Author Topic: What do balloons enable?  (Read 1840 times)

Offline speedevil

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What do balloons enable?
« on: 04/19/2018 03:53 AM »
If F9S2 is recovered with a balloon (ballute, ...) there seem some logical additional capabilities it may give, with slight to larger modifications.

First thought is recovery of very high energy S1s.

Recovery of high energy stage 1s with less fuel use seems plausible.
FH needs enough fuel reserved for full reuse to drop the payload from 63 tons to around 20. Assuming a linear mass scaling of the ballute, and assuming the much lower temperature of entry makes it easier, another 30+ tons with no reuse would be a very useful additional capability. Landing error is a real problem of course, and this may mean the ballute needs cut away earlier to allow for propulsive corrections than optimal.
Needs to cope with 10* the mass at 1/3 the speed. Steeper trajectory may mean the ballute needs to collapse in volume to maintain safe decelleration.

If the ballutes turned out not to be an effective way of reducing entry burns, they could dramatically increase payload to higher energy missions, especially for FH, as well as possibly allow some recoveries that aren't now possible.

Modulo concerns about accuracy of course.

Further thoughts on Mars in the party balloons on Mars thread.

Offline yokem55

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Re: What do balloons enable?
« Reply #1 on: 04/19/2018 04:44 AM »
Stage 1's don't spend enough time in the upper atmosphere to get much benefit from a ballute. They are coming back down into the denser atmosphere before the ballute could slow it down much.

Online Robotbeat

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Re: What do balloons enable?
« Reply #2 on: 04/19/2018 04:48 AM »
Stage 1's don't spend enough time in the upper atmosphere to get much benefit from a ballute. They are coming back down into the denser atmosphere before the ballute could slow it down much.
They would if you gave them a ballistic coefficient 100 times as big. It's not the time, it's the mass of the air column through which you're flying. If the mass of that air column is about the same as the mass of the vehicle or more, the vehicle will slow down a lot. Increasing the ballistic coefficient increases the cross section and the mass of the air column.
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Offline speedevil

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Re: What do balloons enable?
« Reply #3 on: 04/19/2018 04:51 AM »
Stage 1's don't spend enough time in the upper atmosphere to get much benefit from a ballute. They are coming back down into the denser atmosphere before the ballute could slow it down much.

I haven't done the calculations as I should have.
I would guess that you'd want to modify the trajectory to be rather closer to the top of the atmosphere and more horizontal at deploy, taking some extra gravity losses in S1 and S2.
Deceleration may end up too high to save much.

Online Robotbeat

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Re: What do balloons enable?
« Reply #4 on: 04/19/2018 04:58 AM »
A stage with a ballute has a ballistic coefficient of like 5kg/m^2. But the air column at upper atmosphere, i.e. Mars equivalent atmosphere height is 100kg/m^2. So you'll slow down a LOT with a ballute. This is particularly true since you're coming in at an angle, and even more true if you can generate any lift.
« Last Edit: 04/19/2018 04:59 AM by Robotbeat »
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Offline TripD

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Re: What do balloons enable?
« Reply #5 on: 04/19/2018 06:23 AM »
Will you still be able to fine tune trajectory while a ballute is in use?  Or in a passive sense, how much change in trajectory can be expected?

Offline speedevil

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Re: What do balloons enable?
« Reply #6 on: 04/19/2018 07:07 AM »
Will you still be able to fine tune trajectory while a ballute is in use?  Or in a passive sense, how much change in trajectory can be expected?
That rather depends on the design.
There are exotic ones involving weight shift or ... and lifting trajectories.

Naively, if you drop by around 5km, drag doubles.
If you have a stage descending vertically at 1km/s, over 15s, drag rises from 1G to 8G, causing perhaps 400m/s of total deceleration.
If you do the same with a stage with a hundred times the area, or a ballute, this deceleration is basically the same, but if you can vary the area, perhaps by deflation, or more simply cutting ballutes free, you can extend this and maintain deceleration at (say) 4g for not 15 vertical kilometers as the previous example, but 35.

This greatly reduces the heat flux and stress for a given entry.

The big question is how much unpredictable crossrange does the ballute generate.
To a large degree you can compensate for atmospheric variations, by aiming to fall short while under the ballute, and cutting it away at the appropriate time.

This assumes you're not using one of the interesting potential variants that uses the ballute as a hot air balloon, in lieu of parachutes.

Online Robotbeat

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Re: What do balloons enable?
« Reply #7 on: 04/19/2018 01:03 PM »
With a towed ballute, you canít really steer or generate lift. With something like HIAD, you can.
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Offline niwax

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Re: What do balloons enable?
« Reply #8 on: 04/19/2018 03:01 PM »
Someone also proposed sending up a weather balloon to get accurate information on wind patterns, so you can correct using the landing ship instead of the craft.

Offline speedevil

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Re: What do balloons enable?
« Reply #9 on: 04/19/2018 03:41 PM »
Someone also proposed sending up a weather balloon to get accurate information on wind patterns, so you can correct using the landing ship instead of the craft.

Balloons stop at 40km or so, and take an hour or so to get there, making it at least somewhat less useful.

Offline Asteroza

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Re: What do balloons enable?
« Reply #10 on: 04/20/2018 06:35 AM »
Uh, not so obvious is, is this a top first or bottom first HIAD style ballute, of something like a shuttlecock with the engines first?


From a weight perspective being engine down is easier.

Shuttlecock style with the ballute cone poking out of the top in a trailing configuration means your engine is fully exposed though. Maybe release ullage gases to make a buffer gas zone beyond the nozzle during reentry to help?

If a full cone cover, you have either a top deploy or bottom deploy

1. Bottom deploy, you would need to swing down over the engine your core reentry shield and inflation cone, which is a big engineering change.

2. Top deploy you might be able to have a core shield and inflation cone stored under the payload adapter, blow the adapter off and inflate, but all your weight is away from the core shield so you might have very unfavorable center of mass/center of pressure mismatch. But, it might be an easier engineering change in terms of installation.

Offline speedevil

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Re: What do balloons enable?
« Reply #11 on: 04/20/2018 07:09 AM »
Shuttlecock style with the ballute cone poking out of the top in a trailing configuration means your engine is fully exposed though.

I have no info, but a trailing ballute at least seems plausible.
Basically stage first, with ballute on a rope.
Yes, the stage is directly exposed to reentry heat, but the total amount of heat it is exposed to is 1% or so of the total over the whole object, meaning very simple ablatives may be able to cope.
Actively cooling the nozzle - at least the regeneratively cooled bit may be all that is required - the niobium extension may be just fine with no cooling at the heat fluxes involved.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20030106138.pdf
« Last Edit: 04/20/2018 12:54 PM by speedevil »

Online Robotbeat

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Re: What do balloons enable?
« Reply #12 on: 04/20/2018 01:30 PM »
As I said, towed ballute.


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Offline envy887

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Re: What do balloons enable?
« Reply #13 on: 04/20/2018 02:32 PM »
Uh, not so obvious is, is this a top first or bottom first HIAD style ballute, of something like a shuttlecock with the engines first?


From a weight perspective being engine down is easier.

Shuttlecock style with the ballute cone poking out of the top in a trailing configuration means your engine is fully exposed though.
...

The towed ballute can be "poking out" any side you want. It does not matter that the stage is rear-heavy - the ballute will move the center of pressure of the whole assembly FAR behind the engine. If attached to the stage near it's perimeter, the tow rope will easily clear the MVac bell (the stage will actually pitch to move the center of mass below the attachment point, moving the bell further away from the rope).

A 100x increase in BC means a ballute about 30 m in diameter. A 30 m spherical Kapton balloon of 4 mil thickness would mass 322 kg and could hold 26 kg of He at 25 C and 1 kPa (about the air pressure at 30 km). Kapton is usable up to about 400 C. The whole package would be about the size of 2 Falcon 9 COPV bottles at launch assuming the He is 25 C and 375 bar. The ballute would start to crumple from air pressure after passing below 30 km altitude, but at that point it would be subsonic. Maybe they could pump air into it to keep it inflated.

At least, that's what I get at a first run through the numbers.


Offline speedevil

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Re: What do balloons enable?
« Reply #14 on: 04/20/2018 03:36 PM »
A 100x increase in BC means a ballute about 30 m in diameter. A 30 m spherical Kapton balloon of 4 mil thickness would mass 322 kg and could hold 26 kg of He at 25 C and 1 kPa (about the air pressure at 30 km). Kapton is usable up to about 400 C.

Or more like 50m^2 if considering it going in 'belly first'.
I do wonder when we'll first see hardware.

Online docmordrid

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Re: What do balloons enable?
« Reply #15 on: 04/20/2018 05:38 PM »
A 100x increase in BC means a ballute about 30 m in diameter. A 30 m spherical Kapton balloon of 4 mil thickness would mass 322 kg and could hold 26 kg of He at 25 C and 1 kPa (about the air pressure at 30 km). Kapton is usable up to about 400 C.

Or more like 50m^2 if considering it going in 'belly first'.
I do wonder when we'll first see hardware.

If coming in belly first, why not face the incoming side with BFS thermal protection (and winglets?) allowing them to test those? Use a small ballute(s) to keep it oriented.
« Last Edit: 04/20/2018 05:39 PM by docmordrid »
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Offline kraisee

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Re: What do balloons enable?
« Reply #16 on: 04/20/2018 05:48 PM »
A 100x increase in BC means a ballute about 30 m in diameter. A 30 m spherical Kapton balloon of 4 mil thickness would mass 322 kg and could hold 26 kg of He at 25 C and 1 kPa (about the air pressure at 30 km). Kapton is usable up to about 400 C.

Or more like 50m^2 if considering it going in 'belly first'.
I do wonder when we'll first see hardware.

If coming in belly first, why not face the incoming side with BFS thermal protection (and winglets?) allowing them to test those? Use a small ballute(s) to keep it oriented.

I have to agree. They have a perfect 'test bed' already flying, and more importantly they have established a group of customers who seem willing to fly their payloads on vehicles where SpaceX are clearly testing new things.

A mini-BFS on F9 seems like a great way to reduce risks for the full size variant to follow (I'd develop a variant without the fairing though). They could use it to test cryo composite tanks, Raptors, legs, side-entry heatshields, winglets etc. And it would make F9 fully reusable.

I'm sure they've already considered this though, so I guess they found some reason not to.   Probably financial.

Ross.
« Last Edit: 04/20/2018 05:53 PM by kraisee »
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