Big Falcon Hopper - Prototyp in Boca Chica

Das ist eine Dual Bell Düse, wurde bereits früher im Thema disktutiert hier.

Bei einigen Bildern aus dem Internet sollte man wohl auch noch vorsichtig sein. Bei NFS und in den sozialen Netzwerken basht man sich gerade ein wenig mit Vorwürfen wer nun korrekte Infos und wer Fakes verbreitet. Werden wohl heute einige da hin um Bestätigung zu bekommen. Dürfte somit bald neue Bilder/Quellen geben.

PS:
Zumindest für die Düsen findet man auch Videomaterial (ca. bei 1:10):

Warten wir mal ab was es heute so für Infos gibt.

In a rocket nozzle, the pressure of the exhaust drops as it flows from the throat to the nozzle exit. There is high pressure in the chamber to push everything out and then by the time it gets to the exit the pressure is low.

If the pressure of the exhaust at the exit is the same as the ambient pressure, the exhaust coming out of the nozzle will form an orderly, compact, and narrow plume as it moves away from the rocket, because the pressure of the exhaust gases will balance exactly with the pressure of the ambient environment. This balance is called the design condition of the rocket engine and it is the most efficient operating condition for the nozzle. This image shows a Soyuz rocket with its engines firing near the design condition - the exhaust plume under each engine is a compact column of flame travelling straight down and away from the rocket.

If the pressure inside the exhaust plume is greater than the ambient pressure, the plume will expand rapidly in all directions after it exits the nozzle until it reaches an equilibrium pressure with the environment. This condition is called underexpansion or underexpanded flow, because it is created by a nozzle expansion ratio that is too low. Underexpansion can be corrected by re-engineering the engine with a larger nozzle exit to increase the expansion ratio, lowering the exhaust pressure at the exit to more closely match the ambient pressure. An engine that is underexpanded sacrifices performance - further expansion inside the nozzle to reduce exhaust pressure increases the velocity of the exhaust and thus the thrust and efficiency of the engine.

If the exit pressure is below ambient pressure, the exhaust plume will be pressed in on itself by the imbalance. Instead of leaving the nozzle and forming a smooth column, the exhaust will collapse inward and bounce against itself, forming a series of standing shockwaves inside the plume called shock diamonds or sometimes mach diamonds. These shockwaves give the plume a knotted or braided appearance and are caused by overexpansion of the flow inside the nozzle. The remedy is to engineer a smaller nozzle, reducing expansion ratio and thus increasing the exit pressure to more closely align with the ambient pressure. An overexpanded plume with the characteristic braided appearance can be seen in this image.

In all of the cases so far, the exhaust flow follows the contours of the nozzle smoothly from the combustion chamber to the exit, and only separates from the nozzle as it exits. However, in a special case of overexpansion, when the exhaust pressure at the nozzle exit is much lower than the ambient pressure, the air from the environment outside the nozzle can impinge on the flow exiting the nozzle and can creep up the inside of the nozzle wall, coming between the wall and the exhaust flow and causing the flow to separate from the nozzle well before the nozzle exit. At the point where the flow separates from the nozzle, a powerful shock wave is formed which can damage or destroy the nozzle. In addition to the danger, this flow separation drastically reduces the efficiency of the engine. For these reasons, no rocket engines are designed to operate in a state of flow separation. Here is an image showing design condition, underexpansion, overexpansion, and flow separation.

Some minor and transient flow separation is common during the ignition sequence, as shown in this image of the space shuttle main engines’ ignition. The shockwave is visible as a jagged and broken line inside the engine nozzles, with the nozzle on the left side of the image experiencing greater flow separation due to staggered timing of the ignition of each engine.

The characteristics of underexpansion, overexpansion, and flow separation have several consequences that must be considered. First, the risk associated with flow separation is one of the things that forms a limit ability of a rocket to throttle down. Thrust in a rocket engine is reduced by lowering the pressure inside the combustion chamber. Because the drop in pressure between the chamber and the nozzle exit is determined wholly by the nozzle geometry, a reduction in the chamber pressure of a given engine will always correspond with a reduction in exit pressure. While the combustion chamber might be capable of operating at a very low pressure, the downstream effect in the nozzle must be considered, and the threat of flow separation limits how low the pressure at the nozzle can go.

Second, flow through a rocket engine nozzle with a design condition for sea level flight will become increasingly underexpanded as the rocket flies toward space and the ambient pressure falls. If the ambient pressure when the rocket is on the ground matches the nozzle exit pressure, then the ambient pressure at altitude where the air is thinner will be much lower than the nozzle exit pressure, because the nozzle exit pressure does not change in flight. This change from design condition to severe underexpansion is visible during many launches, and very visible on F9 launches. If you watch any launch when the rocket is at sea level, it will at launch produce a narrow and compact column of flame. As the rocket goes up into thin air, the exhaust plume begins to expand outward after it leaves the nozzle, becoming wider and more diffuse - the hallmarks of underexpansion. The expansion of the plume at altitude represents a lost opportunity for greater thrust and efficiency through the use of a much larger expansion ratio in the nozzle to reduce exit pressure.

Third, an engine designed with a very large expansion ratio for use in a vacuum will experience destructive flow separation if operated at sea level. The very thing that makes it ideal for vacuum use - a highly expanded flow and thus very low exit pressure - will lead to collapse of the plume, flow separation, and destruction of the nozzle in the high pressure air on the ground.

Engines designed for use at low altitudes are less efficient at higher altitudes, and engines designed for use at high altitude or in a vacuum cannot be used at low altitude. This is one reason among many why rockets are designed to operate in stages, with the lower stage, often called the boost stage, or simply the booster, using engines designed for operation in the dense atmosphere, and an upper stage using engines designed for operation in the wispy upper atmosphere and the vacuum of space. The boost stage engines operate at a lower expansion ratio, and are used to lift the upper stage out of the dense atmosphere and to high altitude. Once this is achieved, the boost stage is separated and the upper stage continues onward toward orbit using an engine with a high expansion ratio. This type of engine is often called a vacuum engine. The optimum nozzle design for an engine operating in a vacuum is one with an infinite expansion ratio which can bring the exhaust pressure down to zero, matching the ambient pressure. Any smaller and the nozzle suffers from underexpansion. Of course an infinitely large nozzle is not possible, so vacuum engines have to settle for nozzles of a large, but finite size. Typically the nozzle on a vacuum engine is several times the diameter of an equivalent sea level engine.

The Rutherford engine is used on both the boost stage and upper stage of the Electron rocket. As shown here the nozzle on the sea-level engine is so compact that the entire engine can easily be held by a single person. Nine Rutherford engines can fit on the base of the 1.2 meter-wide rocket. Despite an identically-sized combustion chamber and throat, the vacuum variant has a nozzle that is so large that it fills nearly the whole diameter and only a single one can fit on the upper stage. And even this is a compromise; a larger nozzle would be better, but would not fit in the adapter between the two stages of the rocket.

The allure of an engine that has the efficiency of a high expansion ratio combined with the ability to operate at sea level has led to several proposals for what are called altitude compensating nozzles. The most conceptually straightforward is the expandable nozzle - a sea-level nozzle with a large extension that can be pulled up and out of the flow for operation at sea level and then extended into the flow for vacuum use. The concept is shown here. No expandable nozzles have been put into operation for altitude compensation purposes, but some vacuum engines such as the RL-10b-2 use an expandable nozzle to save room inside the rocket before stage separation. The nozzle extension tucks up and away and is only deployed just before the engine ignites at altitude.

Dual-bell nozzles are designed with an abrupt change in the nozzle curvature around the place in the nozzle where the expansion ratio is appropriate for sea level operation. The sharp edge at this place in the nozzle induces predictable and controlled flow separation in sea-level flight. As the rocket climbs, the changing ambient pressure allows the exhaust flow to expand further through the nozzle until the full expansion ratio is achieved. The dual-bell concept is illustrated here.

You can see that characteristic kink in the image at the top of the thread. Here is a crop showing the kink in the nozzle at the point where they want the flow to separate.

Ich frag mich, ob sie die Beine/Rohre noch durch Verkleidung zu den Flügeln umwandeln.

Elon hat das Design bei Hergé gestohlen!!!


Also sowas!

"I love the Tintin rocket design, so I kind of wanted to bias it towards that," Musk added. "If in doubt, go with Tintin."

Die Dual Bell Düse wird dann ja wohl nur im Schiff verbaut oder würde es auch beim Booster Sinn machen?

Die Dual Bell Düse wird dann ja wohl nur im Schiff verbaut oder würde es auch beim Booster Sinn machen?

Ganz interessant, die Falcon 9 im Größenvergleich...
(Pic von der Facebook Fan Gruppe)

Ich habe die Vermutung dass die Dual Bell besonders beim Drosseln der Triebwerke hilft. Demnach kommt die auf SS und SH zum Einsatz.

Der Booster würde von Dual Bell auch profitieren, da der Luftdruck mit der Höhe schnell abfällt.

Zitat von: Steppenwolf am 02. Januar 2019, 15:27:04

Ich habe die Vermutung dass die Dual Bell besonders beim Drosseln der Triebwerke hilft. Demnach kommt die auf SS und SH zum Einsatz.

Hab es eher so verstanden, daß Dual Bell Triebwerke sowohl im Sea-Level- als auch im Vacuum-Einsatz recht optimal laufen:

Hier sieht man nochmal wie "grob" der Aluhut gearbeitet ist.


Quelle: Facebook SpaceX Fan Seite

Hier sieht man nochmal wie "grob" der Aluhut gearbeitet ist.


Quelle: Facebook SpaceX Fan Seite

Vor einiger Zeit hieß es dann, die Vacuum-Triebwerke werden am Anfang erstmal ganz weggelassen.

Jetzt scheint man mit den Dual Bell Triebwerken eine interessante Lösung gefunden zu haben.