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NASA NTRS1988

The central engine of quasars and AGN's - Shock-accelerated relativistic protons

A simple dynamical model is presented in which protons are Fermi-shock-accelerated to relativistic energies and do not readily fall into the black hole. It is shown that, even though the nonthermal radiation from QSOs and AGNs is generally well below the Eddington limit, it is still highly correlated and proportional to the mass. In the present model, the proportionality constant is determined by the p-p cross-section and the minimum shock radius (in units of the Schwarzschild radius) where a stable shock is assumed to exist. The model predicts a rapid drop off in efficiency as this shock radius increases.

Ellison, Donald C.↗

NASA NTRS1992

User's Manual for Aerofcn: a FORTRAN Program to Compute Aerodynamic Parameters

The computer program AeroFcn is discussed. AeroFcn is a utility program that computes the following aerodynamic parameters: geopotential altitude, Mach number, true velocity, dynamic pressure, calibrated airspeed, equivalent airspeed, impact pressure, total pressure, total temperature, Reynolds number, speed of sound, static density, static pressure, static temperature, coefficient of dynamic viscosity, kinematic viscosity, geometric altitude, and specific energy for a standard- or a modified standard-day atmosphere using compressible flow and normal shock relations. Any two parameters that define a unique flight condition are selected, and their values are entered interactively. The remaining parameters are computed, and the solutions are stored in an output file. Multiple cases can be run, and the multiple case solutions can be stored in another output file for plotting. Parameter units, the output format, and primary constants in the atmospheric and aerodynamic equations can also be changed.

Conley, Joseph L.↗

NASA NTRS1999

Strain Rate Dependent Deformation and Strength Modeling of a Polymer Matrix Composite Utilizing a Micromechanics Approach

Potential gas turbine applications will expose polymer matrix composites to very high strain rate loading conditions, requiring an ability to understand and predict the material behavior under extreme conditions. Specifically, analytical methods designed for these applications must have the capability of properly capturing the strain rate sensitivities and nonlinearities that are present in the material response. The Ramaswamy-Stouffer constitutive equations, originally developed to analyze the viscoplastic deformation of metals, have been modified to simulate the nonlinear deformation response of ductile, crystalline polymers. The constitutive model is characterized and correlated for two representative ductile polymers. Fiberite 977-2 and PEEK, and the computed results correlate well with experimental values. The polymer constitutive equations are implemented in a mechanics of materials based composite micromechanics model to predict the nonlinear, rate dependent deformation response of a composite ply. Uniform stress and uniform strain assumptions are applied to compute the effective stresses of a composite unit cell from the applied strains. The micromechanics equations are successfully verified for two polymer matrix composites. IM7/977-2 and AS4/PEEK. The ultimate strength of a composite ply is predicted with the Hashin failure criteria that were implemented in the composite micromechanics model. The failure stresses of the two composite material systems are accurately predicted for a variety of fiber orientations and strain rates. The composite deformation model is implemented in LS-DYNA, a commercially available transient dynamic explicit finite element code. The matrix constitutive equations are converted into an incremental form, and the model is implemented into LS-DYNA through the use of a user defined material subroutine. The deformation response of a bulk polymer and a polymer matrix composite are predicted by finite element analyses. The results compare reasonably well to experimental values, with some discrepancies. The discrepancies are at least partially caused by the method used to integrate the rate equations in the polymer constitutive model.

Goldberg, Robert K.↗

NASA NTRS2006

Mars Aerobraking Spacecraft State Estimation by Processing Inertial Measurement Unit Data

Aerobraking is an efficient technique for orbit adjustment of planetary spacecraft, such as Magellan (Venus), Mars Global Surveyor, and Mars Odyssey. Determination of the vehicle state during the aerobraking phase has conventionally been performed using only radiometric tracking data prior to and following the atmospheric drag pass. This approach is sufficiently accurate and timely to meet current mission operational requirements; however, it is expensive in terms of ground support and leads to delayed results because ofthe need for post-drag pass data. This research presents a new approach to estimation of the vehicle state during the atmospheric pass that sequentially incorporates observations from an Inertial Measurement Unit (IMU) and models of the vehicle and environment. The approach, called Inertial Measurements for Aerobraking Navigation (IMAN), is shown to perform at a level comparable to the conventional methods in terms of navigation accuracy and superior to them in terms of availability of the results immediately after completion ofthe pass. Furthermore, the research shows that IMAN can be used to reliably predict subsequent periapsis times and locations over all aerobraking regimes. IMAN also yields accurate peak dynamic pressure and heating rates, critical for a successful corridor control strategy, comparable to navigation team reconstructed values. This research also provides the first instance of the utilization of the Unscented Kalman Filter for the purpose of estimating an actual spacecraft trajectory arc about another planet.

Unscented Kalman Filter↗

NASA NTRS1975

Recent developments in multiplane-multispeed balancing of flexible rotors in the United States

This paper describes current developments in the evolution of a computer-implemented balancing procedure which permits flexible rotors to be precisely balanced in a cost-effective manner. Corrections in virtually any reasonable number of planes, computed by the procedure using signals from vibration sensors at critical locations, permit rotor operation over any design speed range. Steady-state operation at undamped critical speeds has been demonstrated. Results of recent test efforts indicate that the procedure can be applied with equal effectiveness to rotors of any size. Manufacturing and overhaul cost reductions are expected to flow from its adoption, together with performance advantages from operation in hitherto restricted dynamic regimes.-

Badgley, R. H.↗

NASA NTRSDate not supplied

Design and Technology Maturation of the Stratospheric Projectile Experiment of Entry Dynamics

The supersonic and transonic dynamic stability of blunt-body reentry vehicles currently poses large risks in all of NASA’s ongoing entry missions (MSR SRL, MSR EES, and Dragonfly). These projects have allocated millions of dollars to testing and modeling efforts to buy down risk by using the current state-of-the-art (SoA) facilities at NASA’s disposal. While these facilities have heritage in supplying dynamics data to reentry missions, their availability is severely limited – particularly with the high number of concur-rent projects requesting simultaneous testing– and are costly when considering the science density per dollar. None of the current SoA facility methodologies allow the test model to have the dynamics fully develop through a flight relevant free-stream profile and as such require extrapolations with resultant high uncertainties in order to relate the test dynamics to flight expectations. SPEED is a NASA Ames Center Innovation Fund (CIF) project that is developing a highly tailorable and cost-effective test methodology to better assess the dynamic stability of blunt-body reentry vehicles via a stratospheric balloon flight. This is accomplished by dropping a suite of instrumented capsules from a stratospheric balloon to gain a statistically relevant dataset of scaled reentry vehicles in mission relevant free-flight conditions. This presentation will walk through how the test methodology is being implemented specifically for the Mars Sample Return (MSR) Earth Entry System (EES) geometry in an awarded Flight Opportunities Program (FOP) test flight in early CY24. SPEED Application to MSR: SPEED consists of three main mechanical systems: the Drop Platform, the Projectile, and the test Capsule. SPEED is being developed as a set of guidelines and recommendations for how to test with the proposed Concept of Operations (Conops) since the specific design parameters will vary depending on the specific project’s reference trajectory and entry vehicle design. As such, this presentation will walk through the development time-line as shown in Fig. 2. This is meant to serve as a blueprint for further missions as desired. Mechanical and Avionics Design. The SPEED test platform designed for the MSR-EES capsule geometry with nominal entry parameters has the ability to carry 10 Capsules to altitude instrumented with: 1. 3-Axis Accelerometer 2. IMU 3. Gyroscope 4. Magnetometer 5. Pressure Transducer cruciform 6. Uplook and Horizon Cameras To package the avionics/instrumentation suite, the capsule is approximately 1’ in diameter with the Outer Mold Line (OML) centroid-scaled from the full EES design. The internal volume is gutted and custom-shaped to fit the desired instrumentation suite, as well as to allow for the positioning of ballast mass such that the Center of Gravity is analogous to the flight vehicle. All structural components in the Capsule and Projectile are 3D printed, which significantly reduces the cost of each flight unit to around $1500 including all instrumentation, avionics, and structural components. Flight Conops. The test Capsule is accelerated to the desired altitude and Mach number while stowed in the Projectile, a missile-like vehicle consisting of steel ballast in the nose, a low-drag OML, and an Ejection Mechanism to reliably release the Capsule into the free-flow supersonic conditions. For the MSR-EES design, the capsule employs ~3kg of ballast mass at the nose to accelerate the 1.25kg test Capsule to ~Mach 1.7 at 23km altitude. This requires an initial release altitude of 40km, the quoted limit of a 80kg payload by the FOP-contracted balloon provider. Once the Ejection Mechanism avionics detect the proper conditions, the spring-loaded Ejection Mechanism will release and – guided by the sabot – expose the test Capsule to the desired test conditions for ~5 seconds of free-flight in the supersonic/transonic regimes. Dynamics in the subsonic regime will also be captured with the instrumentation suite with post-flight recovery operations aimed at recovering the high-G-load capable SD cards after the planned hard impact landings. Testing and Development: In the few months the SPEED project has worked the development of MSR-EES flight test, the team has performed lab and drone based testing which this presentation will overview. After the first design phase, the team fabricated Engineering Demonstration Units (EDUs) of all subsystems to perform validation testing shown in Fig. 5. After validation was completed on the subsystem level, a drone-drop test was performed at the recreational flight ceiling of 400ft altitude to assess the SPEED systems in a flight environment. Parameters such as in-flight stability, hard impact landing performance, and avionics performance were quantified and qualified. The FY23 CIF will culminate in a helicopter drop test aboard an Air National Guard Blackhawk. This will prepare the team for the CY24 FOP stratospheric balloon flight that should provide the final verification to begin offering the test platform for mission support. Focus of Presentation: This presentation will outline the technology maturation path of the SPEED implementation to the MSR-EES capsule baseline as well as the details regarding the mechanical system, avionics and instrumentation, and flight operations. Note that a complementary presentation is being submitted for a methodology overview of the SPEED test platform, introducing the testing technique and benefits as well as the full application space of the technology.

pitch damping coefficient↗

NASA NTRS1990

Corrosion Prevention in Copper Combustion Chamber Liners of Liquid Oxygen/Methane Booster Engines

Prior research conducted by United Technologies Research Center and Rockwell International Rocketdyne Division encountered severe copper corrosion while flowing hydrocarbon fuels through copper cooling tubes. These experimental results have very important implications for the development of low-cost, regeneratively cooled, oxygen/hydrocarbon bipropellant booster engines. Aerojet TechSystems undertook a program sponsored by NASA/Lewis Research Center in 1986 with two objectives: (1) to define the corrosive interaction process that occurs between hydrocarbon fuels and copper combustion chamber liner materials, using both static and dynamic test methods, and (2) to identify and demonstrate protective measures against this corrosive process. The results of the first program task demonstrated that the most damaging corrosive process was caused by trace amounts of sulfur-containing impurities in the hydrocarbon fuel which react with the copper chamber liner material to form cuprous sulfide (Cu2S), as reported in Paper No. AlAA 88-3215. Experiments were conducted which demonstrated this corrosive process with Mil-Spec RP-1, propane, and methane fuels in contact with OFHC, NASA-Z, and ZrCu coppers. The preliminary results of the second program task demonstrated that electrodeposited gold and platinum coatings held promise of preventing corrosion of copper chamber liners by methane contaminated with sulfur-containing impurities, as reported in Paper No. AIAA 89-2738. Experiments were conducted which demonstrated greatly reduced corrosion of copper by hydrogen sulfide (H2S) and methyl mercaptan (CH3SH) as measured impurities in methane. This paper presents the concluding, confirmatory results from the second program task. The efficacy of a protective gold coating on NASA-Z copper cooling channel walls was conclusively demonstrated. Tests were conducted in the Aerojet Carbothermal Test Facility which provide realistic simulations of booster engine cooling channel conditions such as temperature, pressure, flow velocity, and heat flux. Dynamic test series were performed with methane containing 5 and 10 ppm (by volume) hydrogen sulfide. Gold-plated and unplated copper alloy specimens were evaluated. The tests demonstrated that gold coatings were effective in preventing corrosion of the copper. Posttest metallographic examinations of the unplated specimens showed severe corrosion as a result of reaction with the sulfur containing contaminant in the fuel. In contrast, posttest metallographic examination of the gold-coated specimens showed no corrosion under similar operating conditions. The findings from metallurgical and chemical analyses of the copper specimens, chemical analysis of the methane, and description of all test conditions are provided. Conclusions are reached as to the efficacy of the plating methods and the effectiveness of the metal coatings in the prevention of corrosion in hydrocarbon-fueled booster engine combustion chamber liners.

S D Rosenberg↗

NASA NTRS1957

Theoretical and Experimental Investigation of the Subsonic-Flow Fields Beneath Swept and Unswept Wings with Tables or Vortex-induced Velocities

The flow-field characteristics beneath swept and unswept wings as determined by potential-flow theory are compared with the experimentally determined flow fields beneath swept and unswept wing-fuselage combinations. The potential-flow theory utilized considered both spanwise and chordwise distributions of vorticity as well as the wing-thickness effects. The perturbation velocities induced by a unit horseshoe vortex are included in tabular form. The theoretical predictions of the flow-field characteristics were qualitatively correct in all cases considered, although there were indications that the magnitudes of the downwash angles tended to be overpredicted as the tip of the swept wing was approached and that the sidewash angles ahead of the unswept wing were underpredicted. The calculated effects of compressibility indicated that significant increases in the chordwise variation of flow angles and dynamic-pressure ratios should be expected in going from low to high subsonic speeds.

Alford, William J , Jr↗

NASA NTRS2021

On the Settling Depth of Meltwater Escaping From Beneath Antarctic Ice Shelves

Antarctic glacial meltwater is thought to play an important role in determining large-scale Southern Ocean climate trends, yet recent modeling efforts have proceeded without a good understanding of how its vertical distribution in the water column is set. To rectify this, here we conduct new large-eddy simulations of the ascent of a buoyant meltwater plume after its escape from beneath an Antarctic ice shelf. We find that the meltwater’s settling depth is primarily a function of the buoyancy forcing per unit width of the source and the ambient stratification, consistent with the classical theory of turbulent buoyant plumes and in contrast to previous work that suggested an important role for centrifugal instability. Our results further highlight the significant role played by localized variability in stratification; this helps explain observed interannual variability in the vertical meltwater distribution near Pine Island Glacier. Because of the vast heterogeneity in mass loss rates and ambient conditions at different Antarctic ice shelves, a dynamic parameterization of meltwater settling depth may be crucial for accurately simulating high-latitude climate in a warming world; we discuss how this may be developed following this work, and where the remaining challenges lie.

Antarctic glacial meltwater↗

NASA NTRSDate not supplied

Visualization of Freestream Vortices in the NASA Langley 4-Foot Supersonic Unitary Plan Wind Tunnel

Streamwise vortices present in the freestream of the NASA Langley 4-Foot Supersonic Unitary Plan Wind Tunnel have been observed experimentally for the first time. A high-energy, burst-mode laser system was used to conduct time-resolved planar laser Mie scattering in the freestream of this facility. The freestream of the facility was observed over Mach numbers ranging from 2.4 to 4.6 and (unit) Reynolds number between 3.3 × 10 6 to 14.8 × 10 6 1/m. Apparent vortical structures were observed in the tunnel freestream at Mach numbers ≤ 3 with little difference observed for varying Reynolds number. With further-increasing Mach number, protrusive and oscillatory vertical ‘sprite’ structures were observed emanating from the tunnel floor boundary layer. Neither of these structural phenomena have been observed before experimentally in the facility and act to confirm and inform predictions made in computational fluid dynamic simulations of the facility.

Flow visualization↗

NASA NTRSDate not supplied

Visualization of Freestream Vortices in the NASA Langley 4-Foot Supersonic Unitary Plan Wind Tunnel

Flow visualization↗

NASA NTRS2015

Regolith and Environment Science and Oxygen and Lunar Volatile Extraction (RESOLVE): Lunar Advanced Volatile Analysis (LAVA) Capillary Fluid Dynamic Restriction Effects on Gas Chromatography

The Resource Prospector (RP) mission with the Regolith and Environment Science and Oxygen Lunar Volatile Extraction (RESOLVE) payload aims to show the presence of water in lunar regolith, and establish a proving ground for NASAs mission to Mars. One of the analysis is performed by the Lunar Advanced Volatiles Analysis (LAVA) subsystem, which consists of a fluid network that facilitates the transport of volatile samples to a gas chromatograph and mass spectrometer (GC-MS) instrument. The understanding of fluid dynamics directed from the GC to the MS is important due to the influence of flow rates and pressures that affect the accuracy of and prevent the damage to the overall GC-MS instrument. The micro-scale capillary fluid network within the GC alone has various lengths and inner-diameters; therefore, determination of pressure differentials and flow rates are difficult to model computationally, with additional complexity from the vacuum conditions in space and lack of a lunar atmosphere. A series of tests were performed on an experimental set-up of the system where the inner diameters of the GC transfer line connecting to the MS were varied. The effect on chromatography readings were also studied by applying these lines onto a GC instrument. It was found that a smaller inner diameter transfer line resulted in a lower flow rate, as well as a lower pressure differential across the thermal conductivity detector (TCD) unit of the GC and a negligible pressure drop across the mock-up capillary column. The chromatography was affected with longer retention times and broader peak integrations. It was concluded that a 0.050 mm inner diameter line still proved most suitable for the systems flow rate preferences. In addition, it was evident that this small transfer line portrayed some expense to GC signal characteristics and the wait time for steady-state operation.

RESOLVE↗

NASA NTRS1994

STS-62 Space Shuttle mission report

The STS-62 Space Shuttle Program Mission Report summarizes the Payload activities as well as the Orbiter, External Tank (ET), Solid Rocket Booster (SRB), Redesigned Solid Rocket Motor (RSRM), and the Space Shuttle main engine (SSHE) systems performance during the sixty-first flight of the Space Shuttle Program and sixteenth flight of the Orbiter vehicle Columbia (OV-102). In addition to the Orbiter, the flight vehicle consisted of an ET designated as ET-62; three SSME's which were designated as serial numbers 2031, 2109, and 2029 in positions 1, 2, and 3, respectively; and two SRB's which were designated BI-064. The RSRM's that were installed in each SRB were designated as 360L036A (lightweight) for the left SRB, and 36OWO36B (welterweight) for the right SRB. This STS-62 Space Shuttle Program Mission Report fulfills the Space Shuttle Program requirement as documented in NSTS 07700, Volume 8, Appendix E. That document requires that each major organizational element supporting the Program report the results of its hardware evaluation and mission performance plus identify all related in-flight anomalies. The primary objectives of the STS-62 mission were to perform the operations of the United States Microgravity Payload-2 (USMP-2) and the Office of Aeronautics and Space Technology-2 (OAST-2) payload. The secondary objectives of this flight were to perform the operations of the Dexterous End Effector (DEE), the Shuttle Solar Backscatter Ultraviolet/A (SSBUV/A), the Limited Duration Space Environment Candidate Material Exposure (LDCE), the Advanced Protein Crystal Growth (APCG), the Physiological Systems Experiments (PSE), the Commercial Protein Crystal Growth (CPCG), the Commercial Generic Bioprocessing Apparatus (CGBA), the Middeck Zero-Gravity Dynamics Experiment (MODE), the Bioreactor Demonstration System (BDS), the Air Force Maui Optical Site Calibration Test (AMOS), and the Auroral Photography Experiment (APE-B).

Fricke, Robert W., Jr.↗

NASA NTRS2006

Experiments Conducted Aboard the International Space Station: The Pore Formation and Mobility Investigation (PFMI) and the In-Space Soldering Investigation (ISSI): A Current Study of Results

Experiments in support of the Pore Formation and Mobility Investigation (PFMI) and the In-Space Soldering Investigation (ISSI) were conducted aboard the International Space Station (ISS) with the goal of promoting our fundamental understanding of melting dynamics , solidification phenomena, and defect generation during materials processing in a microgravity environment. Through the course of many experiments a number of observations, expected and unexpected, have been directly made. These include gradient-driven bubble migration, thermocapillary flow, and novel microstructural development. The experimental results are presented and found to be in good agreement with models pertinent to a microgravity environment. Based on the space station results, and noting the futility of duplicating them in Earth s unit-gravity environment, attention is drawn to the role ISS experimentslhardware can play to provide insight to potential materials processing techniques and/or repair scenarios that might arise during long duration space transport and/or on the lunar/Mars surface.

Grugel, R. N.↗

NASA NTRS2008

Generic Spacecraft Model for Real-Time Simulation

Generic Spacecraft is the name of an evolving library of software that provides for simulation of a generic spacecraft that can orbit the Earth and land on the Moon (and, eventually, on Mars). This library is incorporated into the Langley Standard Realtime Simulation in C++ (LaSRS++) software framework. The generic-spacecraft simulation serves as a test bed for modeling spacecraft dynamics, propulsion, control systems, guidance, and displays. The Generic Spacecraft library supplements the LaSRS++ framework with an interface that facilitates the connection of new models into the LaSRS++ simulation by eliminating what would otherwise be the necessity of writing additional C++ classes to record data from the models and code to display values on graphical user interfaces (GUIs): The library includes routines for integrating new models into the LaSRS++ framework, identifying model inputs and outputs with full descriptions and units identified, recording data, and automatically generating graphical user interfaces (GUIs). The library is designed to be used in a manner similar to that of LaSRS++ software components for simulating vehicles other than the generic spacecraft. The user specifies (1) a spacecraft and individual models to be constructed and (2) connections between individual model inputs and outputs.

Kenney, Patrick S.↗

NASA NTRS2003

Model of Fluidized Bed Containing Reacting Solids and Gases

A mathematical model has been developed for describing the thermofluid dynamics of a dense, chemically reacting mixture of solid particles and gases. As used here, "dense" signifies having a large volume fraction of particles, as for example in a bubbling fluidized bed. The model is intended especially for application to fluidized beds that contain mixtures of carrier gases, biomass undergoing pyrolysis, and sand. So far, the design of fluidized beds and other gas/solid industrial processing equipment has been based on empirical correlations derived from laboratory- and pilot-scale units. The present mathematical model is a product of continuing efforts to develop a computational capability for optimizing the designs of fluidized beds and related equipment on the basis of first principles. Such a capability could eliminate the need for expensive, time-consuming predesign testing.

Bellan, Josette↗

NASA NTRS1971

Project: Apollo 15

The 12-day Apollo 15 mission, scheduled for launch on July 26 to carry out the fourth United States manned exploration of the Moon, will: Double the time and extend tenfold the range of lunar surface exploration as compared with earlier missions; Deploy the third in a network of automatic scientific stations; Conduct a new group of experiments in lunar orbit; and Return to Earth a variety of lunar rock and soil samples. Scientists expect the results will greatly increase man's knowledge both of the Moon's history and composition and of the evolution and dynamic interaction of the Sun-Earth system. This is so because the dry, airless, lifeless Moon still bears records of solar radiation and the early years of solar system history that have been erased from Earth. Observations of current lunar events also may increase understanding of similar processes on Earth, such as earthquakes. The Apollo 15 Lunar module will make its descent over the Apennine peaks, one of the highest mountain ranges on the Moon, to land near the rim of the canyon-like Hadley Rille. From this Hadley-Apennine lunar base, between the mountain range and the rille, Commander David R. Scott and Lunar Module Pilot James B. Irwin will explore several kilometers from the lunar module, driving an electric-powered lunar roving vehicle for the first time on the Moon. Scott and Irwin will leave the lunar module for three exploration periods to emplace scientific experiments on the lunar surface and to make detailed geologic investigations of formations in the Apennine foothills, along the Hadley Rille rim, and to other geologic structures. The three previous manned landings were made by Apollo 11 at Tranquillity Base, Apollo 12 in the Ocean of Storms and Apollo 14 at Fra Mauro.

Source record↗

NASA NTRS2015

The Legacy of Episodic Climatic Events in Shaping Temperate, Broadleaf Forests

In humid, broadleaf-dominated forests where gap dynamics and partial canopy mortality appears to dominate the disturbance regime at local scales, paleoecological evidence shows alteration at regional-scales associated with climatic change. Yet, little evidence of these broad-scale events exists in extant forests. To evaluate the potential for the occurrence of large-scale disturbance, we used 76 tree-ring collections spanning approx. 840 000 sq km and 5327 tree recruitment dates spanning approx. 1.4 million sq km across the humid eastern United States. Rotated principal component analysis indicated a common growth pattern of a simultaneous reduction in competition in 22 populations across 61 000 km2. Growth-release analysis of these populations reveals an intense and coherent canopy disturbance from 1775 to 1780, peaking in 1776. The resulting time series of canopy disturbance is so poorly described by a Gaussian distribution that it can be described as ''heavy tailed,'' with most of the years from 1775 to 1780 comprising the heavy-tail portion of the distribution. Historical documents provide no evidence that hurricanes or ice storms triggered the 1775-1780 event. Instead, we identify a significant relationship between prior drought and years with elevated rates of disturbance with an intense drought occurring from 1772 to 1775. We further find that years with high rates of canopy disturbance have a propensity to create larger canopy gaps indicating repeated opportunities for rapid change in species composition beyond the landscape scale. Evidence of elevated, regional-scale disturbance reveals how rare events can potentially alter system trajectory: a substantial portion of old-growth forests examined here originated or were substantially altered more than two centuries ago following events lasting just a few years. Our recruitment data, comprised of at least 21 species and several shade-intolerant species, document a pulse of tree recruitment at the subcontinental scale during the late-1600s suggesting that this event was severe enough to open large canopy gaps. These disturbances and their climatic drivers support the hypothesis that punctuated, episodic, climatic events impart a legacy in broadleaf-dominated forests centuries after their occurrence. Given projections of future drought, these results also reveal the potential for abrupt, meso- to large-scale forest change in broadleaf-dominated forests over future decades.

Climatology↗