888[/ame]
 

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IronRain

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--- NOTE ---
The previous thread these posts were posted to was the LauncherOne thread. I've moved the SpaceShipTwo posts to a separate thread.

Also, congrats VG! \o/
 
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jedidia

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:rofl: I even think 100km is already short, how long can you keep yourself in orbit at that altitude without assistance?


Anyway I would stick with FAI definition rather than USAF
 

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:rofl: I even think 100km is already short, how long can you keep yourself in orbit at that altitude without assistance?

I believe 1 orbit, but I can imagine that depends on many factors.

Another nice article: https://www.livescience.com/63166-outer-space-border-karman-line.html

About 50 of these satellites, however, stood out. While re-entering the atmosphere at the end of their missions, each of these satellites successfully completed at least two full rotations around the Earth at altitudes below 62 miles (100 km). The Soviet Elektron-4 satellite, for example, circled the planet 10 times at around 52 miles (85 km) before tumbling into the atmosphere and burning up in 1997.
 
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kuddel

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:rofl: I even think 100km is already short, how long can you keep yourself in orbit at that altitude without assistance?

It also depends on your type of orbit. A highly elliptic orbit with only the perigee as low as 85 km can be circled more than once ...

I personally prefer the 100 km, but only because it's a nice value and easier to remember.

Also on this topic:
 

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Is it even with 2 pilots and no passengers it couldn't make the 100 km, or was the engine not operating at maximum thrust or burn time?

I've long been an advocate of altitude compensating nozzles, such as the aerospike. At 30,000 feet the air density is still a third that at sea level. But at 100,000 feet it's only 100th that at sea level, which is near vacuum as far as nozzle efficiency is concerned.

This means for most of the flight to 300,000 feet it would be operating at near vacuum conditions. An altitude compensating nozzle would allow you to achieve that maximum efficiency for most of the flight.

Bob Clark
 

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This means for most of the flight to 300,000 feet it would be operating at near vacuum conditions.
I don't know what Virgin does, but Blue Origin shuts the engine down at about 180kft, so they don't spend much time burning in a vacuum. Still your point is totally valid
 

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An altitude compensating nozzle would set them back years in order to rework their ship(s), they'd need to redesign the craft in order to hold the fuels.


From what I'd read, the engine was not planned to operate at it's max burn time. This was an up/down test flight.
 
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MaverickSawyer

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Besides, they're launching from, what, 45,000 feet? That's pretty close to vacuum, compared to sea level... only 2-3 psi, if memory serves, versus 14.7 at sea level. So they can run an altitude-optimized nozzle from the outset.

Blue Origin is launching from much closer to sea level, so they use a sea level nozzle. Yes, there's some efficiency losses... But it's mechanically simpler and much better understood than an aerospike or other altitude-optimized nozzle.

Just because it's more efficient, doesn't necessarily mean that it's the ultimate, "everyone should strive for this" technology. ;)
 

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An altitude compensating nozzle would set them back years in order to rework their ship(s), they'd need to redesign the craft in order to hold the fuels.
From what I'd read, the engine was not planned to operate at it's max burn time. This was an up/down test flight.

Not really. The aerospike has been known about for 50 years. But all orbital vehicles are multistage and its improvement in payload is not as important in that case since the upper stage(s) already have vacuum optimized nozzles.

Bob Clark
 

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I understand the aerospike has been around for a while. but...

IIRC, their current engine is a drop-n-swap (slide old engine out, slide new engine in). To change all that around to put a linear aerospike, or a conical one, there would be a need to change the structure to hold the new tanks, plus a new thrust structure. All this would need extensive testing before the public could hitch a ride to "space".

Plus there'd be a need for increased manpower and support equipment.

I would think it'd be cheaper to clean sheet a new craft than retrofit an existing one.
 

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I understand the aerospike has been around for a while. but...

IIRC, their current engine is a drop-n-swap (slide old engine out, slide new engine in). To change all that around to put a linear aerospike, or a conical one, there would be a need to change the structure to hold the new tanks, plus a new thrust structure. All this would need extensive testing before the public could hitch a ride to "space".
Plus there'd be a need for increased manpower and support equipment.
I would think it'd be cheaper to clean sheet a new craft than retrofit an existing one.

You're right. To change from the usual cylindrically shaped combustion chamber to a toroidal shaped one for the aerospike would be an expensive retrofit.

What might work instead would be an deployable nozzle extension, such as on the RL10-B2 engine:

rl10b-2.jpg


You couldn't have it be fully extended from the beginning because of the large air drag. But it could be deployed at high altitude when the air density is much reduced.

Bob Clark
 

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Again, SS2 is already vacuum optimized, so a nozzle extension wouldn't produce noticeable performance gains... especially given the already low specific impulse of the hybrid motor, the increased mechanical complexity, and the fact that it would likely disrupt the entry aerodynamics.

Honest opinion: the choice to use a hybrid motor instead of a liquid fuel engine is going to be the Achilles heel of SS2, if it isn't already.
 

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Again, SS2 is already vacuum optimized, so a nozzle extension wouldn't produce noticeable performance gains... especially given the already low specific impulse of the hybrid motor, the increased mechanical complexity, and the fact that it would likely disrupt the entry aerodynamics.

Honest opinion: the choice to use a hybrid motor instead of a liquid fuel engine is going to be the Achilles heel of SS2, if it isn't already.

Agree with you they should have used a liquid fuel engine from the beginning.

I was trying to find engine specs on SpaceShipTwo but they weren't available. I was only able to find them on SpaceShipOne here:

SpaceShipOne.
Propulsion was by a SpaceDev hybrid rocket engine, selected after competitive testing against an eEC design. The enormous liquid nitrous oxide tank dominated the fuselage interior. The HTPB fuel burned in the presence of the nitrous oxide to produce a chamber pressure of 37 atm, expanded through a 25:1 nozzle. Thrust loads were transmitted from the fuel casing, through the nitrous oxide tank, and then to the spacecraft through rubber that bonded the tank to the fuselage.
http://www.astronautix.com/t/tierone.html


I assume the SS2 engine specs will be analogous. Virgin Galactic was going to switch the fuel for SS2 to a type of nylon for better performance, but decided to switch back to the same rubber-based fuel of SS1.

Then a 25:1 expansion ratio is larger than that common for a sea level engine but is not that of an optimized vacuum engine. For instance the Merlin 1D sea level engine has a 16:1 expansion ratio, and for the Merlin Vacuum it's at 165:1.

A 25:1 ratio for the SS2 engine is rather at an intermediate level, appropriate for the 45,000 feet ignition point.

That Astronautix page gives the SS1 engine Isp as 250s. For whatever reason, though this was sufficient for SS1 to reach 100 km, it's not for SS2.

Hybrid engines do not scale up easily, which Virgin Galactic discovered much to their chagrin. So perhaps the SS2 engine simply can't reach the 250s performance level. Or it might simply be that having a larger passenger cabin on SS2, instead of just a pilot section, increased the dry weight and therefore lowered the mass ratio.

In any case I used the Rocket Propulsion Analysis(RPA) program to estimate the vacuum Isp for a vacuum optimized nozzle for the N2O/HTPB propellant combination. I set the vacuum expansion ratio at the 280:1 level of the RL10-B2. Then I got a 318.5s Isp at optimum expansion.

SS1 at a 3,600 kg gross weight and 1,200 kg dry weight has a mass ratio of 3 to 1. Then at its 250s Isp it had an ideal velocity, i.e., the velocity without gravity loss, air drag loss, and air pressure loss on Isp, of 250*9.81Ln(3) = 2,690 m/s.

But at 318.52s Isp, the ideal velocity would be 318.52*9.81Ln(3) = 3,430 m/s. That 740 m/s increase in the ideal velocity would translate to a significant increase in altitude.

Bob Clark
 
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